ncert science viii book

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FOREWORD iii

PREFACE v

Chapter 1

CROP PRODUCTION AND MANAGEMENT 1

Chapter 2

MICROORGANISMS : FRIEND AND FOE 17

Chapter 3

SYNTHETIC FIBRES AND PLASTICS 32

Chapter 4

MATERIALS : METALS AND NON-METALS 44

Chapter 5

COAL AND PETROLEUM 56

Chapter 6

COMBUSTION AND FLAME 64

Chapter 7

CONSERVATION OF PLANTS AND ANIMALS 77

Chapter 8

CELL — STRUCTURE AND FUNCTIONS 90

Chapter 9

REPRODUCTION IN ANIMALS 100

Chapter 10

REACHING THE AGE OF ADOLESCENCE 113CO

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Chapter 11

FORCE AND PRESSURE 127

Chapter 12

FRICTION 146

Chapter 13

SOUND 157

Chapter 14

CHEMICAL EFFECTS OF ELECTRIC CURRENT 172

Chapter 15

SOME NATURAL PHENOMENA 184

Chapter 16

LIGHT 199

Chapter 17

STARS AND THE SOLAR SYSTEM 215

Chapter 18

POLLUTION OF AIR AND WATER 239

INDEX 253

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/book_publishing/class8/science/Index.pdf

CROP PRODUCTION AND MANAGEMENTCROP PRODUCTION AND MANAGEMENT

Paheli and Boojho went to theiruncle’s house during the summervacation. Their uncle is a farmer.

One day they saw some tools like khurpi,sickle, shovel, plough, etc., in the field.

In order to provide food for a largepopulation— regular production, propermanagement and distribution of food isnecessary.

1.1 Agricultural Practices

Till 10,000 B.C. people were nomadic.They were wandering in groups fromplace to place in search of food andshelter. They ate raw fruits andvegetables and started hunting foranimals for food. Later, they couldcultivate land and produce rice, wheatand other food crops. Thus, was born‘Agriculture’.

When plants of the same kind are grownand cultivated at one place on a largescale, it is called a crop. For example,crop of wheat means that all the plantsgrown in a field are that of wheat.

You already know that crops are ofdifferent types like cereals, vegetablesand fruits. These can be classified on thebasis of the season in which they grow.

India is a vast country. The climaticconditions like temperature, humidityand rainfall vary from one region toanother. Accordingly, there is a rich

You have learnt that all livingorganisms require food. Plants can maketheir food themselves. Can you recallhow green plants synthesise their ownfood? Animals including humans cannot make their own food. So, where doanimals get their food from?

But, first of all why do we have to eatfood?

You already know that the energyfrom food is utilised by organisms forcarrying out their various bodyfunctions, such as digestion, respirationand excretion. We get our food fromplants, or animals, or both.

Since we all need food,how can we provide

food to a large numberof people in our

country?

Food has to beproduced on a large

scale.

I want to knowwhere and how we

use these tools.

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variety of crops grown in different partsof the country. Despite this diversity,two broad cropping patterns can beidentified. These are:

(i) Kharif Crops : The crops which aresown in the rainy season are calledkharif crops. The rainy season in Indiais generally from June to September.Paddy, maize, soyabean, groundnut,cotton, etc., are kharif crops.(ii) Rabi Crops : The crops grown in thewinter season are called rabi crops.Their time period is generally fromOctober to March. Examples of rabicrops are wheat, gram, pea, mustardand linseed.

Besides these, pulses and vegetablesare grown during summer at manyplaces.

1.2 Basic Practices of CropProduction

to as agricultural practices. Theseactivities are listed below.

(i) Preparation of soil(ii) Sowing(iii) Adding manure and fertilisers(iv) Irrigation(v) Protecting from weeds(vi) Harvesting(vii) Storage

1.3 Preparation of Soil

The preparation of soil is the first stepbefore growing a crop. One of the mostimportant tasks in agriculture is to turnthe soil and loosen it. This allows theroots to penetrate deep into the soil. Theloose soil allows the roots to breatheeasily even when they go deep into thesoil. Why does the loosening of soil allowthe roots to breathe easily?

The loosened soil helps in the growthof earthworms and microbes present inthe soil. These organisms are friends ofthe farmer since they further turn andloosen the soil and add humus to it.But why does the soil need to be turnedand loosened?

You have learnt in the previousclasses that soil contains minerals,water, air and some living organisms.In addition, dead plants and animalsget decomposed by soil organisms. Inthis way, various nutrients held in thedead organisms are released back intothe soil. These nutrients are againabsorbed by plants.

Since only a few centimetres of thetop layer of soil supports plant growth,turning and loosening of soil brings thenutrient-rich soil to the top so thatplants can use these nutrients. Thus,

Why can paddy not begrown in the winter season?

Paddy requires a lot ofwater. Therefore, it is grown

only in the rainy season.

Cultivation of crops involves severalactivities undertaken by farmers over aperiod of time. You may find that theseactivities are similar to those carried outby a gardener or even by you when yougrow ornamental plants in your house.These activities or tasks are referred


turning and loosening of soil is veryimportant for cultivation of crops.

The process of loosening and turningof the soil is called tilling or ploughing.This is done by using a plough. Ploughsare made of wood or iron. If the soil isvery dry, it may need watering beforeploughing. The ploughed field may havebig pieces of soil called crumbs. It isnecessary to break these crumbs with aplank. The field is levelled for sowing aswell as for irrigation purposes. Thelevelling of soil is done with the help ofa leveller.

Sometimes, manure is added to thesoil before tilling. This helps in propermixing of manure with soil. The soil iswatered before sowing.

Agricultural Implements

Before sowing the seeds, it is necessary tobreak soil to the size of grains to get betteryield. This is done with the help ofvarious tools. The main tools used for thispurpose are the plough, hoe and cultivator.

Plough : This is being used sinceancient times for tilling the soil, addingfertilisers to the crop, removing theweeds, scraping of soil, etc. Thisimplement is made of wood and is drawnby a pair of bulls or other animals(horses, camels, etc.). It contains astrong triangular iron strip calledploughshare. The main part of theplough is a long log of wood which iscalled a ploughshaft. There is a handleat one end of the shaft. The other end isattached to a beam which is placed onthe bulls’ necks. One pair of bulls and aman can easily operate the plough[Fig. 1.1 (a)].

The indigenous wooden plough isincreasingly being replaced by ironploughs nowadays.Hoe : It is a simple tool which is usedfor removing weeds and for looseningthe soil. It has a long rod of wood oriron. A strong, broad and bent plate ofiron is fixed to one of its ends and

Fig. 1.1 (a) : The plough

ploughshaft

ploughshare

beam

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Activity 1.1

Take a beaker and fill half of itwith water. Put a handful of wheatseeds and stir well. Wait for sometime.

Are there seeds which float onwater? Would those be lighter orheavier than those which sink? Whywould they be lighter? Damaged seedsbecome hollow and are thus lighter.Therefore, they float on water.

This is a good method forseparating good, healthy seeds fromthe damaged ones.

Before sowing, one of the importanttasks is to know about the tools usedfor sowing seeds [Fig. 1.2 (a), (b)].

Traditional tool : The tool usedtraditionally for sowing seeds isshaped like a funnel [Fig. 1.2 (a)]. Theseeds are fi l led into the funnel,passed down through two or threepipes having sharp ends. Theseends pierce into the soil and placeseeds there.

Cultivator : Nowadays ploughing isdone by tractor driven cultivator. Theuse of cultivator saves labour and time.[Fig. 1.1 (c)].

Fig. 1.1 (b) : A hoe

Fig. 1.2 (a) : Traditional method of sowing

Fig. 1.1 (c) : Cultivator driven by a tractor

1.4 Sowing

Sowing is the most important part ofcrop production. Before sowing, goodquality seeds are selected. Good qualityseeds are clean and healthy seeds of agood variety. Farmers prefer to use seedswhich give a high yield.

Selection of Seeds

grip beam

bent platerod

handle

One day I saw my mother putsome gram seeds in a vessel

and pour some water onthem. After a few minutessome seeds started to floaton top. I wonder why some

seeds float on water!

works like a blade. It is pulled byanimals [Fig. 1.1 (b)].


Seed drill : Nowadays the seed drill [Fig.1.2 (b)] is used for sowing with the helpof tractors. This tool sows the seedsuniformly at proper distances anddepths. It ensures that seeds get coveredby the soil after sowing. This preventsdamage caused by birds. Sowing byusing a seed drill saves time and labour.

Fig. 1.2 (b) : A seed drill

There is a nursery near myschool. I found that littleplants were kept in small

bags. Why are theykept like this?

Seeds of a few plantssuch as paddy are first

grown in a nursery. Whenthey grow into plantlets,they are transplanted inthe field manually. Some

forest plants and floweringplants are also grown in

the nursery.

An appropriate distance between theseeds is important to avoid overcrowdingof plants. This allows plants to get

I saw a healthy crop growingin a farm. In the neighbouringfarm, the plants were weak.

Why do some plants growbetter than others?

sufficient sunlight, nutrients and waterfrom the soil. Sometimes a few plantshave to be removed to preventovercrowding.

1.5 Adding Manure andFertilisers

The substances which are added to thesoil in the form of nutrients for thehealthy growth of plants are calledmanure and fertilisers.

Soil supplies mineral nutrients to thecrop. These nutrients are essential forthe growth of plants. In certain areas,farmers grow crop after crop in the samefield. The field is never left uncultivatedor fallow. Imagine what happens to thenutrients?

Continuous growing of crops makesthe soil poorer in certain nutrients.Therefore, farmers have to add manureto the fields to replenish the soil withnutrients. This process is calledmanuring. Improper or insufficientmanuring results in weak plants.

Manure is an organic substanceobtained from the decomposition ofplant or animal wastes. Farmers dumpplant and animal waste in pits at openplaces and allow it to decompose. Thedecomposition is caused by somemicroorganisms. The decomposedmatter is used as organic manure.You have already learnt aboutvermicomposting in Class VI.

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Activity 1.2

Take moong or gram seeds andgerminate them. Select three equalsized seedlings out of these. Nowtake three empty glasses or similarvessels. Mark them A, B and C. Toglass A add little amount of soilmixed with a little cow dung manure.In glass B put the same amount ofsoil mixed with a little urea. Takethe same amount of soil in glass Cwithout adding anything [Fig.1.3(a)]. Now pour the same amountof water in each glass and plant theseedlings in them. Keep them in asafe place and water them daily. After7 to 10 days observe their growth[Fig. 1.3(b)].

showed better growth of plants? Inwhich glass was the growth fastest?

Fertilisers are chemical substanceswhich are r ich in a particularnutrient. How are these different frommanure? Fertilisers are produced infactories. Some examples of fertilisersare— urea, ammonium sulphate,super phosphate, potash, NPK(Nitrogen, Phosphorus, Potassium).

The use of fertilisers has helpedfarmers to get better yield of cropssuch as wheat, paddy and maize. Butexcessive use of fertilisers has madethe soil less fertile. Fertilisers have alsobecome a source of water pollution.Therefore, in order to maintain thefert i l i ty of the soi l , we have tosubstitute ferti l isers by organicmanure or leave the field uncultivated(fallow) in between two crops.

The use of manure improves soiltexture as well as its water retainingcapacity. It replenishes the soil withall the nutrients.

Another method of replenishingthe soil with nutrients is through croprotation. This can be done by growingdifferent crops alternately. Earlier,farmers in northern India used togrow legumes as fodder in one seasonand wheat in the next season. Thishelped in the replenishment of the soilwith nitrogen. Farmers are beingencouraged to adopt this practice.

In the previous classes, you havelearnt about Rhizobium bacteria.These are present in the nodules ofthe roots of leguminous plants. Theyfix atmospheric nitrogen.

Fig. 1.3 (a) : Preparation of the experiment

Did all the plants in all the glassesgrow at the same pace? Which glass

Fig. 1.3 (b) : Growing seedlings with manureand fertiliser


Table 1.1 gives the differencesbetween a fertiliser and manure.

Advantages of Manure : The organicmanure is considered better thanfertilisers. This is because

it enhances the water holdingcapacity of the soil.it makes the soil porous due to whichexchange of gases becomes easy.it increases the number of friendlymicrobes.it improves the texture of the soil.

1.6 IrrigationAll living beings need water to live.Water is important for proper growthand development of flowers, fruits andseeds of plants. Water is absorbed bythe plant roots. Along with water,minerals and fertilisers are alsoabsorbed. Plants contain nearly 90%water. Water is essential becausegermination of seeds does not take placeunder dry conditions. Nutrientsdissolved in water get transported to

each part of the plant. Water alsoprotects the crop from both frost andhot air currents. To maintain themoisture of the soil for healthy cropgrowth, fields have to be wateredregularly.

The supply of water to crops atdifferent intervals is called irrigation.The time and frequency of irrigationvaries from crop to crop, soil to soil andseason to season. In summer, thefrequency of watering is higher. Why isit so? Could it be due to the increasedrate of evaporation of water from the soiland the leaves?

Table 1.1 : Differences between Fertiliser and Manure

S. No. Fertiliser Manure

1. A fertiliser is an inorganic Manure is a natural substance obtainedsalt. by the decomposition of cattle dung,

human waste and plant residues.

2. A fertiliser is prepared in Manure can be prepared in the fields.factories.

3. A fertiliser does not provide Manure provides a lot of humus to the soil.any humus to the soil.

4. Fertilisers are very rich in Manure is relatively less rich in plantplant nutrients like nitrogen, nutrients.phosphorus and potassium.

Sources of irrigation : The sources ofirrigation are— wells, tubewells, ponds,lakes, rivers, dams and canals.

I am very careful thisyear about watering

the plants. Lastsummer my plantsdried up and died.

SCIENCE8

Fig. 1.4 (a) : Moat

Fig. 1.4 (b) : Chain pump

Fig. 1.4 (d) : Rahat

Fig. 1.4 (c) : Dhekli

Traditional Methods ofIrrigation

The water available in wells,lakes and canals is lifted upby different methods indifferent regions, for taking itto the fields.

Cattle or human labour isused in these methods. Sothese methods are cheaper,but less efficient. The varioustraditional ways are:

(i) moat (pulley-system)(ii) chain pump

(iii) dhekli, and(iv) rahat (Lever system)

[Figs. 1.4 (a)- (d)].

Pumps are commonlyused for lifting water. Diesel,biogas, electricity andsolar energy is used to runthese pumps.


Modern Methods ofIrrigation

Modern methods of irrigationhelp us to use watereconomically. The mainmethods used are as follows:

(i) Sprinkler System: Thissystem is more useful on theuneven land where sufficientwater is not available. Theperpendicular pipes, havingrotating nozzles on top, arejoined to the main pipelineat regular intervals. Whenwater is allowed to flowthrough the main pipe underpressure with the help of a pump, itescapes from the rotating nozzles. It getssprinkled on the crop as if it is raining.Sprinkler is very useful for sandy soil[Fig. 1.5 (a)].

(ii) Drip system : In this system, thewater falls drop by drop just at the

Fig. 1.5 (b) : Drip System

Fig. 1.5 (a) : Sprinkler system

position of the roots. So it is called dripsystem. It is the best technique forwatering fruit plants, gardens and trees.The system provides water to plants dropby drop [Fig. 1.5(b)]. Water is not wastedat all. It is a boon in regions whereavailability of water is poor.

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1.7 Protection from Weeds

Boojho and Paheli went to a nearbywheat field and saw that there were someother plants in the field.

Have these otherplants been planted

purposely?

In a field many other undesirableplants may grow naturally along withthe crop. These undesirable plants arecalled weeds.

The removal of weeds is calledweeding. Weeding is necessary sinceweeds compete with the crop plants forwater, nutrients, space and light. Thus,they affect the growth of the crop. Someweeds interfere even in harvesting andmay be poisonous for animals andhuman beings.

Farmers adopt many ways to removeweeds and control their growth. Tillingbefore sowing of crops helps inuprooting and killing of weeds, whichmay then dry up and get mixed withthe soil. The best time for the removalof weeds is before they produce flowersand seeds. The manual removal includesphysical removal of weeds by uprootingor cutting them close to the ground,from time to time. This is done with thehelp of a khurpi. A seed drill [Fig. 1.2(b)]is also used to uproot weeds.

Weeds are also controlled by usingcertain chemicals, called weedicides,like 2,4-D. These are sprayed in thefields to kill the weeds. They do notdamage the crops. The weedicides arediluted with water to the extent requiredand sprayed in the fields with a sprayer.(Fig. 1.6).

Do weedicides have anyeffect on the person handling

the weedicide sprayer?

Fig. 1.6 : Spraying weedicide

As already mentioned, the weedicidesare sprayed during the vegetative growthof weeds before flowering and seedformation. Spraying of weedicides mayaffect the health of farmers. So theyshould use these chemicals very carefully.They should cover their nose and mouthwith a piece of cloth during spraying ofthese chemicals.

1.8 Harvesting

Harvesting of a crop is an importanttask. The cutting of crop after it ismature is called harvesting. Inharvesting, crops are pulled out or cutclose to the ground. It usually takes 3to 4 months for a cereal crop to mature.

Harvesting in ourcountry is either donemanually by sickle(Fig. 1.7) or by a machinecalled harvester. In theharvested crop, the grainseeds need to be separated

Fig. 1.7 :Sickle


from the chaff. This process is calledthreshing. This is carried out with thehelp of a machine called ‘combine’ whichis in fact a combined harvester andthresher (Fig. 1.8).

Fig. 1.8 : Combine

After harvesting, sometimesstubs are left in the field, whichare burnt by farmers. Paheli is

worried. She knows that itcauses pollution. It may also

catch fire and damage the cropslying in the fields.

Harvest Festivals

After three or four months of hardwork there comes the day of theharvest. The sight of golden fieldsof standing crop, laden with grain,fills the hearts of farmers with joyand a sense of well-being. Theefforts of the past season haveborne fruit and it is time to relaxand enjoy a little. The period ofharvest is, thus, of great joy andhappiness in all parts of India.Men and women celebrate it withgreat enthusiasm. Specialfest ivals associated with theharvest season are Pongal ,Baisakhi, Holi, Diwali, Nabanyaand Bihu.

1.9 Storage

Storage of produce is an important task.If the crop grains are to be kept forlonger time, they should be safe frommoisture, insects, rats andmicroorganisms. The fresh crop hasmore moisture. If freshly harvestedgrains (seeds) are stored without drying,they may get spoilt or attacked byorganisms, losing their germinationcapacity. Hence, before storing them,the grains are properly dried in the sunto reduce the moisture in them. Thisprevents the attack by insect pests,bacteria and fungi. Farmers store

Farmers with small holdings of landdo the separation of grain and chaff bywinnowing (Fig. 1.9). You have alreadystudied this in Class VI.

Fig. 1.9 : Winnowing machine

I saw my mother puttingsome dried neem leaves

in an iron drumcontaining wheat.

I wonder why?

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1.10Food from AnimalsFig. 1.10 (a) : Silos for storage of grains

Fig. 1.10 (b) : Storage of grains in granaries

grains in jute bags or metallic bins.However, large scale storage of grainsis done in silos and granaries to protectthem from pests like rats and insects[Fig. 1.10 (a) and (b)].

Dried neem leaves are used forstoring food grains at home. For storinglarge quantities of grains in biggodowns, specific chemical treatmentsare required to protect them from pestsand microorganisms.

Activity 1.3

Make the following Table in yournote book and complete it.

S.No. Food Sources

1. Milk Cow, Buffalo, She-goat, She-camel . . .

2.

3.

4.

Fish is good for health.We get cod liver oil from fishwhich is rich in vitamin D.

After completing this Table, youmust have seen that, like plants,animals also provide us with differentkinds of food. Many people living in thecoastal areas consume fish as a majorpart of their diet. In the previous classesyou have learnt about the food that weobtain from plants. We have just seenthat the process of crop productioninvolves a number of steps like selectionof seeds, sowing, etc. Similarly, animalsreared at home or in farms, have to beprovided with proper food, shelter andcare. When this is done on a large scale,it is called animal husbandry.


KEYWORDS

AGRICULTURAL

PRACTICES

ANIMAL HUSBANDRY

CROP

FERTILISER

GRANARIES

HARVESTING

IRRIGATION

KHARIF

MANURE

PLOUGH

RABI

SEEDS

SILO

SOWING

STORAGE

THRESHING

WEEDS

WEEDICIDE

WINNOWING

WHAT YOU HAVE LEARNT

In order to provide food to our growingpopulation, we need to adopt certainagricultural practices.

Same kind of plants grown and cultivated ata place constitute a crop.In India, crops can be broadly categorised into

two types based on seasons - rabi and kharifcrops.

It is necessary to prepare soil by tilling andlevelling. Ploughs and levellers are used forthis purpose.

Sowing of seeds at appropriate depths anddistances gives good yield. Good variety of

seeds are sown after selection of healthy seeds.Sowing is done by seed drills.

Soil needs replenishment and enrichmentthrough the use of organic manure andfertilisers. Use of chemical fertilisers has

increased tremendously with the introductionof new crop varieties.

Supply of water to crops at appropriateintervals is called irrigation.Weeding involves removal of unwanted and

uncultivated plants called weeds.Harvesting is the cutting of the mature crop

manually or by machines.Separation of the grains from the chaff is calledthreshing.

Proper storage of grains is necessary to protectthem from pests and microorganisms.

Food is also obtained from animals for whichanimals are reared. This is called animal

husbandry.

Exercises

1. Select the correct word from the following list and fill in the blanks.float, water, crop, nutrients, preparation

(a) The same kind of plants grown and cultivated on a large scale at aplace is called _____________.

(b) The first step before growing crops is _____________ of the soil.

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(c) Damaged seeds would _____________ on top of water.(d) For growing a crop, sufficient sunlight and _____________ and

_____________ from the soil are essential.

2. Match items in column A with those in column B.

A B

(i) Kharif crops (a) Food for cattle

(ii) Rabi crops (b) Urea and super phosphate

(iii) Chemical fertilisers (c) Animal excreta, cow dungurine and plant waste

(iv) Organic manure (d) Wheat, gram, pea

(e) Paddy and maize

3. Give two examples of each.(a) Kharif crop(b) Rabi crop

4. Write a paragraph in your own words on each of the following.

(a) Preparation of soil (b) Sowing

(c) Weeding (d) Threshing

5. Explain how fertilisers are different from manure.

6. What is irrigation? Describe two methods of irrigation which conserve water.

7. If wheat is sown in the kharif season, what would happen? Discuss.

8. Explain how soil gets affected by the continuous plantation of crops ina field.

9. What are weeds? How can we control them?

10. Arrange the following boxes in proper order to make a flow chart ofsugarcane crop production.

E X

E R

C I

S E

S

Sending crop tosugar factory Irrigation Harvesting Sowing

1 2 3 4

Preparation ofsoil

Ploughing thefield

Manuring

5 6 7


11. Complete the following word puzzle with the help of clues given below.

Down1. Providing water to the crops.2. Keeping crop grains for a long time under proper conditions.5. Certain plants of the same kind grown on a large scale.

Across3. A machine used for cutting the matured crop.4. A rabi crop that is also one of the pulses.6. A process of separating the grain from chaff.

E X

E R

C I

S E

S

Extended Learning — Activities and Projects

1. Sow some seeds in the soil and arrange to water them by dripirrigation. Observe daily.(i) Do you think it can save water?(ii) Note the changes in the seed.

2. Collect different types of seeds and put them in small bags. Attachthese bags in a herbarium file and label them.

3. Collect new agricultural machine pictures and paste in a file withtheir names and uses.

4. Project Work

Visit a farm, nursery or a garden nearby. Gather information about(i) importance of seed selection.(ii) method of irrigation.

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An Example for Field Trip Work

Himanshu and his friends were very anxious and curious to go to Thikri village.They went to Shri Jiwan Patel’s farmhouse. They had taken bags to collect someseeds and other things.

Himanshu : Sir namaskar, I am Himanshu. Here are my friends Mohan, Davidand Sabiha. We want some information about crops. Please guideus.

Shri Patel : Namaskar and welcome all of you. What are your queries?

Sabiha : When did you start this work and what are the main crops thatyou grow?

Shri Patel : About 75 years ago, my grandfather started this work. The maincrops that we grow are wheat, gram, soyabean and moong.

David : Sir, can you tell us the difference between traditional and modernagricultural practices?

Shri Patel : Earlier we used traditional tools like sickle, bullock plough, trowel,etc., and depended on rain water for irrigation. But now we usemodern methods of irrigation. We use implements like tractors,cultivators, seed drill and harvester. We get good quality seeds.We carry out soil testing and use manure and fertilisers. Newinformation about agriculture is obtained through radio, T.V. andother sources. As a result we are able to get good crops on a largescale. This year we got 9 to 11 quintals of gram crop/acre and 20to 25 quintals of wheat/acre. In my opinion awareness of newtechnology is important for better crop yield.

Mohan : Sabiha, come here and see some earthworms. Are they helpful tothe farmers?

Sabiha : Oh Mohan! we learnt about it in Class VI.

Shri Patel : Earthworms turn the soil and loosen it for proper aeration, sothey help the farmer.

David : Can we have some seeds of the crops you grow here?

[They put some seeds, fertilisers and soil sample in the bags.]

Himanshu : Sir, we are thankful to you for making this visit pleasant and forproviding useful information.

(iii) effect of extreme cold and extreme hot weather on the plants.(iv) effect of continuous rain on the plants.(v) fertilisers/manure used.

For more information, visit :www.krishiworld.com/html/balanced fertiliser. htm.www.ikis.com/links/ap.cultivation.html

MICROORGANISMS : FRIEND AND FOEMICROORGANISMS : FRIEND AND FOE

You have seen several kinds ofplants and animals. However,there are other living organisms

around us which we cannot see witheyes alone. These are calledmicroorganisms or microbes. Forexample, you might have observed thatduring rainy season moist bread getsspoilt and its surface gets covered withgreyish white patches. Observe thesepatches through a magnifying glass. Youwill see tiny, black rounded structures.Do you know what these structures areand where did these come from?

2.1 Microorganisms

Activity 2.1

Collect some moist soil from thefield in a beaker and add water toit. After soil particles have settled,observe a drop of water from thebeaker under a microscope. Whatdo you see ?

Activity 2.2

Take a few drops of water from apond. Spread on a glass slide andobserve through a microscope.

Do you find tiny organisms movingaround?

These observations show that waterand soil are full of tiny organisms,though not all of them fall into thecategory of microbes. Thesemicroorganisms or microbes are sosmall in size that they cannot be seenwith the unaided eye. Some of these,such as the fungus that grows on bread,can be seen with a magnifying glass.Others cannot be seen without the helpof a microscope. That is why these arecalled microorganisms or microbes.

Microorganisms are classified intofour major groups. These groups arebacteria, fungi, protozoa and somealgae. Some of these commonmicroorganisms are shown inFigs. 2.1 - 2.4.

Viruses are also microscopic. They,however, reproduce only inside the cellsof the host organism, which may be abacterium, plant or animal. Some of theviruses are shown in Fig. 2.5. Commonailments like cold, influenza (flu) andmost coughs are caused by viruses.Serious diseases like polio and chickenpox are also caused by viruses.

Diseases like dysentery and malariaare caused by protozoans whereastyphoid and tuberculosis (TB) arebacterial diseases.

You have learnt about some ofthese microorganisms in Classes VIand VII.

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Fig. 2.4: Fungi

Spiral bacteria Rod shaped bacteria

Fig. 2.1: Bacteria

Chlamydomonas Spirogyra

Fig. 2.2 : Algae

Amoeba

Paramecium

Fig. 2.3 : Protozoa

Bread mould Penicillium Aspergillus


2.2 Where do MicroorganismsLive?

Microorganisms may be single-celledlike bacteria, some algae and protozoa,or multicellular, such as algae and fungi.They can survive under all types ofenvironment, ranging from ice coldclimate to hot springs and deserts tomarshy lands. They are also foundinside the bodies of animals includinghumans. Some microorganisms grow onother organisms while others existfreely. Microorganisms like amoeba canlive alone, while fungi and bacteria maylive in colonies.

2.3 Microorganisms and Us

Microorganisms play an important rolein our lives. Some of them are beneficialin many ways whereas some others areharmful and cause diseases. Let us studyabout them in detail.

Microorganisms have been used forthe production of alcohol since ages.

Friendly Microorganisms

Microorganisms are used for variouspurposes. They are used in thepreparation of curd, bread and cake.

Curd contains several micro-organisms. Of these, the bacteriumLactobacillus promotes the formationof curd. It multiplies in milk and convertsit into curd. Bacteria are also involvedin the making of cheese, pickles andmany other food items. An importantingredient of rava (sooji ) idlis andbhaturas is curd. Can you guess why?

Activity 2.3

Take ½ kg flour (atta or maida),add some sugar and mix with

Fig. 2.5 : Viruses

I saw that my motheradded a little curd to warm

milk to set curd for thenext day. I wonder why!

They are also used in cleaning upof the environment. For example, theorganic wastes (vegetable peels, remainsof animals, faeces, etc.) are brokendown into harmless and usablesubstances by bacteria. Recall thatbacteria are also used in thepreparation of medicines. In agriculturethey are used to increase soil fertilityby fixing nitrogen.

Making of Curd and Bread

You have learnt in Class VII that milk isturned into curd by bacteria.

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yeast powder to the sugar solution.Keep it covered in a warm place for4-5 hours. Now smell the solution.Could you get a smell?

Louis Pasteurdiscovered

fermentationin 1857.

In 1929, AlexanderFleming was workingon a culture of disease-causing bacteria.Suddenly he found the

spores of a little green mould in one ofhis culture plates. He observed thatthe presence of mould prevented thegrowth of bacteria. In fact, it also killedmany of these bacteria. From this themould penicillin was prepared.

This is the smell of alcohol as sugarhas been converted into alcohol by yeast.This process of conversion of sugar intoalcohol is known as fermentation.

warm water. Add a small amountof yeast powder and knead tomake a soft dough. What do youobserve after two hours? Did youfind the dough rising?

Maida with Yeast Powder

Raised maida

Fig. 2.6

Yeast reproduces rapidly andproduces carbon dioxide duringrespiration. Bubbles of the gas fill thedough and increase its volume (Fig. 2.6).This is the basis of the use of yeast inthe baking industry for making breads,pastries and cakes.

Commercial Use of Microorganisms

Microorganisms are used for the largescale production of alcohol, wine andacetic acid (vinegar). Yeast is used forcommercial production of alcoholand wine. For this purpose yeast isgrown on natural sugars present ingrains like barley, wheat, rice andcrushed fruit juices, etc.

Activity 2.4

Take a 500 mL beaker filled upto ¾with water. Dissolve 2-3 teaspoonsof sugar in it. Add half a spoon of

Medicinal Use of Microorganisms

Whenever you fall ill the doctor maygive you some antibiotic tablets,capsules or injections such as ofpenicillin. The source of thesemedicines is microorganisms. Thesemedicines kill or stop the growth of thedisease-causing microorganisms.Such medicines are called antibiotics.These days a number of antibiotics arebeing produced from bacteria andfungi. Streptomycin, tetracycline anderythromycin are some of the


commonly known antibiotics which aremade from fungi and bacteria. Theantibiotics are manufactured bygrowing specific microorganisms andare used to cure a variety of diseases.

Antibiotics are even mixed with thefeed of livestock and poultry to checkmicrobial infection in animals. They arealso used to control many plantdiseases.

It is important to remember thatantibiotics should be taken only onthe advice of a qualified doctor. Alsoyou must finish the courseprescribed by the doctor. If you takeantibiotics when not needed or inwrong doses, it may make the drugless effective when you might needit in future. Also antibiotics takenunnecessarily may kill the beneficialbacteria in the body. Antibiotics,however, are not effective againstcold and flu as these are caused byviruses.

Vaccine

we are protected from the disease-causing microbes. This is how a vaccineworks. Several diseases, includingcholera, tuberculosis, smallpox andhepatitis can be prevented byvaccination.

Edward Jennerdiscovered thevaccine for small-pox in 1798.

Why are children/infants givenvaccination?

When a disease-carrying microbe entersour body, the body produces antibodiesto fight the invader. The body alsoremembers how to fight the microbe ifit enters again. So, if dead or weakenedmicrobes are introduced in a healthybody, the body fights and killsthem by producing suitable antibodies.The antibodies remain in the body and

In your childhood, you must havebeen given injections to protect yourselfagainst several diseases. Can youprepare a list of these diseases? Youmay take help from your parents.

It is essential to protect all childrenagainst these diseases. Necessary vaccinesare available in the nearby hospitals. Youmight have seen the advertisement on T.V.and newspapers regarding protection ofchildren against polio under Pulse PolioProgram. Polio drops given to children areactually a vaccine.

A worldwide campaign againstsmallpox has finally led to itseradication from most parts of theworld.

These days vaccines are made on alarge scale from microorganisms toprotect humans and other animals fromseveral diseases.

Increasing Soil Fertility

Some bacteria and blue green algae(Fig. 2.7) are able to fix nitrogen fromthe atmosphere to enrich soil withnitrogen and increase its fertility. Thesemicrobes are commonly calledbiological nitrogen fixers.

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Cleaning the Environment

Boojho and Paheli had observed theschool gardener making manure. Alongwith their friends, they collected wastesof plants, vegetables and fruits fromnearby houses and gardens. They putthem in a pit meant for waste disposal.After some time, it decomposed and gotconverted to manure. Boojho andPaheli wanted to know how this couldhappen.

Activity 2.5

Take two pots and fill each pot halfwith soil. Mark them A and B. Putplant waste in pot A and things likepolythene bags, empty glass bottlesand broken plastic toys in pot B.Put the pots aside. Observe themafter 3-4 weeks.

Do you find any difference in thecontents of the two pots? If so, what isthe difference? You will find that plantwaste in pot A, has been decomposed.How could this happen? The plant wastehas been converted into manure by theaction of microbes. The nutrients

released in the process could be usedby the plants again.

Did you notice that in pot B, thepolythene bags, empty glasses, bottlesand broken toy parts did not undergoany such change? The microbes couldnot ‘act’ on them and convert them intomanure.

You often see large amounts of deadorganic matter in the form of decayingplants and sometimes dead animals onthe ground. You find that theydisappear after some time. This isbecause the microorganismsdecompose dead organic waste of plantsand animals converting them intosimple substances. These substancesare again used by other plants andanimals. Thus, microorganisms canbe used to degrade the harmful andsmelly substances and thereby cleanup the environment.

2.4 Harmful Microorganisms

Microorganisms are harmful in manyways. Some of the microorganismscause diseases in human beings, plantsand animals. Such disease-causing

Fig. 2.7 : The Nitrogen fixing blue green algae


microorganisms are called pathogens.Some microorganisms spoil food,clothing and leather. Let us study moreabout their harmful activities.

Disease— causing Microorganismsin Humans

Pathogens enter our body through theair we breathe, the water we drink orthe food we eat. They can also gettransmitted by direct contact with aninfected person or carried through ananimal. Microbial diseases that canspread from an infected person to ahealthy person through air, water, foodor physical contact are calledcommunicable diseases. Examples ofsuch diseases include cholera, commoncold, chicken pox and tuberculosis.

When a person suffering fromcommon cold sneezes, fine droplets ofmoisture carrying thousands of virusesare spread in the air. The virus may enterthe body of a healthy person whilebreathing.

There are some insects and animalswhich act as carriers of disease-causing microbes. Housefly is one suchcarrier. The flies sit on the garbage andanimal excreta. Pathogens stick to theirbodies. When these flies sit on uncoveredfood they may transfer the pathogens.Whoever eats the contaminated food islikely to get sick. So, it is advisable toalways keep food covered. Avoidconsuming uncovered items of food.Another example of a carrier is thefemale Anopheles mosquito (Fig. 2.8),which carries the parasite of malaria.Female Aedes mosquito acts as carrierof dengue virus. How can we control thespread of malaria or dengue?

Then how do youprevent the spread of

communicablediseases?

We should keep ahandkerchief on the

nose and mouth whilesneezing. It is better tokeep a distance from

infected persons.

Fig. 2.8 : Female Anopheles mosquito

Why does the teacherkeep telling us notto let water collectanywhere in theneighbourhood?

All mosquitoes breed in water. Hence,one should not let water collectanywhere, in coolers, tyres, flower potetc. By keeping the surroundings cleanand dry we can prevent mosquitoes frombreeding. Try to make a list of measureswhich help to avoid the spread ofmalaria.

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Some of the common diseasesaffecting humans, their mode oftransmission and few general methodsof prevention are given in Table 2.1.

Disease— causing Microorganismsin Animals

Several microorganisms not only causediseases in humans and plants, but also

in other animals. For example, anthraxis a dangerous human and cattledisease caused by a bacterium. Footand mouth disease of cattle is causedby a virus.

Disease— causing Microorganismsin Plants

Several microorganisms causediseases in plants like wheat, rice, potato,sugarcane, orange, apple and others.The diseases reduce the yield of crops.See Table 2.2 for some such plantdiseases. They can be controlled by the

Human Disease Causative Mode of Preventive measuresMicroorganism Transmission (General)

Tuberculosis Bacteria Air

Measles Virus Air

Chicken Pox Virus Air/Contact

Polio Virus Air/Water

Cholera Bacteria Water/Food

Typhoid Bacteria Water

Hepatitis B Virus Water

Malaria Protozoa Mosquito

Robert Köch (1876)discovered the bacterium(Bacillus anthracis) whichcauses anthrax disease.

Keep the patient in completeisolation. Keep the personalbelongings of the patient awayfrom those of the others.Vaccination to be given atsuitable age.

Maintain personal hygieneand good sanitary habits.Consume properly cooked foodand boiled drinking water.Vaccination.

Drink boiled drinking water.Vaccination.

Use mosquito net andrepellents. Spray insecticidesand control breeding ofmosquitoes by not allowingwater to collect in thesurroundings.

Table 2.1: Some Common Human Diseases caused by Microorganisms


use of certain chemicals which kill themicrobes.

Food Poisoning

Boojho was invited by his friend to aparty and he ate a variety of foodstuff.On reaching home he started vomiting.He had to be taken to a hospital. Thedoctor said that this condition could bedue to food poisoning.

make the food poisonous causingserious illness and even death. So, itis very important that we preservefood to prevent it from being spoilt.

2.5 Food Preservation

In Chapter 1, we have learnt about themethods used to preserve and store foodgrains. How do we preserve cooked foodat home? You know that bread leftunused under moist conditions isattacked by fungus. Microorganismsspoil our food. Spoiled food emits badsmell and has a bad taste and changedcolour. Is spoiling of food a chemicalreaction?

Paheli bought some mangoes but shecould not eat them for a few days. Latershe found that they were spoilt androtten. But she knows that the mangopickle her grandmother makes does notspoil for a long time. She is confused.

Plant Micro- Mode of FiguresDiseases organism Transmission

Citrus Bacteria Air

canker

Rust of Fungi Air,

wheat seeds

Yellow vein Virus Insect

mosaic of

bhindi (Okra)

Paheli wonders howfood can become a

‘poison’.

Table 2.2: Some Common Plant Diseases caused by Microorganisms

Food poisoning could be due to theconsumption of food spoilt by somemicroorganisms. Microorganismsthat grow on our food sometimesproduce toxic substances. These

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Let us study the common methodsto preserve food in our homes. We haveto prevent it from the attack ofmicroorganisms.

Chemical Method

Salts and edible oils are the commonchemicals generally used to check thegrowth of microorganisms. Thereforethey are called preservatives. We addsalt or acid preservatives to pickles toprevent the attack of microbes. Sodiumbenzoate and sodium metabisulphite arecommon preservatives. These are alsoused in the jams and squashes to checktheir spoilage.

Preservation by Common Salt

Common salt has been used to preservemeat and fish for ages. Meat and fishare covered with dry salt to check thegrowth of bacteria. Salting is also usedto preserve amla, raw mangoes,tamarind, etc.

Preservation by Sugar

Jams, jellies and squashes are preservedby sugar. Sugar reduces the moisturecontent which inhibits the growth ofbacteria which spoil food.

Preservation by Oil and Vinegar

Use of oil and vinegar prevents spoilageof pickles because bacteria cannot livein such an environment. Vegetables,fruits, fish and meat are often preservedby this method.

Heat and Cold Treatments

You must have observed your motherboiling milk before it is stored or used.Boiling kills many microorganisms.

Similarly, we keep our food in therefrigerator. Low temperature inhibitsthe growth of microbes.

Pasteurized milk can be consumedwithout boiling as it is free from harmfulmicrobes. The milk is heated to about700C for 15 to 30 seconds and thensuddenly chilled and stored. By doingso, it prevents the growth of microbes.This process was discovered by LouisPasteur. It is called pasteurization.

Storage and Packing

These days dry fruits and evenvegetables are sold in sealed air tightpackets to prevent the attack ofmicrobes.

2.6 Nitrogen Fixation

You have learnt about the bacteriumRhizobium in Classes VI and VII. It isinvolved in the fixation of nitrogen inleguminous plants (pulses). Recall thatRhizobium lives in the root nodules ofleguminous plants (Fig. 2.9), such asbeans and peas, with which it has asymbiotic relationship. Sometimesnitrogen gets fixed through the actionof lightning. But you know that theamount of nitrogen in the atmosphereremains constant. You may wonderhow? Let us understand this in the nextsection.

Why does the milk thatcomes in packets not spoil?My mother told me that themilk is ‘pasteurized’. What

is pasteurization?


nitrogen cannot be taken directly byplants and animals. Certain bacteria andblue green algae present in the soil fixnitrogen from the atmosphere andconvert into compounds of nitrogen.Once nitrogen is converted into theseusable compounds, it can be utilised byplants from the soil through their rootsystem. Nitrogen is then used for thesynthesis of plant proteins and othercompounds. Animals feeding on plantsget these proteins and other nitrogencompounds (Fig. 2.10).

When plants and animals die,bacteria and fungi present in the soilconvert the nitrogenous wastes intonitrogenous compounds to be used byplants again. Certain other bacteriaconvert some part of them to nitrogengas which goes back into theatmosphere. As a result, the percentageof nitrogen in the atmosphere remainsmore or less constant.

Fig. 2.10 : Nitrogen cycle

Fig. 2.9 : Roots of a leguminous plant with rootnodules

2.7 Nitrogen cycle

Our atmosphere has 78% nitrogen gas.Nitrogen is one of the essentialconstituents of all living organisms aspart of proteins, chlorophyll, nucleicacids and vitamins. The atmospheric

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KEYWORDS

ALGAE

ANTIBIOTICS

ANTIBODIES

BACTERIA

CARRIER

COMMUNICABLE-

DISEASES

FERMENTATION

FUNGI

LACTOBACILLUS

MICROORGANISM

NITROGEN CYCLE

NITROGEN FIXATION

PASTEURIZATION

PATHOGEN

PRESERVATION

PROTOZOA

RHIZOBIUM

VACCINE

VIRUS

YEAST


� Microorganisms are too small and are not

visible to the unaided eye.

� They can live in all kinds of environment,

ranging from ice cold climate to hot springs

and deserts to marshy lands.

� Microorganisms are found in air, water and

in the bodies of plants and animals.

� They may be unicellular or multicellular.

� Microorganisms include bacteria, fungi,

protozoa and some algae. Viruses, though

different from the above mentioned living

organisms, are considered microbes.

� Viruses are quite different from other

microorganisms. They reproduce only inside

the host organism; bacterium, plant or animal

cell.

� Some microorganisms are useful for

commercial production of medicines and

alcohol.

� Some microorganisms decompose the organic

waste and dead plants and animals into simple

substances and clean up the environment.

� Protozoans cause serious diseases like

dysentery and malaria.

� Some of the microorganisms grow on our food

and cause food poisoning.

� Some microorganisms reside in the root

nodules of leguminous plants. They can fix

nitrogen from air into soil and increase the

soil fertility.

� Some bacteria and blue green algae present

in the soil fix nitrogen from the atmosphere

and convert into nitrogenous compounds.

� Certain bacteria convert compounds of

nitrogen present in the soil into nitrogen gas

which is released to the atmosphere.


Exercises

1. Fill in the blanks:

(a) Microorganisms can be seen with the help of a ____________.

(b) Blue green algae fix __________ directly from air to enhance fertilityof soil.

(c) Alcohol is produced with the help of __________.

(d) Cholera is caused by __________.

2. Tick the correct answer:

(a) Yeast is used in the production of

(i) sugar (ii) alcohol (iii) hydrochloric acid (iv) oxygen

(b) The following is an antibiotic

(i) Sodium bicarbonate (ii) Streptomycin (iii) Alcohol (iv) Yeast

(c) Carrier of malaria-causing protozoan is

(i) female Anopheles mosquito (ii) cockroach(iii) housefly (iv) butterfly

(d) The most common carrier of communicable diseases is

(i) ant (ii) housefly (iii) dragonfly (iv) spider

(e) The bread or idli dough rises because of

(i) heat (ii) grinding (iii) growth of yeast cells (iv) kneading

(f) The process of conversion of sugar into alcohol is called

(i) nitrogen fixation (ii) moulding (iii) fermentation (iv) infection

3. Match the organisms in Column I with their action inColumn II.

Column I Column II

(i) Bacteria (a) Fixing Nitrogen

(ii) Rhizobium (b) Setting of curd

(iii) Lactobacillus (c) Baking of bread

(iv) Yeast (d) Causing Malaria

(v) A protozoan (e) Causing Cholera

(vi) A Virus (f) Causing AIDS

(g) Producing antibodies

4. Can microorganisms be seen with the naked eye? If not, how can they beseen?

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5. What are the major groups of microorganisms?

6. Name the microorganisms which can fix atmospheric nitrogen in the soil.

7. Write 10 lines on the usefulness of microorganisms in our lives.

8. Write a short paragraph on the harms caused by microorganisms.

9. What are antibiotics? What precautions must be taken while takingantibiotics?E

X E

R C

I S

E S

A B


1. Pull out a gram or bean plant from the field. Observe its roots. Youwill find round structures called root nodules on the roots. Draw adiagram of the root and show the root nodules.

2. Collect the labels from the bottles of jams and jellies. Write downthe list of contents printed on the labels.

3. Visit a doctor. Find out why antibiotics should not be overused.Prepare a short report.

4. Project : Requirements – 2 test tubes, marker pen, sugar, yeastpowder, 2 balloons and lime water.

Take two test tubes and mark them A and B. Clamp these tubes ina stand and fill them with water leaving some space at the top. Puttwo spoonfuls of sugar in eachof the test tubes. Add a spoonfulof yeast in test tube B. Inflate thetwo balloons incompletely. Nowtie the balloons on the mouthsof each test tube. Keep them ina warm place, away fromsunlight. Watch the setup everyday for next 3-4 days. Recordyour observations and think ofan explanation.

Now take another test tubefilled 1/4 with lime water.Remove the balloon from testtube B in such a manner thatgas inside the balloon does notescape. Fit the balloon on thetest tube and shake well.Observe and explain.


Did You Know?

Bacteria have lived on the earth for much longer than human beings.They are such hardy organisms that they can live under extreme conditions.They have been found living in boiling mudpots and extremly cold icy waters.They have been found in lakes of caustic soda and in pools of concentratedsulphuric acid. They can survive at depths of several kilometres. Theyprobably can survive in space, too. A kind of bacterium was recoveredfrom a camera which stood on the moon for two years.There is probably noenvironment in which bacteria cannot survive.

5. For more information, visit the following websites:� www.microorganisms� www.biology4kids.com/files/micro_main html

SYNTHETIC FIBRES AND PLASTICSSYNTHETIC FIBRES AND PLASTICS

The clothes which we wear aremade of fabrics. Fabricsare made from fibres obtained

from natural or artificial sources. Canyou name some natural fibres? Fibresare also used for making a large varietyof household articles. Make a list of somecommon articles made from fibres. Tryto separate them into those made fromnatural fibres and those made fromartificial fibres. Make entries in Table 3.1.

Why did you label some fibres asartificial?

You have read in your previousclasses that natural fibres like cotton,wool, silk, etc., are obtained from plantsor animals. The synthetic fibres, on theother hand, are made by human beings.That is why these are called syntheticor man-made fibres.

3.1 What are SyntheticFibres?

Try to recall the uniform pattern foundin a necklace of beads joined with thehelp of a thread [Fig. 3.1(a)]. Or, try tojoin a number of paper clips together tomake a long chain, as in Fig. 3.1 (b). Isthere any similarity between the two?

Fig. 3.1 : (a) Beads and(b) paper clips joined toform long chains

(a) (b)

Table 3.1 : Natural and artificialfibres

S. No. Name of Type of FibreArticle (Natural/

artificial)

A synthetic fibre is also a chain ofsmall units joined together. Each smallunit is actually a chemical substance.Many such small units combine to forma large single unit called a polymer. Theword ‘polymer’ comes from two Greekwords; poly meaning many and mermeaning part/unit. So, a polymer is madeof many repeating units.

Polymers occur in nature also. Cotton,for example, is a polymer calledcellulose. Cellulose is made up of alarge number of glucose units.

3.2 Types of Synthetic Fibres

A. Rayon

You have read in Class VII that silk fibreobtained from silkworm was discoveredin China and was kept as a closelyguarded secret for a long time. Fabricobtained from silk fibre was very costly.But its beautiful texture fascinatedeverybody. Attempts were made to makesilk artificially. Towards the end of thenineteenth century, scientists weresuccessful in obtaining a fibre havingproperties similar to that of silk. Such afibre was obtained by chemicaltreatment of wood pulp. This fibre wascalled rayon or artificial silk. Althoughrayon is obtained from a natural source,wood pulp, yet it is a man-made fibre. Itis cheaper than silk and can be wovenlike silk fibres. It can also be dyed in awide variety of colours. Rayon is mixedwith cotton to make bed sheets or mixedwith wool to make carpets. (Fig. 3.2.)

B. Nylon

Nylon is another man-made fibre. In1931, it was made without using anynatural raw material (from plant oranimal). It was prepared from coal, waterand air. It was the first fully syntheticfibre.

Nylon fibre was strong, elastic andlight. It was lustrous and easy to wash.So, it became very popular for makingclothes.

We use many articles made fromnylon, such as socks, ropes, tents,toothbrushes, car seat belts, sleepingbags, curtains etc. (Fig. 3.3). Nylon is


Fig. 3.4: Use of Nylon Fibres

Fig. 3.3: Various Articlesmade from Nylon

Is nylon fibrereally so strong

that we can makenylon parachutesand ropes for rock

climbing?

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Fig. 3.4: Use of Nylon Fibres

Fig. 3.5: An iron stand with a thread hangingfrom the clamp.

also used for making parachutes andropes for rock climbing (Fig. 3.4). Anylon thread is actually stronger thana steel wire.

Let us find out.

Activity: 3.1

Take an iron stand with a clamp.Take a cotton thread of about 60cm length. Tie it to the clamp sothat it hangs freely from it as shownin Fig. 3.5. At the free end suspand

a pan so that weights can be placedin it. Add weights one by one tillthe thread breaks. Note down thetotal weight required to break thethread. This weight indicates thestrength of the fibre. Repeat thesame activity with threads of wool,polyester, silk and nylon. Tabulatethe data as shown in Table 3.2.Arrange the threads in order of theirincreasing strength.

Observation Table 3.2

S. No. Type of Total weight requiredthread/fibre to break the thread

1. Cotton

2. Wool

3. Silk

4. Nylon

You may use a hook or a nail onthe wall for hanging the fibres anda polythene bag at the other end.In place of weights you may usemarbles (or pebbles) of similarsize.

Precaution : Note that all threadsshould be of the same lengthand almost of the samethickness.

C. Polyester and Acrylic

Polyester is another synthetic fibre.Fabric made from this fibre does not getwrinkled easily. It remains crisp and iseasy to wash. So, it is quite suitable formaking dress material. You must haveseen people wearing nice polyester shirtsand other dresses. Terylene is a popularpolyester. It can be drawn into very finefibres that can be woven like any otheryarn.


My mother alwaysbuys PET bottles andPET jars for storing

rice and sugar. Iwonder what PET is !

Oh! Now I understand whymy mother never wearspolyester clothes whileworking in the kitchen.

PET is a very familiar form ofpolyester. It is used for making bottles,utensils, films, wires and many otheruseful products.

Look around and make a list ofthings made of polyester.

Polyester (Poly+ester) is actually madeup of the repeating units of achemical called an ester. Esters arethe chemicals which give fruits theirsmell. Fabrics are sold by names likepolycot, polywool, terrycot etc. As thename suggests, these are made bymixing two types of fibres. Polycot isa mixture of polyester and cotton.Polywool is a mixture of polyester andwool.

We wear sweaters and use shawls orblankets in the winter. Many of theseare actually not made from natural wool,though they appear to resemble wool.These are prepared from another typeof synthetic fibre called acrylic. Thewool obtained from natural sources isquite expensive, whereas clothes madefrom acrylic are relatively cheap. Theyare available in a variety of colours.Synthetic fibres are more durable andaffordable which makes them morepopular than natural fibres.

You have already performed anactivity of burning natural and syntheticfibres (Activity 3.6 of Class VII). What

did you observe? When you burnsynthetic fibres you find that theirbehaviour is different from that of thenatural fibres. You must have noticedthat synthetic fibres melt on heating.This is actually a disadvantage ofsynthetic fibres. If the clothes catch fire,it can be disastrous. The fabric meltsand sticks to the body of the personwearing it. We should, therefore, notwear synthetic clothes while working inin the kitchen or in a laboratory.

All the synthetic fibres are preparedby a number of processes using rawmaterials of petroleum origin, calledpetrochemicals.

3.3 Characteristics ofSynthetic Fibres

Imagine that it is a rainy day. What kindof umbrella would you use and why?Synthetic fibres possess uniquecharacteristics which make thempopular dress materials. They dry upquickly, are durable, less expensive,readily available and easy to maintain.Perform the following activity and learnfor yourself.

Activity 3.2

Take two cloth pieces of the samesize, roughly half a metre squareeach. One of these should be fromnatural fibre. The other could be a

SCIENCE36

(a)

Fig. 3.6 : (a) Linear (b) Cross-linkedarrangements

(b)

Fig. 3.7 : Various articles made of plastics

synthetic fibre. You can take helpof your parents in selecting thesepieces. Soak the pieces in differentmugs each containing the sameamount of water. Take the piecesout of the containers after fiveminutes and spread them in the sunfor a few minutes. Compare thevolume of the water remaining ineach container.

Do synthetic fabrics soak less/morewater than the natural fabrics? Do theytake less/more time to dry?

What does this activity tell you aboutthe characteristics of the synthetic fabrics?

Find out from your parents about thedurability, cost and maintenance of thesefabrics, compared with the natural fabrics.

3.4 Plastics

You must be familiar with many plasticarticles used everyday. Make a list ofsuch items and their uses.

Plastic is also a polymer like thesynthetic fibre. All plastics do nothave the same type of arrangement ofunits. In some it is linear, whereas inothers it is cross-linked. (Fig. 3.6).

Plastic articles are available in allpossible shapes and sizes as you cansee in Fig. 3.7. Have you everwondered how this is possible? Thefact is that plastic is easily mouldablei.e. can be shaped in any form. Plasticcan be recycled, reused, coloured,melted, rolled into sheets or made intowires. That is why it finds such avariety of uses.

Polythene (Poly+ethene) is anexample of a plastic. It is used formaking commonly used polythenebags.

Now, try to bend a piece of plasticyourself. Can all the plastic articles bebent easily?

You will observe that some plasticarticles can bend easily while somebreak when forced to bend. When weadd hot water to a plastic bottle, it getsdeformed. Such plastic which getsdeformed easily on heating and can be


3.4 Plastics as Materials ofChoice

Today if we think of storing a food item,water, milk, pickles, dry food, etc.,plastic containers seem mostconvenient. This is because of their lightweight, lower price, good strength andeasy handling. Being lighter ascompared to metals, plastics are usedin cars, aircrafts and spacecrafts, too.The list is endless if we start countingarticles like slippers, furniture anddecoration pieces, etc.

Now, let us discuss the characteristicproperties of plastics.

A. Plastic is non-reactive

You know that metals like iron getrusted when left exposed to moistureand air. But plastics do not react withwater and air. They are not corrodedeasily. That is why they are used to storevarious kinds of material, includingmany chemicals.

B. Plastic is light, strong and durable

Talk to your parents or grandparentsabout the types of buckets that wereused in the past. What is the materialof the buckets or mugs you are usingtoday? What are the advantages of usinga plastic container? Since plastic is verylight, strong, durable and can bemoulded into different shapes and sizes,it is used for various purposes. Plasticsare generally cheaper than metals. Theyare widely used in industry and forhousehold articles. Make a list ofdifferent kind of plastic containers thatyou use in daily life.

bent easily are known asthermoplastics. Polythene and PVCare some of the examples ofthermoplastics. These are used formanufacturing toys, combs andvarious types of containers.

On the other hand, there are someplastics which when moulded once, cannot be softened by heating. These arecalled thermosetting plastics. Twoexamples are bakelite and melamine.Bakelite is a poor conductor of heat andelectricity. It is used for makingelectrical switches, handles of variousutensils, etc. Melamine is a versatilematerial. It resists fire and can tolerateheat better than other plastics. It is usedfor making floor tiles, kitchenware andfabrics which resist fire. Fig. 3.8 showsthe various uses of thermoplastics andthermosetting plastics.

Articles made of thermosetting plastics

Articles made of thermoplastics

Fig : 3.8: Some articles made of plastic

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Table 3.3

Type of waste Approximate time Nature of materialtaken to degenerate

Peels of vegetable and fruits, 1 to 2 weeks. Biodegradable

leftover foodstuff, etc.

Paper 10–30 days Biodegradable

Cotton cloth 2 to 5 months Biodegradable

Wood 10 to15 years Biodegradable

Woollen clothes About a year Biodegradable

Tin, aluminium, and other 100 to 500 years Non-biodegradable

metal cans

Plastic bags Several years Non-biodegradable

C. Plastics are poor conductors

You have learnt above that plastics are poorconductors of heat and electricity. That iswhy electrical wires have plastic covering,and handles of screw drivers are made ofplastic. As mentioned above, handles offrying pans are also made of plastic.

Did You Know?� Plastics find extensive use in the

health-care industry. Someexamples of their use are thepackaging of tablets, threads usedfor stitching wounds, syringes,doctors’ gloves and a number ofmedical instruments.

� Special plastic cookware is usedin microwave ovens for cookingfood. In microwave ovens, the heatcooks the food but does not affectthe plastic vessel.

� Teflon is a special plastic on whichoil and water do not stick. It isused for nonstick coating oncookwares.

� Fire-proof plastics: Althoughsynthetic fibre catches fire easily,it is interesting to know that theuniforms of firemen have coatingof melamine plastic to make themflame resistant.

3.7 Plastics and theEnvironment

When we go to the market, we get usuallythings wrapped in plastic or packed inpolythene bags. That is one reason whyplastic waste keeps getting accumulatedin our homes. Ultimately, plastic findsits way in the garbage. Disposal of plasticis a major problem. Why?

A material which gets decomposedthrough natural processes, such asaction by bacteria, is calledbiodegradable. A material which is noteasily decomposed by natural processesis termed as non-biodegradable.

Look at the Table 3.3.

� Source: http://edugreen.teri.res.in/explore/solwaste/types.htm


Since plastic takes several years todecompose, it is not environmentfriendly. It causes environmentalpollution. Besides, the burning processin the synthetic material is quite slowand it does not get completely burnteasily. In the process it releases lots ofpoisonous fumes into the atmospherecausing air pollution. How can thisproblem be solved?

Avoid the use of plastics as far aspossible. Make use of bags made ofcotton or jute when you go for shopping.The biodegradable and non-biodegradable wastes should becollected separately and disposed offseparately. Practise this in your homes.Can you suggest some other ways inwhich you can contribute towardsreducing the use of plastic materials?

It is better to recycle the plastic waste.Most of the thermoplastics can be recycled.Make a list of items that can be recycled.However, during recycling certaincolouring agents are added. This limits itsusage especially for storage of food.

As a responsible citizen rememberthe 4 R principle. Reduce, Reuse,Recycle and Recover. Develop habitswhich are environment friendly.

Have you ever seen a garbage dumpwhere animals, especially cows, areeating garbage? In the process ofeating the food waste they swallowmaterials like polythene bags andwrappers of food. Can you imagine theconsequences? The plastic materialchokes the respiratory system of theseanimals, or forms a lining in theirstomachs and can be the cause of theirdeath.

The polybags carelessly thrown hereand there are responsible for cloggingthe drains, too. Sometimes we are verycareless and throw the wrappers ofchips, biscuits and other eatables onthe road or in parks or picnic places.Should we not think twice before doingso ? As a responsible citizen whatmeasures do you suggest to keep publicplaces clean and free of plastic?

Fibre-wise

� Do not throw plastic bags in thewater bodies or on the road.

� Take a cotton carry-bag or a jutebag while going for shopping.

� Try to minimise the use of plasticmaterials e.g., use a steel lunchbox instead of a plastic one.

SCIENCE40

KEYWORDS

ACRYLIC

ARTIFICIAL SILK

NYLON

PLASTIC

POLYESTER

POLYMER

POLYTHENE

RAYON

SYNTHETIC FIBRES

TERYLENE

THERMOPLASTICS

THERMOSETTING-

PLASTICS


� Synthetic fibres and plastics, like natural

fibres, are made of very large units called

polymers. Polymers are made up of many

smaller units.

� While natural fibres are obtained from plants

and animals, synthetic fibres are obtained by

chemical processing of petrochemicals. Like

natural fibres, these fibres can also be woven

into fabrics.

� Synthetic fibres find uses ranging from many

household articles like ropes, buckets,

furniture, containers, etc. to highly specialized

uses in aircrafts, ships, spacecrafts,

healthcare, etc.

� Depending upon the types of chemicals used

for manufacturing synthetic fibres, they are

named as Rayon, Nylon, Polyester and Acrylic.

� The different types of fibres differ from one

another in their strength, water absorbing

capacity, nature of burning, cost, durability,

etc.

� Today, life without plastics cannot be imagined.

Be it home, or outside, plastic is every where.

� The waste created by plastics is not

environment friendly. On burning plastics

release poisonous gases. On dumping in the

ground they may take years to degenerate.

This is because of their non-biodegradable

nature.

� We need to use synthetic fibres and plastics

in such a manner that we can enjoy their good

qualities and at the same time minimise the

environmental hazards for the living

communities.


Exercises

1. Explain why some fibres are called synthetic.

2. Mark (� ) the correct answer:

Rayon is different from synthetic fibres because

(a) it has a silk like appearance

(b) it is obtained from wood pulp

(c) its fibres can also be woven like those of natural fibres

3. Fill in the blanks with appropriate words:

(a) Synthetic fibres are also called ______ or _______ fibres.

(b) Synthetic fibres are synthesised from raw material called ________.

(c) Like synthetic fibres, plastic is also a __________ .

4. Give examples which indicate that nylon fibres are very strong.

5. Explain why plastic containers are favoured for storing food.

6. Explain the difference between the thermoplastic and thermosettingplastics.

7. Explain why the following are made of thermosetting plastics.

(a) Saucepan handles

(b) Electric plugs/switches/plug boards

8. Categorise the materials of the following products into ‘can be recycled’and ‘cannot be recycled’:

Telephone instruments, plastic toys, cooker handles, carry bags, ball pointpens, plastic bowls, plastic covering on electrical wires, plastic chairs,electrical switches.

9. Rana wants to buy shirts for summer. Should he buy cotton shirts orshirts made from synthetic material? Advise Rana, giving your reason.

10. Give examples to show that plastics are noncorrosive in nature.

11. Should the handle and bristles of a tooth brush be made of the samematerial? Explain your answer.

12. ‘Avoid plastics as far as possible’. Comment on this advice.

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13. Match the terms of column I correctly with the phrases given incolumn II.

Column I Column II

(i) Polyester (a) Prepared by using wood pulp(ii) Teflon (b) Used for making parachutes and stockings(iii) Rayon (c) Used to make non-stick cookwares(iv) Nylon (d) Fabrics do not wrinkle easily

14. ‘Manufacturing synthetic fibres is actually helping conservation of forests’.Comment.

15. Describe an activity to show that thermoplastic is a poor conductor ofelectricity.


1. Have you heard of the campaign : ‘‘Say No To Plastics”. Coin a fewmore slogans of this kind. There are certain governmental and non-governmental organisations who educate general public on how tomake a wise use of plastics and develop environment friendly habits.Find out organisations in your area which are carrying outawareness programmes. If there is none, form one.

2. Organise a debate in the school. The children may be given anoption to role play as manufacturers of synthetic fabrics or those offabrics from natural sources. They can then debate on the topic‘My Fabric is Superior’.

3. Visit five families in your neighbourhood and enquire about thekind of clothes they use, the reason for their choice and advantagesof using them in terms of cost, durability and maintenance. Make ashort report and submit it to your teacher.

4. Devise an activity to show that organic waste is biodegradable whileplastic is not.

5. If you wish to know more about fibres and plastics and the productsmade from them, you may explore the following web sites:

� http://www.pslc.ws/macrog/index.htm

� http://www.edugreen.teri.res.in/exploresolwaste/types/htm

� http://www.nationalgeographic.com/resources/ngo/education/

plastics

� http://www.packagingtoday.com/

� http://www.bbc.co.uk/schools.gcsebitesize/design/textiles/fibresrev/

html/


Nylon appears like silk. It is strong and flexible. These endearing qualities ofnylon created a public sensation, or nylon mania, when it was introduced in1939. Women’s stockings made from this new fibre were in great demand. But,unfortunately, most of the nylon production had to be diverted to makingparachutes during the Second World War (1939-1945). After the war, whenproduction of stockings resumed, supply did not match the demand. There wasa huge black market for this product. Women had to wait for hours in queues toget a pair. Often there were nylon riots.

Did You Know

MATERIALS : METALS AND NON-METALSMATERIALS : METALS AND NON-METALS

You are familiar with a number ofmaterials like iron, aluminium,copper, etc. Some materials have

been given in Table 4.1.

similar change if we try to beat awood log ?

Let us find out.

Activity 4.1

Take a small iron nail, a coal piece,a piece of thick aluminium wire anda pencil lead. Beat the iron nail witha hammer (Fig. 4.1). (But take carethat you don’t hurt yourself in theprocess). Try to hit hard. Hit hard

Table 4.1 : Appearance andHardness of materials

Object Appearance Hardness/Material (Shiny/Dull) (Very hard/

Not veryhard)

Iron

Coal

Sulphur

Aluminium

Copper

-----

Can you name the materials whichare metals? The rest of the materials inTable 4.1 are non-metals. Metals can bedistinguished from non-metals on thebasis of their physical and chemicalproperties. Recall that lustre andhardness are physical properties.

4.1 Physical Properties ofMetals and Non-metals

Have you ever seen a blacksmith beatingan iron piece or an article made up ofiron, like a spade, a shovel, an axe? Doyou find a change in the shape of thesearticles on beating? Would you expect a

Table 4.2 Malleability of Materials

Object/ Change in ShapeMaterial (Flattens/Breaks

into pieces)Iron nail

Coal piece

Aluminium wire

Pencil lead

also the aluminium wire. Thenrepeat the same kind of treatmenton the coal piece and pencillead. Record your observations inTable 4.2.

Fig. 4.1 : Beating an iron nail with hammer

Fig. 4.2 : Electric tester

Where do you find the use ofaluminium and copper wires? Have youseen wires of coal? Definitely not!

The property of metal by which it canbe drawn into wires is called ductility.

Have you ever noticed the differencein sound on dropping an iron sheet/plate, a metal coin, and a piece of coalon the floor? If not, you can try it now.

Do you note any difference in the soundproduced?

Oh! The meaning of recallingour experiences and then of

this activity was to show thatmetals are good conductors ofheat and electricity. We learnt

this in Class VI.


You saw that the shape of the iron nailand the aluminium wire changed onbeating. If they were beaten harder thesecould be changed into sheets. You mightbe familiar with silver foil used fordecorating sweets. You must also befamiliar with the aluminium foil used forwrapping food. The property of metals bywhich they can be beaten into thin sheetsis called malleability. This is acharacteristic property of metals. As youmust have noticed, materials like coal andpencil lead do not show this property. Canwe call these as metals?

Can you hold a hot metallic pan whichis without a plastic or a wooden handleand not get hurt? Perhaps not! Why? Tryto list some other experiences in which awooden or plastic handle protects youfrom being hurt while handling hotthings. On the basis of these experienceswhat can you say about the conductionof heat by wood and plastic?

You must have seen an electricianusing his screw driver. What kind ofhandle does it have? Why?

Let us find out.

Activity 4.2

Recall how to make an electriccircuit to test whether electricity canpass through an object or not(Fig. 4.2). You might have performed

the activity with various objects inClass VI. Now, repeat the activitywith the materials mentioned inTable 4.3. Observe and group thesematerials into good conductors andpoor conductors.

Table 4.3 : Electricalconductivity of materials

S.No. Materials Good Conductor /PoorConductor

1. Iron rod/nail

2. Sulphur

3. Coal piece

4. Copper wire

You observe that iron rod, nail andcopper wire are good conductors whilerolled sulphur piece and coal piece arepoor conductors.

SCIENCE46

Have you seen wooden bells intemples? Can you give reason?

The things made of metals produceringing sound when struck hard.Suppose you have two boxes similar inappearance, one made of wood and theother of metal. Can you tell which boxis made of metal by striking both theboxes?

Since metals produce ringingsounds, they are said to be sonorous.The materials other than metals are notsonorous.

After performing the above activities,we can say that some materials are hard,lustrous, malleable, ductile, sonorousand good conductors of heat andelectricity. The materials whichgenerally posses these properties arecalled metals. The examples of metalsare iron, copper, aluminium, calcium,magnesium, etc. In contrast, materialslike coal and sulphur are soft and dullin appearance. They break down intopowdery mass on tapping with hammer.They are not sonorous and are poorconductors of heat and electricity. Thesematerials are called non-metals. Theexamples of non-metals are sulphur,carbon, oxygen, phosphorus, etc.

which rust is formed. You had alsoperformed in Class VII an activity ofburning a magnesium ribbon in air. Youhad learnt that in both the processesoxide formation takes place. Completethe following reactions of iron andmagnesium with oxygen.

Iron (Fe) + Oxygen (O2) + Water (H2O) → ?Magnesium (Mg) + Oxygen (O2) → ?

Activity 4.3

Let us check the nature of rustformed as a result of the reactionbetween iron, oxygen and water.Collect a spoonful of rust anddissolve it in a very little amount ofwater. You will find that the rustremains suspended in water. Shakethe suspension well. Test thesolution with red and blue litmuspapers (Fig. 4.3). What do youobserve? Is the solution acidic orbasic?

Metals like sodium and potassium aresoft and can be cut with a knife.Mercury is the only metal which isfound in liquid state at roomtemperature. These are exceptions.

4.2 Chemical Properties ofMetals and Non-metals

A. Reaction with Oxygen

You are familiar with the phenomenonof rusting of iron. Recall the reaction by

Rust

Rustsuspension

Red litmuspaper

Fig. 4.3 : Testing the nature of rust


Fig. 4.4 (a) : Burning of sulphur powder

Improviseddeflagrating

spoonNow recall the activity of burningmagnesium ribbon. The ash obtainedon burning magnesium ribbon isdissolved in water and tested for itsacidic / basic nature.

Is the solution acidic or basic? Howdo you ascertain this?

You must have observed that the redlitmus turns blue. So, oxide of magnesiumis also basic in nature. In general, metallicoxides are basic in nature.

Let us now observe the reaction ofnon-metals with oxygen.

Activity 4.4

(To be demonstrated by the teacherin the class)

Take a small amount of powderedsulphur in a deflagrating spoon andheat it. If deflagrating spoon is notavailable, you may take a metallic capof any bottle and wrap a metallic wirearound it and give it the shapeshown in Fig. 4.4 (a).

As soon as sulphur starts burning,introduce the spoon into a gas jar/glass tumbler [Fig. 4.4 (a)]. Cover thetumbler with a lid to ensure thatthe gas produced does not escape.Remove the spoon after some time.Add a small quantity of water intothe tumbler and quickly replace thelid. Shake the tumbler well. Checkthe solution with red and bluelitmus papers [Fig. 4.4 (b)].

When a copper vessel is exposed tomoist air for long, it acquires a dullgreen coating. The green material isa mixture of copper hydroxide(Cu(OH)2) and copper carbonate(CuCO3). The following is the reaction2Cu+H2O+CO2+O2→Cu (OH)2 + CuCO3

moist air

Does copper also getrusted? I have seena greenish depositon the surface ofcopper vessels.

Fig. 4.4 (b) : Testing of solution with litmuspapers

SCIENCE48

The name of the product formed inthe reaction of sulphur and oxygen issulphur dioxide gas. When sulphurdioxide is dissolved in water sulphurousacid is formed. The reaction can be givenas follows:

Sulphur dioxide (SO2) + Water (H2O) →Sulphurous acid (H2SO3)

The sulphurous acid turns bluelitmus paper red. Generally, oxides ofnon-metals are acidic in nature.

Recall the name of some of thelaboratory acids and bases you have readin Class VII. Note down their names inTable 4.4. Identify the metal or non-metal present in them which formsoxides with oxygen.

B. Reaction with Water

Let us see how metals and non-metalsreact with water.

Table 4.4 : Metals and Non-metals in Acids and Bases

S.No. Name of the base Metal Name of the acid Non-metal

1. Calcium hydroxide Calcium Sulphuric acid Sulphur

2.

3.

4.

5.

Sodium metal is very reactive. Itreacts vigorously with oxygen andwater. A lot of heat is generated inthe reaction. It is, therefore, storedin kerosene.

Activity 4.5

To be demonstrated by the teacher.During demonstration special careshould be taken that the size of thesodium metal piece is roughly thesize of a wheat grain. It should beheld with a pair of tongs.)Take a 250 mL beaker/glass tumbler.Fill half of it with water. Nowcarefully cut a small piece of sodiummetal. Dry it using filter paper andwrap it in a small piece of cotton.Put the sodium piece wrapped incotton into the beaker. Observecarefully. During observation keepaway from the beaker. When reactionstops touch the beaker. What do youfeel? Has the beaker become hot?Test the solution with red and bluelitmus papers. Is the solution acidicor basic?

Fig. 4.5 : Reaction of sodium with water


You observed that sodium reactsvigorously with water. Some other metalsdo not do so. For example, iron reactswith water slowly.

Generally, non-metals do not reactwith water though they may be veryreactive in air. Such non-metals arestored in water. For example,phosphorus is a very reactive non-metal.It catches fire if exposed to air. To preventthe contact of phosphorus withatmospheric oxygen, it is stored in water.

C. Reactions with Acids

Let us see how metals and non-metalsbehave with acids.

Activity 4.6

Warning : Keep the mouth of thetest tube away from your face. Usetest tube holder to hold the testtube.Take samples of metals and non-metals listed in Table 4.5 in separate

Table 4.5 : Reaction of metals and non-metals with acids

Test Metal/ Reaction with dilute Reaction with dilutetube Non-Metal Hydrochloric acid Sulphuric acidLabel room warm room warm

temperature temperature

A Magnesium

(ribbon)

B Aluminium (foil)

C Iron (filings)

D Copper (peeled

flexible wire)

E Charcoal (powder)

F Sulphur (powder)

test tubes and label them as A, B,C, D, E, and F. With the help of adropper add 5 mL of dilutehydrochloric acid to each test tubeone by one. Observe the reactionscarefully. If no reaction occurs in acold solution, warm the test tubegently. Bring a burning matchsticknear the mouth of each test tube.Repeat the same activity usingdilute sulphuric acid instead of thedilute hydrocholoric acid. Recordyour observations in Table 4.5.

Is there a difference in the way metalsand non-metals react with acids? Whatcould the ‘pop’ sound in some cases bedue to when a burning match stick isbrought near the mouth of the testtubes?

You must have found that non-metals generally do not react with acidsbut metals react with acids and producehydrogen gas that burns with a ‘pop’sound. You must have noticed that

SCIENCE50

Cu Cu

No Change

ZnSO4 FeSO4

copper does not react with dilutehydrochloric acid even on heating butit reacts with sulphuric acid.

D. Reactions with Bases

Activity 4.7

To be demonstrated by the teacher.During the preparation of sodiumhydroxide solution, care should betaken that pellets of sodiumhydroxide are handled with aplastic spatula.

Prepare a fresh solution of sodiumhydroxide in a test tube bydissolving 3-4 pellets of it in 5 mLof water. Drop a piece of aluminiumfoil into it. Bring a burning matchstick near the mouth of the testtube. Observe carefully.

What does the ‘pop’ sound indicate?As before, the ‘pop’ sound indicates thepresence of hydrogen gas.

Metals react with sodium hydroxideto produce hydrogen gas.

Reactions of non-metals with basesare complex.

E. Displacement Reactions

Recall the activity of the reaction betweencopper sulphate and iron that youperformed in Class VII. Let us observesome more reactions of that kind.

Activity 4.8

Take five 100 mL beakers and labelthem A, B, C, D and E. Take about50 mL of water in each beaker.Dissolve in each beaker ateaspoonful of each substance asindicated in Fig. 4.6 (a).

(a)

Beaker A : Copper sulphate (CuSO4) + Zinc granule (Zn),

Beaker B : Copper sulphate (CuSO4) + Iron nail (Fe)

Beaker C : Zinc sulphate (ZnSO4) + Copper turnings (Cu),

Beaker D : Iron sulphate (FeSO4) + Copper turnings (Cu)

Beaker E : Zinc sulphate (ZnSO4) + Iron nail (Fe)

Fig. 4.6 (a) and (b) : Displacement reactions

A B C D E

A B C D E

(b)


replace copper in beaker ‘A’ why cannotcopper replace zinc in beaker ‘C’?Remember that science is not arbitrary.It follows definite rules based on facts.And the rule here is that zinc is morereactive than copper and iron. A morereactive metal can replace a less reactivemetal, but a less reactive one cannotreplace a more reactive metal. Now youcan understand why there are nodisplacement reactions in beakers Dand E also. Can you guess thesequence of metals from more reactiveto less reactive among zinc, iron andcopper?

4.3 Uses of Metals and Non-metals

You should be able to guess why metalsare used in making machinery,automobiles, aeroplanes, trains,satellites, industrial gadgets, cookingutensils, water boilers, etc. You are alsofamiliar with the uses of some non-metals. Here are some interesting ones.We are sure that you will guess themright:� Non-metal essential for our life which

all living beings inhale duringbreathing,

� Non-metals used in fertilisers toenhance the growth of plants,

� Non-metal used in water purificationprocess,

� Non-metal used in the purplecoloured solution which is appliedon wounds as an antiseptic,

� Non-metals used in crackers.

You may add some more uses ofmetals and non-metals from yourexperiences.

� Keep the beakers undisturbed forsome time.

� Record your observations in yournote book.

What changes do you observe in thevarious beakers? You have read that onemetal displaces another metal from itscompound in aqueous solution. Inbeaker ‘A’ zinc (Zn) replaces copper (Cu)from copper sulphate (CuSO4). That iswhy the blue colour of copper sulphatedisappears and a powdery red mass ofcopper is deposited at the bottom of thebeaker. The reaction can be representedas follows:

Copper Sulphate (CuSO4) + Zinc (Zn) (Blue)→ Zinc Sulphate (ZnSO4) + Copper (Cu)

(Colourless) (Red)

You can write down the reactiontaking place in beaker ‘B’ in a similarmanner.

I have understood the reactionstaking place in beakers ‘A’ and‘B’. But I am still confused whythere is no change in beakers

‘C’, ‘D’ and ‘E’?

There could have been displacementof zinc by copper in beaker ‘C’ and byiron in beaker ‘E’. Similarly iron couldbe displaced by copper in beaker ‘D’.

Since we do not see any change inbeaker C, we can infer that copper isnot able to replace zinc from zincsulphate. But why? When zinc can

SCIENCE52

I heard that magnesium isfound in plants. In whatform is it found in them?

Doctor reported irondeficiency in my body.

Where is iron in my body?

In Class VII, you have learnt that in a chemical reaction, new substances are formed.These substances are different from those which underwent the reaction. Now, if asubstance cannot be broken down further by chemical reactions, by cooling, heating,or by electrolysis, it is called ‘element’. Sulphur is an element. So is iron. Carbon, too,is an element. The smallest unit of an element is atom. A sample of an elementcontains only one kind of atoms. The atom of an element remains unaffected by physicalchanges in the element. For example, an atom of liquid sulphur would be exactly thesame as the atom of solid or vapour sulphur.

Although we have an infinite variety of substances in the universe, the number ofelements forming these substances is limited. There are no more than 92 naturallyoccurring elements. An important classification of elements is in terms of metals andnon-metals. Most of the elements are metals. Less than 20 are non-metals. A few aremetalloids which possess characters of both metals and non-metals.


KEYWORDS

ATOM

CONDUCTOR

DISPLACEMENT

REACTION

DUCTILITY

ELEMENTS

HARDNESS

MALLEABILITY

METALS

METALLOIDS

NON-METALS

SONOROUS


� Metals are lustrous whereas non-metals have

no lustre.

� Generally, metals are malleable and ductile.

Non-metals do not have these properties.

� Generally, metals are good conductors of heat

and electricity but non-metals are poor

conductors.

� On burning, metals react with oxygen to

produce metal oxides which are basic in

nature. Non-metals react with oxygen to

produce non- metallic oxides which are acidic

in nature.

� Some metals react with water to produce metal

hydroxides and hydrogen gas. Generally, non-

metals do not react with water.

� Metals react with acids and produce metal

salts and hydrogen gas. Generally, non-metals

do not react with acids.

� Some metals react with bases to produce

hydrogen gas.

� More reactive metals displace less reactive

metals from their compounds in aqueous

solutions.

� Metals and non-metals are used widely in

every day life.

Exercises

1. Which of the following can be beaten into thin sheets?

(a) Zinc (b) Phosphorus (c) Sulphur (d) Oxygen

2. Which of the following statements is correct?

(a) All metals are ductile.

(b) All non-metals are ductile.

(c) Generally, metals are ductile.

(d) Some non-metals are ductile.

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3. Fill in the blanks :

(a) Phosphorus is very non-metal.

(b) Metals are conductors of heat and .

(c) Iron is reactive than copper.

(d) Metals react with acids to produce gas.

4. Mark ‘T’ if the statement is true and ‘F’ if it is false.

(a) Generally, non-metals react with acids. ( )

(b) Sodium is a very reactive metal. ( )

(c) Copper displaces zinc from zinc sulphate solution. ( )

(d) Coal can be drawn into wires. ( )

5. Some properties are listed in the following Table. Distinguish betweenmetals and non-metals on the basis of these properties.

Properties Metals Non-metals

1. Appearance

2. Hardness

3. Malleability

4. Ductility

5. Heat Conduction

6. Conduction of Electricity

6. Give reasons for the following :

(a) Aluminium foils are used to wrap food items.(b) Immersion rods for heating liquids are made up of metallic substances.(c) Copper cannot displace zinc from its salt solution.(d) Sodium and potassium are stored in kerosene.

7. Can you store lemon pickle in an aluminium utensil? Explain.

8. In the following Table some substances are given in Column I. In Column IIsome uses are given. Match the items in column I with those in Column II.

Column I Column II

(i) Gold (a) Thermometers

(ii) Iron (b) Electric wire

(iii) Aluminium (c) Wrapping food

(iv) Carbon (d) Jewellery

(v) Copper (e) Machinary

(vi) Mercury (f) Fuel


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S9. What happens when

(a) Dilute sulphuric acid is poured on a copper plate?(b) Iron nails are placed in copper sulphate solution?

Write word equations of the reactions involved.

10. Saloni took a piece of burning charcoal and collected the gas evolved in atest tube.

(a) How will she find the nature of the gas ?(b) Write down word equations of all the reactions taking place in this

process.

11. One day Reeta went to a jeweller’s shop with her mother. Her mother gaveold gold jewellery to the goldsmith to polish. Next day when they broughtthe jewellery back, they found that there was a slight loss in its weight.Can you suggest a reason for the loss in weight?


1. Prepare Index Cards for any four metals and four non-metals. Thecard should have information like name of metal/non-metal; itsphysical properties, chemical properties and its uses.

2. Visit a blacksmith and observe how metals are moulded.

3. Suggest an experiment to compare the conductivity of electricity byiron, copper, aluminium and zinc. Perform the experiment andprepare a short report on the results.

4. Find out the locations of the deposits of iron, aluminium and zincin India. Mark these in an outline map of India. In which form arethe deposits found? Discuss in the class.

5. Discuss with your parents/neighbours/goldsmiths why gold ispreferred for making jewellery.

6. Visit the following websites and enjoy the quiz on metals and non-metals:

� chemistry.about.com/library/weekly/bl050303a.htm� chemistry.about.com/od/testsquizzes/Chemistry_Tests_

Quizzes.htm� www.syvum.com/cgi/online/mult.cgi/squizzes/science/

metals.tdf?0� www.gcsescience.com/q/qusemet.html� www.corrosionsource.com/handbook/periodic/metals.htm

COAL AND PETROLEUMCOAL AND PETROLEUM

Can air, water and soil be exhaustedby human activities? You have alreadystudied about water in Class VII. Is watera limitless resource?

In the light of the availability ofvarious resources in nature, naturalresources can be broadly classifiedinto two kinds:

Inexhaustible Natural Resources

These resources are present in unlimitedquantity in nature and are not likely tobe exhausted by human activities.Examples are: sunlight, air.

Exhaustible Natural Resources

The amount of these resources innature is limited. They can beexhausted by human activities.Examples of these resources are forests,wildlife, minerals, coal, petroleum,natural gas etc.

Activity 5.2

It is a group activity

Take some containers. Fill themwith popcorn/peanuts/roastedgram/toffees. Divide students intogroups of seven each. Further divideeach group into three subgroupscontaining 1, 2 and 4 students.Label them as first, second andthird generation respectively. These

We use various materials for ourbasic needs. Some of them arefound in nature and some

have been made by human efforts.

Activity 5.1

Make a list of various materials usedby us in daily life and classify themas natural and man-made.

Natural Man-made

Can we use all ournatural resources

forever ?

Does this list include air, water, soiland minerals? Since all these areobtained from nature, they are callednatural resources.

sub-groups represent theconsumers. As population isgrowing, second and thirdgenerations have larger number ofconsumers.

Put one full container for eachgroup on a table. Ask consumers ofthe first generation from each groupto consume eatables from thecontainer of their group. Now, askthe second generation consumersfrom each group to do the same. Askstudents to observe carefully theavailability of eatables in eachcontainer. If some thing is left in thecontainers, ask third generationfrom each group to consume it. Now,finally observe whether all theconsumers of the third generationgot the eatables or not. Also observeif any thing is still left in any of thecontainers.

Assume that the eatables in theconta iner represent the tota lavailability of an exhausible naturalresource like coal, petroleum ornatural gas. Each group may havedifferent consumption pattern. Arethe earlier generations of any grouptoo greedy? It may be that the earliergenerations in some groups wereconcerned about the cominggeneration(s) and left something forthem.

In this chapter we will learn aboutsome exhaustible natural resourceslike coal, petroleum and natural gas.These were formed from the deadremains of living organisms (fossils).So, these are all known as fossilfuels.

5.1 Coal

You may have seen coal, or heard aboutit (Fig. 5.1). It is as hard as stone and isblack in colour.

Fig. 5.1: Coal

Coal is one of the fuels used to cookfood. Earlier, it was used in railwayengines to produce steam to run theengine. It is also used in thermal powerplants to produce electricity. Coal is alsoused as a fuel in various industries.

Story of Coal

Where do we getcoal from and how

is it formed?

About 300 million years ago theearth had dense forests in low lyingwetland areas. Due to natural processes,like flooding, these forests got buriedunder the soil. As more soil depositedover them, they were compressed. Thetemperature also rose as they sankdeeper and deeper. Under high pressureand high temperature, dead plants gotslowly converted to coal. As coalcontains mainly carbon, the slowprocess of conversion of dead vegetationinto coal is called carbonisation. Sinceit was formed from the remains ofvegetation, coal is also called a fossil fuel.A coal mine is shown in Fig. 5.2.


SCIENCE58

Fig. 5.2: A coal mine

C. Coal gas

Coal gas is obtained during theprocessing of coal to get coke. It is used

Coal gas was used for street lightingfor the first time in London in 1810and in New York around 1820. Now adays, it is used as a source of heatrather than light.

These days, bitumen, a petroleumproduct, is used in place of coal-tarfor metalling the roads.

Fig. 5.3: Coal-tar

When heated in air, coalburns and produces mainly carbondioxide gas.

Coal is processed in industry to getsome useful products such as coke, coaltar and coal gas.

A. Coke

It is a tough, porous and blacksubstance. It is almost pure form ofcarbon. Coke is used in the manufactureof steel and in the extraction of manymetals.

B. Coal tar

It is a black, thick liquid (Fig. 5.3) withunpleasant smell. It is a mixture of

about 200 substances. Productsobtained from coal tar are used asstarting materials for manufacturingvarious substances used in everyday lifeand in industry, like synthetic dyes,drugs, explosives, perfumes, plastics,paints, photographic materials, roofingmaterials, etc. Interestingly,naphthalene balls used to repel mothsand other insects are also obtained fromcoal tar.


as a fuel in many industries situatednear the coal processing plants.

5.2 Petroleum

You know that petrol is used as a fuel inlight automobiles such as motor cycles/scooters and cars. Heavy motor vehicleslike trucks and tractors run on diesel.Petrol and diesel are obtained from anatural resource called petroleum.

Do you know how petroleum isformed?

Petroleum was formed fromorganisms living in the sea. As theseorganisms died, their bodies settled atthe bottom of the sea and got coveredwith layers of sand and clay. Overmillions of years, absence of air, hightemperature and high pressuretransformed the dead organisms intopetroleum and natural gas.

Refining of Petroleum

Petroleum is a dark oily liquid. It hasan unpleasant odour. It is a mixture ofvarious constituents such as petroleumgas, petrol, diesel, lubricating oil,paraffin wax, etc. The process ofseparating the various constituents/

Fig. 5.4 : Petroleum and natural gas deposits

The world’s first oil well was drilledin Pennsylvania, USA, in 1859. Eightyears later, in 1867, oil was stuck atMakum in Assam. In India, oil isfound in Assam, Gujarat, MumbaiHigh and in the river basins ofGodavari and Krishna.

Look at Fig. 5.4. It shows the depositsof petroleum and natural gas. You seethat the layer containing petroleum oiland gas is above that of water. Why is itso? Recall that oil and gas are lighterthan water and do not mix with it.

Wells

Gas

Oil

Water

Reservoir rock

Imperviousrock

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fractions of petroleum is known asrefining. It is carried out in a petroleumrefinery (Fig. 5.5).

Various constituents of petroleumand their uses are given in Table 5.1.

Many useful substances are obtainedfrom petroleum and natural gas.These are termed as ‘Petrochemicals’.These are used in the manufactureof detergents, fibres (polyester, nylon,acrylic etc.), polythene and otherman-made plastics. Hydrogen gasobtained from natural gas, is used inthe production of fertilisers (urea).Due to its great commercialimportance, petroleum is also called‘black gold’.

5.3 Natural Gas

Natural gas is a very important fossilfuel because it is easy to transportthrough pipes. Natural gas is storedunder high pressure as compressednatural gas (CNG). CNG is used forpower generation. It is now being usedas a fuel for transport vehicles becauseit is less polluting. It is a cleaner fuel.

The great advantage of CNG is thatit can be used directly for burning in

Table 5.1 Various constituents of petroleum and their uses

S.No. Constituents of petroleum Uses

1. Petroleum Gas in Liquid form Fuel for home and industry

(LPG)

2. Petrol Motor fuel, aviation fuel, solvent for

dry cleaning

3. Kerosene Fuel for stoves, lamps and for jet aircrafts

4. Diesel Fuel for heavy motor vehicles, electric

generators

5. Lubricating oil Lubrication

6. Paraffin wax Ointments, candles, vaseline etc.

7. Bitumen Paints, road surfacing

Fig. 5.5: A petroleum refinery


homes and factories where it can besupplied through pipes. Such a networkof pipelines exists in Vadodara (Gujarat),some parts of Delhi and other places.

Natural gas is also used as a startingmaterial for the manufacture of anumber of chemicals and fertilisers.India has vast reserves of natural gas.In our country, natural gas has beenfound in Tripura, Rajasthan,Maharashtra and in the KrishnaGodavari delta.

5.4 Some Natural Resourcesare Limited

You have studied in the beginning ofthe chapter that some natural resourcesare exhaustible like fossil fuels, forests,minerals etc.

You know that coal and petroleumare fossil fuels. It required the deadorganisms millions of years to getconverted into these fuels. On the otherhand, the known reserves of these willlast atmost a few hundred years.Moreover, burning of these fuels is amajor cause of air pollution. Their useis also linked to global warming. It istherefore necessary that we use thesefuels only when absolutely necessary.This will result in better environment,smaller risk of global warming and theiravailability for a longer period of time.

In India, the Petroleum ConservationResearch Association (PCRA) advisespeople how to save petrol/diesel whiledriving. Their tips are:

� Drive at a constant and moderate speedas far as possible,

� Switch off the engine at traffic lights orat a place where you have to wait,

� Ensure correct tyre pressure, and� Ensure regular maintenance of the

vehicle.

Can coal, petroleumand natural gas be

prepared in thelaboratory from dead

organisms?

No. Theirformation is a veryslow process and

conditions fortheir formation

cannot be createdin the laboratory.

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KEYWORDS

COAL

COAL GAS

COAL TAR

COKE

FOSSIL FUEL

NATURAL GAS

PETROLEUM

PETROLEUM-

REFINERY


� Coal, petroleum and natural gas are fossil

fuels.

� Fossil fuels were formed from the dead remains

of living organisms millions of years ago.

� Fossil fuels are exhaustible resources.

� Coke, coal tar and coal gas are the products

of coal.

� Petroleum gas, petrol, diesel, kerosene,

paraffin wax, lubricating oil are obtained by

refining petroleum.

� Coal and petroleum resources are limited. We

should use them judiciously.

Exercises

1. What are the advantages of using CNG and LPG as fuels?

2. Name the petroleum product used for surfacing of roads.

3. Describe how coal is formed from dead vegetation. What is this processcalled?

4. Fill in the blanks :

(a) Fossil fuels are , and .

(b) Process of separation of different constituents from petroleum iscalled .

(c) Least polluting fuel for vehicle is .

5. Tick True/False against the following statements :

(a) Fossil fuels can be made in the laboratory. (T/F)

(b) CNG is more polluting fuel than petrol. (T/F)

(c) Coke is almost pure form of carbon. (T/F)

(d) Coal tar is a mixture of various substances. (T/F)

(e) Kerosene is not a fossil fuel. (T/F)

6. Explain why fossil fuels are exhaustible natural resources.


7. Describe characteristics and uses of coke.

8. Explain the process of formation of petroleum.

9. The following Table shows the total power shortage in India from 1991–1997. Show the data in the form of a graph. Plot shortage percentage forthe years on the Y-axis and the year on the X-axis.

S. No. Year Shortage (%)

1 1991 7.9

2 1992 7.8

3 1993 8.3

4 1994 7.4

5 1995 7.1

6 1996 9.2

7 1997 11.5

E X

E R

C I

S E

S


1. Get an outline map of India. Mark the places in the map wherecoal, petroleum and natural gas are found. Show the places wherepetroleum refineries are situated.

2. Choose any five families of your neighbourhood. Enquire whethertheir energy consumption (coal, gas, electricity, petrol, kerosene)has increased or decreased in the last five years. Enquire also aboutthe measures they adopt to conserve energy.

3. Find out the location of major thermal power plants in India. Whatcould be the reasons for their being located at those places?

For more information, visit:� www.energyquest.ca.gov/story/chapter08.html� en.wikipedia.org/wiki/Non-renewable_resources� http://lsa.colorado.edu/summarystreet/texts/coal.htm� http://www.eia.doe.gov/kids/energyfacts/sources/non-

renewable/oil.html

COMBUSTION AND FLAMECOMBUSTION AND FLAME

We use different kinds of fuel forvarious purposes at home, inindustry and for running

automobiles. Can you name a few fuelsused in our homes? Name a few fuelsused in trade and industry. What fuelsare used for running automobiles? Yourlist will contain fuels like cowdung,wood, coal, charcoal, petrol, diesel,compressed natural gas (CNG), etc.

You are familiar with the burning of acandle. What is the difference between theburning of a candle and the burning of afuel like coal? May be you were able toguess right: candle burns with a flamewhereas coal does not. Similarly, you willfind many other materials burningwithout a flame. Let us study the chemicalprocess of burning and the types of flameproduced during this process.

6.1 What is Combustion?

Recall the activity of burning ofmagnesium ribbon performed in ClassVII. We learnt that magnesium burns toform magnesium oxide and producesheat and light (Fig. 6.1).

We can perform a similar activity witha piece of charcoal. Hold the piece witha pair of tongs and bring it near theflame of a candle or a Bunsen burner.What do you observe?

We find that charcoal burns in air.We know that coal, too, burns in airproducing carbon dioxide, heat andlight.

We were told thatfood is a fuel for

our body.

Fig. 6.1 : Burning of magnesium

A chemical process in which asubstance reacts with oxygen to give offheat is called combustion. Thesubstance that undergoes combustionis said to be combustible. It is also calleda fuel. The fuel may be solid, liquid orgas. Sometimes, light is also given offduring combustion, either as a flame oras a glow.

In the reactions mentioned abovemagnesium and charcoal arecombustible substances.

Activity 6.2

Caution : Be careful while handlingburning candle.

Fix a lighted candle on a table. Puta glass chimney over the candle andrest it on a few wooden blocks insuch a way that air can enter the

Table 6.1 Combustible and non-combustible Substances

Material Combustible Non-combustible

Wood

Paper

Iron nails

Kerosene oil

Stone piece

Straw

Charcoal

Matchsticks

Glass

Activity 6.1

Collect some materials like straw,matchsticks, kerosene oil, paper,iron nails, stone pieces, glass, etc.Under the supervision of yourteacher try to burn each of thesematerials one by one. If combustiontakes place mark the materialcombustible, otherwise mark it asnon-combustible (Table 6.1).

(a) (b) (c)Fig. 6.2: Experiment to show that air is

essential for burning

Rightly so. In our bodyfood is broken down by

reaction with oxygen andheat is produced. We

learnt that in Class VII.

Can you name some moresubstances which are combustible? Youcan add those to Table 6.1.

Let us investigate conditions underwhich combustion takes place.


chimney [Fig. 6.2(a)]. Observe whathappens to the flame. Now removethe blocks and let the chimney reston the table [Fig. 6.2(b)]. Againobserve the flame. Finally, put aglass plate over the chimney [Fig.6.2(c)]. Watch the flame again. Whathappens in the three cases? Doesthe flame flicker off? Does it flickerand give smoke? Does it burnunaffected? Can you infer anythingat all about the role played by air inthe process of burning?

We find that for combustion, air isnecessary. The candle burns freely incase (a) when air can enter thechimney from below. In case (b), whenair does not enter the chimney frombelow, the flame flickers and producessmoke. In case (c), the flame finallygoes off because the air is notavailable.

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Activity 6.3

Place a piece of burning wood orcharcoal on an iron plate or Tawa.Cover it with a glass jar or atumbler, or a transparent plastic jar.Observe what happens. Doescharcoal stop burning aftersometime? Can you think of thereason why it stops burning?

You might have heard that when theclothes of a person catch fire, the person

We have read that the sunproduces its own heat and

light. Is it also some kind ofcombustion?

In the sun, heat and light areproduced by nuclear reactions. Youwill learn about this process in higherclasses.

During extreme heatof summer, at someplaces dry grassescatch fire. Fromgrasses, it spreads totrees, and very soonthe whole forest is onfire (Fig. 6.4). It isvery difficult tocontrol such fires.

Fig. 6.4 : Forest fire

Fig. 6.3 : Blanket wrapped around a personwhose clothes caught fire

Now recall some of your experiences.

Does a matchstick burn by itself?How does it burn?

You must have had an experience ofburning a piece of paper. Does it burnwhen a burning matchstick is broughtnear it?

Can you burn a piece of wood bybringing a lighted matchstick near it?

Why do you have to use paper orkerosene oil to start fire in wood or coal?

Have you heard of forest fires?

is covered with a blanket to extinguishfire (Fig. 6.3). Can you guess why?


We find that a combustible substancecannot catch fire or burn as long as itstemperature is lower than its ignitiontemperature. Have you ever seencooking oil catching fire when a fryingpan is kept for long on a burning stove?Kerosene oil and wood do not catch fireon their own at room temperature. But,if kerosene oil is heated a little, it willcatches fire. But if wood is heated alittle, it would still not catch fire. Doesit mean that ignition temperature ofkerosene oil is lower than that of wood?Does it mean that we need to takespecial care in storing kerosene oil? Thefollowing activity shows that it isessential for a substance to reachignition temperature to burn.

Activity 6.4

Caution : Be careful while handlingburning candle.

Make two paper cups by folding asheet of paper. Pour about 50 mL ofwater in one of the cups. Heat boththe cups separately with a candle(Fig. 6.5). What do you observe?

The history of the matchstick is veryold. More than five thousand yearsago small pieces of pinewood dippedin sulphur were used as matches inancient Egypt. The modern safetymatch was developed only about twohundred years ago.

A mixture of antimony trisulphide,potassium chlorate and whitephosphorus with some glue andstarch was applied on the head of amatch made of suitable wood. Whenstruck against a rough surface, whitephosphorus got ignited due to the heatof friction. This started thecombustion of the match. However,white phosphorus proved to bedangerous both for the workersinvolved in the manufacturing ofmatches and for the users.

These days the head of the safetymatch contains only antimonytrisulphide and potassium chlorate.The rubbing surface has powderedglass and a little red phosphorus(which is much less dangerous).When the match is struck against therubbing surface, some redphosphorus gets converted into whitephosphorus. This immediately reactswith potassium chlorate in thematchstick head to produce enoughheat to ignite antimony trisulphideand start the combustion.

Do these experiences tell you thatdifferent substances catch fire atdifferent temperatures?

The lowest temperature at which asubstance catches fire is called itsignition temperature.

Can you tell now why a matchstickdoes not catch fire on its own at roomtemperature? Why does the matchstickstart burning on rubbing it on the sideof the matchbox?

Fig. 6.5 : Heating water in a paper cup

water

papercups

What happens to the empty papercup? What happens to the paper cupwith water? Does water in this cupbecome hot?

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If we continue heatingthe cup, we can even boilwater in the paper cup.

Can you think of anexplanation for thisphenomenon?

The heat supplied tothe paper cup istransferred to water byconduction. So, in thepresence of water, theignition temperature ofpaper is not reached.Hence, it does not burn.

The substances whichhave very low ignitiontemperature and caneasily catch fire with aflame are calledinflammable substances. Examples ofinflammable substances are petrol,alcohol, Liquified Petroleum Gas (LPG),etc. Can you list some more inflammablesubstances?

6.2 How do We Control Fire?

You must have seen or heard of firebreaking out in homes, shops andfactories. If you have seen such anaccident, write a short description inyour note book. Also, share theexperience with your classmates.

It is important that all of usknow the telephone numbers

of the fire service.

Fig. 6.6: Firemen extinguish the fire by throwing water under pressure

Find out the telephone number of thefire service in your area. If a firebreaks out in your house or in yourneighbourhood, the first thing to dois to call the fire service.

Does your city/town have a firebrigade station?

When a fire brigade arrives, whatdoes it do? It pours water on the fire(Fig. 6.6). Water cools the combustiblematerial so that its temperature isbrought below its ignition temperature.This prevents the fire from spreading.Water vapours also surround thecombustible material, helping in cuttingoff the supply of air. So, the fire isextinguished.

You have learnt that there are threeessential requirements for producingfire. Can you list these requirements?

These are: fuel, air (to supply oxygen)and heat (to raise the temperature of thefuel beyond the ignition temperature).Fire can be controlled by removing oneor more of these requirements. The jobof a fire extinguisher is to cut off thesupply of air, or to bring down thetemperature of the fuel, or both. Noticethat the fuel in most cases cannot be


The most common fireextinguisher is water. Butwater works only whenthings like wood and paperare on fire. If electricalequipment is on fire, watermay conduct electricityand harm those trying todouse the fire. Water isalso not suitable for firesinvolving oil and petrol. Doyou recall that water isheavier than oil? So, itsinks below the oil, and oilkeeps burning on top.

For fires involving electrical equipment and inflammable materials likepetrol, carbon dioxide (CO2) is the best extinguisher. CO2, being heavier thanoxygen, covers the fire like a blanket. Since the contact between the fuel andoxygen is cut off, the fire is controlled. The added advantage of CO2 is that inmost cases it does not harm the electrical equipment.

How do we get the supply of carbon dioxide? It can be stored at high pressureas a liquid in cylinders. In what form is the LPG stored in cylinders? Whenreleased from the cylinder, CO2 expands enormously in volume and cools down.So, it not only forms a blanket around the fire, it also brings down thetemperature of the fuel. That is why it is an excellent fire extinguisher. Anotherway to get CO2 is to release a lot of dry powder of chemicals like sodiumbicarbonate (baking soda) or potassium bicarbonate. Near the fire, thesechemicals give off CO2.

CAUTION : Do not handle the gasstove yourself. Ask your parentsto help.

Fig. 6.7 : Fire extinguisher

eliminated. If, for instance, a buildingcatches fire, the whole building is thefuel.

6.3 Types of CombustionBring a burning matchstick or a gaslighter near a gas stove in the kitchen.Turn on the knob of the gas stove. Whatdo you observe?

We find that the gas burns rapidlyand produces heat and light. Such

combustion is known as rapidcombustion.

There are substances likephosphorus which burn in air at roomtemperature.

The type of combustion in which amaterial suddenly bursts into flames,without the application of any apparentcause is called spontaneouscombustion.

Spontaneous combustion of coal dusthas resulted in many disastrous firesin coal mines. Spontaneous forestfires are sometimes due to the heat

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of the sun or due to lightning strike.However, most forest fires are due tothe carelessness of human beings. Itis important to remember that thecampfires must be completelyextinguished before leaving a forestafter a picnic, or a visit.

We generally have fireworks onfestival days. When a cracker is ignited,a sudden reaction takes place with theevolution of heat, light and sound. Alarge amount of gas formed in thereaction is liberated. Such a reaction iscalled explosion. Explosion can also takeplace if pressure is applied on thecracker.

6.4 Flame

Observe an LPG flame. Can you tell thecolour of the flame. What is the colourof a candle flame?

Recall your experience of burning amagnesium ribbon in Class VII. If youdo not have experience of burning theremaining items in Table 6.2 you cando that now.

Record your observations andmention whether on burning thematerial forms a flame or not.

Table 6.2 Materials forming Flame on Burning

S.No. Material Forms flame Does not form flame

1. Candle

2. Magnesium

3. Camphor

4. Kerosene Stove

5. Charcoal

Fig. 6.9 : Flames of kerosene lamp, candle andBunsen Burner

Fig. 6.8: Colours of a candle flame and theflame of a kitchen stove

6.5 Structure of a Flame

Activity 6.6

Light a candle (Caution : Be careful).Hold a glass tube with a pair oftongs and introduce its one end inthe dark zone of a non-flickering


candle flame [Fig. 6.10 (a)]. Bring alighted matchstick near the otherend of the glass tube. Do you see aflame? If so, what is it that producesa flame? Notice that the wax nearthe heated wick melts quickly.

The substances which vapouriseduring burning, give flames. Forexample, kerosene oil and molten waxrise through the wick and are vapourisedduring burning and form flames.Charcoal, on the other hand, does notvapourise and so does not produce aflame. In Activity 6.6, could the vapoursof wax coming out of the glass tube bethe cause of the flame produced?

When the candle flame is steady,introduce a clean glass plate/slide intothe luminous zone of the flame [Fig. 6.10(b)]. Hold it there with a pair of tongsfor about 10 seconds. Then remove it.What do you observe?

Fig. 6.10 (a)

Fig. 6.10 (c)

A circular blackish ring is formed onthe glass plate/slide. It indicates thedeposition of unburnt carbon particlespresent in the luminous zone of theflame.

Hold a thin long copper wire justinside the flame for about 30 seconds[Fig. 6.10 (c)].

Fig. 6.10 (b)

Notice that the portion of the copperwire just outside the flame gets red hot.Does it indicate that the non-luminouszone of the flame has a hightemperature? In fact, this part of theflame is the hottest part [Fig. 6.10(d)].

Goldsmiths blow the outermost zoneof a flame with a metallic blow-pipe for

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Fig. 6.11 : Goldsmith blowing through ametallic pipe

are mainly wood, charcoal, petrol,kerosene, etc. These substances arecalled fuels. A good fuel is one which isreadily available. It is cheap. It burnseasily in air at a moderate rate. Itproduces a large amount of heat. It doesnot leave behind any undersirablesubstances.

There is probably no fuel that couldbe considered as an ideal fuel. Weshould look for a fuel which fulfils mostof the requirements for a particular use.

Fuels differ in their cost. Some fuelsare cheaper than others.

Make a list of fuels familiar to you.Group them as solid, liquid and gaseousfuels as in Table 6.3.

6.7 Fuel Efficiency

Suppose you were asked to boil a givenquantity of water using cow dung, coaland LPG as fuel. Which fuel would youprefer? Give your reason. You may takethe help of your parents. Do these threefuels produce the same amount of heat?The amount of heat energy produced oncomplete combustion of 1 kg of a fuel iscalled its calorific value. The calorificvalue of a fuel is expressed in a unitcalled kilojoule per kg (kJ/kg).Calorific values of some fuels are givenin Table 6.4.

Fig. 6.10 (d): Different Zones of Candle flame

Table 6.3 Types of Fuels

S. No. Solid Fuels Liquid Fuels Gaseous fuels

1. Coal Kerosene oil Natural gas

2.

3.

hottestpart

moderatelyhot

leasthot

wax candle

outer zone ofcomplete

combustion (blue)

middle zone ofpartial combustion

(yellow)

innermost zone ofunburnt wax

vapours (black)

melting gold and silver (Fig. 6.11). Whydo they use the outermost zone of theflame?

6.6 What is a Fuel?

Recall that the sources of heat energyfor domestic and industrial purposes


2. Incomplete combustion of these fuelsgives carbon monoxide gas. It is avery poisonous gas. It is dangerousto burn coal in a closed room. Thecarbon monoxide gas produced cankill persons sleeping in that room.

For centuries, wood was used asdomestic and industrial fuel. But nowit has been replaced by coal and otherfuels like LPG. In many rural parts ofour country, people still use wood asa fuel because of its easy availabilityand low cost. However, burning ofwood gives a lot of smoke which isvery harmful for human beings. Itcauses respiratory problem. Also,trees provide us with usefulsubstances which are lost when woodis used as fuel. Moreover cutting oftrees leads to deforestation whichis quite harmful to the environment,as you learnt in Class VII.

Table 6.4 : Calorific Values ofDifferent Fuels

Fuel Calorific Value(kJ/kg)

Cow dung cake 6000-8000

Wood 17000-22000

Coal 25000-33000

Petrol 45000

Kerosene 45000

Diesel 45000

Methane 50000

CNG 50000

LPG 55000

Biogas 35000-40000

Hydrogen 150000

Oh! So, that is why we areadvised never to sleep in a

room with burning orsmouldering coal fire in it.

Burning of Fuels Leads to HarmfulProducts

The increasing fuel consumption hasharmful effects on the environment.1. Carbon fuels like wood, coal,

petroleum release unburnt carbonparticles. These fine particles aredangerous pollutants causingrespiratory diseases, such as asthma.

3. Combustion of most fuels releasescarbon dioxide in the environment.Increased concentration of carbondioxide in the air is believed to causeglobal warming.

Global warming is the rise intemperature of the atmosphere of theearth. This results, among otherthings, in the melting of polar glaciers,which leads to a rise in the sea level,causing floods in the coastal areas.Low lying coastal areas may even bepermanently submerged under water.

4. Burning of coal and diesel releasessulphur dioxide gas. It is anextremely suffocating and corrosivegas. Moreover, petrol engines give offgaseous oxides of nitrogen. Oxidesof sulphur and nitrogen dissolve inrain water and form acids. Such rainis called acid rain. It is very harmfulfor crops, buildings and soil. You havealready learnt about it in Class VII.The use of diesel and petrol as fuels

in automobiles is being replaced byCNG (Compressed Natural Gas), becauseCNG produces the harmful products invery small amounts. CNG is a cleaner fuel.

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KEYWORDS

ACID RAIN

CALORIFIC VALUE

COMBUSTION

DEFORESTATION

EXPLOSION

FLAME

FIRE EXTINGUISHER

FUEL

FUEL EFFICIENCY

GLOBAL WARMING

IDEAL FUEL

IGNITION

TEMPERATURE

INFLAMMABLE

SUBSTANCES


� The substances which burn in air are called

combustible.

� Oxygen (in air) is essential for combustion.

� During the process of combustion, heat and

light are given out.

� Ignition temperature is the lowest temperature

at which a combustible substance catches

fire.

� Inflammable substances have very low

ignition temperature.

� Fire can be controlled by removing one or more

requirements essential for producing fire.

� Water is commonly used to control fires.

� Water cannot be used to control fires involving

electrical equipments or oils.

� There are various types of combustions such

as rapid combustion, spontaneous

combustion, explosion, etc.

� There are three different zones of a flame -

dark zone, luminous zone and non-luminous

zone.

� An ideal fuel is cheap, readily available, readily

combustible and easy to transport. It has high

calorific value. It does not produce gases or

residues that pollute the environment.

� Fuels differ in their efficiency and cost.

� Fuel efficiency is expressed in terms of its

calorific value which is expressed in units of

kilojoule per kg.

� Unburnt carbon particles in air are dangerous

pollutants causing respiratory problems.

� Incomplete combustion of a fuel gives

poisonous carbon monoxide gas.

� Increased percentage of carbon dioxide in air

has been linked to global warming.

� Oxides of sulphur and nitrogen produced by

the burning of coal, diesel and petrol cause

acid rain which is harmful for crops, buildings

and soil.


Exercises

1. List conditions under which combustion can take place.

2. Fill in the blanks:(a) Burning of wood and coal causes of air.(b) A liquid fuel, used in homes is .(c) Fuel must be heated to its before it starts

burning.(d) Fire produced by oil cannot be controlled by .

3. Explain how the use of CNG in automobiles has reduced pollution in ourcities.

4. Compare LPG and wood as fuels.

5. Give reasons:(a) Water is not used to control fires involving electrical equipment.(b) LPG is a better domestic fuel than wood.(c) Paper by itself catches fire easily whereas a piece of paper wrapped

around an aluminium pipe does not.

6. Make a labelled diagram of a candle flame.

7. Name the unit in which the calorific value of a fuel is expressed.

8. Explain how CO2 is able to control fires.

9. It is difficult to burn a heap of green leaves but dry leaves catch fire easily.Explain.

10. Which zone of a flame does a goldsmith use for melting gold and silver andwhy?

11. In an experiment 4.5 kg of a fuel was completely burnt. The heat producedwas measured to be 180,000 kJ. Calculate the calorific value of the fuel.

12. Can the process of rusting be called combustion? Discuss.

13. Abida and Ramesh were doing an experiment in which water was to beheated in a beaker. Abida kept the beaker near the wick in the yellow partof the candle flame. Ramesh kept the beaker in the outermost part of theflame. Whose water will get heated in a shorter time?

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baking soda +vinegar


1. Survey the availability of various fuels in your locality. Find outtheir cost per kg and prepare a tabular chart showing how manykJ of various fuels you can get for every rupee.

2. Find out the number, type and location of fire extinguishers availablein your school, nearby shops and factories. Write a brief report aboutthe preparedness of these establishments to fight fire.

3. Survey 100 houses in your area. Find the percentage of householdsusing LPG, kerosene, wood and cattledung as fuel.

4. Talk to people who use LPG at home. Find out what precautionsthey take in using LPG.

5. Make a model of a fire extinguisher. Place a short candle and aslightly taller candle in a small dish filled with baking soda. Placethe dish at the bottom of a large bowl. Light both the candles. Thenpour vinegar into the dish of baking soda. Take care. Do not pourvinegar on the candles. Observe the foaming reaction. What happensto the candles? Why? In what order?

Fig. 6.12

For more information, visit:� www.newton.dep.anl.gov/askasci/chem03/chem03767.htm� http://www.einstrumentsgroup.com/gas_analyzers/combustion/

what-is-combustion.php� ht tp :// l i b ra ry .kcc .hawa i i . edu/ex te rna l/chemis t r y/

everyday_combustion.html� http://en.wikipedia.org/wiki/combustion� http://wwwchem.csustan.edu/consumer/fuels/heats%20.htm

CONSERVATION OF PLANTS AND ANIMALSCONSERVATION OF PLANTS AND ANIMALS

We saw in Class VII thatPaheli and Boojho had visitedthe forest along with Prof.

Ahmad and Tibu. They were eager toshare their experiences with theirclassmates. Other children in the classwere also eager to share their experiencesas some of them had visited BharatpurSanctuary. Some others had heardabout Kaziranga National Park,Lockchao Wildlife Sanctuary, GreatNicobar Biosphere Reserve and TigerReserve, etc.

Some natural causes of deforestationare forest fires and severe droughts.

Activity 7.1

Add more causes of deforestation toyour list and classify them intonatural and man-made.

7.2 Consequences ofDeforestation

Paheli and Boojho recalled theconsequences of deforestation. Theyremembered that deforestation increasesthe temperature and pollution level onthe earth. It increases the level of carbondioxide in the atmosphere. Ground waterlevel also gets lowered. They know thatdeforestation disturbs the balance innature. They were told by Prof. Ahmadthat if cutting of trees continues, rainfalland the fertility of the soil will decrease.

7.1 Deforestation and ItsCauses

A great variety of plants and animalsexists on earth. They are essential forthe wellbeing and survival of mankind.Today, a major threat to survival of theseorganisms is deforestation. We knowthat deforestation means clearing offorests and using that land for otherpurposes. Trees in the forest are cut forsome of the purposes mentioned below:

� Procuring land for cultivation.� Building houses and factories.� Making furniture or using wood as

fuel.

How does deforestation reducerainfall on the one hand andlead to floods on the other?

Moreover, there will be increasedchances of natural calamities such asfloods and droughts.

Recall that plants need carbondioxide for photosynthesis. Fewer treeswould mean that less carbon dioxide willbe used up resulting in its increased

What is the purpose ofmaking national parks,wildlife sanctuaries and

biosphere reserves?

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amount in the atmosphere. This will leadto global warming as carbon dioxidetraps the heat rays reflected by the earth.The increase in temperature on the earthdisturbs the water cycle and may reducerainfall. This could cause droughts.

Deforestation is a major cause whichleads to the change in soil properties.Physical properties of the soil get affectedby plantation and vegetation. Recallfrom Class VII how trees prevent soilerosion. Fewer trees result in more soilerosion. Removal of the top layer of thesoil exposes the lower, hard and rockylayers. This soil has less humus and isless fertile. Gradually the fertile landgets converted into deserts. It is calleddesertification.

Deforestation also leads to a decreasein the water holding capacity of the soil.The movement of water from the soilsurface into the ground (infiltration rate)is reduced. So, there are floods. Theother properties of the soil like nutrientcontent, texture, etc., also changebecause of deforestation.

We have studied in Class VII that weget many products from forests. List theseproducts. Will we face the shortage of theseproducts if we continue cutting trees?

Activity 7.2

Animal life is also affected bydeforestation. How? List the pointsand discuss them in your class.

7.3 Conservation of Forestand Wildlife

Having become aware of the effects ofdeforestation, Paheli and Boojho areworried. They go to Prof. Ahmad and askhim how forests and wildlife can be saved.

Prof. Ahmad organises a visit to abiosphere reserve for Paheli, Boojho andtheir classmates. He selects a placenamed Pachmarhi Biosphere Reserve. Heknows that the plants and animals foundhere are similar to those of the upperHimalayan peaks and to those belongingto the lower western ghats. Prof. Ahmadbelieves that the biodiversity found hereis unique. He requests Madhavji, a forestemployee, to guide the children insidethe biosphere reserve. He explains thatpreserving areas of such biologicalimportance make them a part of ournational heritage.

Biosphere is that part of the earth inwhich living organisms exist or whichsupports life. Biological diversity orbiodiversity, refers to the variety oforganisms existing on the earth, theirinterrelationships and theirrelationship with the environment.

Madhavji explains to the childrenthat apart from our personal efforts andefforts of the society, governmentagencies also take care of the forests andanimals. The government lays downrules, methods and policies to protectand conserve them. Wildlife sanctuaries,national parks, biosphere reserves, etc.,are protected areas for conservation ofplants and animals present in that area.

To protect our flora and fauna andtheir habitats, protected areascalled sanctuaries, national parks andbiosphere reserves have beenearmarked. Plantation, cultivation,grazing, felling trees, hunting andpoaching are prohibited there.Sanctuary : Areas where animals areprotected from any disturbance tothem and their habitat.


Activity 7.3

Find out the number of nationalparks, wildlife sanctuaries andbiosphere reserves in your district,state and country. Record in Table7.1. Show these areas in an outlinemap of your state and India.

Fig. 7.1 : Pachmarhi Biosphere Reserve

7.4 Biosphere Reserve

Children along with Prof. Ahmad andMadhavji enter the biosphere reservearea. Madhavji explains that biospherereserves are the areas meant forconservation of biodiversity. As you areaware that biodiversity is the variety ofplants, animals and microorganismsgenerally found in an area. Thebiosphere reserves help to maintain thebiodiversity and culture of that area. Abiosphere reserve may also containother protected areas in it. ThePachmarhi Biosphere Reserve consistsof one national park named Satpura andtwo wildlife sanctuaries named Bori andPachmarhi (Fig. 7.1).

Table 7.1 : Protected areas for conservation.

Protected Areas — National Park Wildlife Sanctuary Biosphere Reserve

In my district

In my state

In my country

PACHMARHI SANCTUARY

TO PIPARIYATAWA

RESERVIOR

CHURNA

BORI SANCTUARYDHAIN BORI

SATPURA NATIONAL PARK

NEEMGHAN PANARPANI GATE

PACHMARHI

↑N

National Park : Areas reserved forwild life where they can freely usethe habitats and natural resources.Biosphere Reserve : Large areas ofprotected land for conservation of wildlife, plant and animal resources andtraditional life of the tribals living inthe area.

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(d) (e) (f)

Fig. 7.2 : (a) Wild dog (b) Cheetal (c) Wolf (d) Leopard (e) Fern (f) Jamun

Activity 7.4

List the factors disturbing thebiodiversity of your area. Some ofthese factors and human activitiesmay disturb the biodiversityunknowingly. List these humanactivities. How can these be checked?Discuss in your class and write abrief report in your notebook.

7.5 Flora and Fauna

As the children walk around thebiosphere reserve they appreciate thegreen wealth of the forest. They are veryhappy to see tall teak trees and animalsinside the forest. Suddenly, Paheli finds

a rabbit and wants to catch it. Shestarts running after it. Prof. Ahmadstops her. He explains that animals arecomfortable and happy in their ownhabitat. We should not disturb them.Madhavji explains that some animalsand plants typically belong to aparticular area. The plants and animalsfound in a particular area are termedflora and fauna of that area.

Sal, teak, mango, jamun, silver ferns,arjun, etc are the flora and chinkara,blue-bull, barking deer, cheetal,leopard, wild dog, wolf, etc. areexamples of the fauna of thePachmarhi Biosphere Reserve(Fig. 7.2).

(a) (b) (c)


I have heard that some of theendemic species may vanish.

Is it true?

Activity 7.5

Try to identify the flora and faunaof your area and list them.

7.6 Endemic SpeciesSoon the group quietly enters the deepforest. Children are surprised to see avery big squirrel. This squirrel has a bigfluffy tail. They are very curious to knowabout it. Madhavji tells them that thisis known as the giant squirrel and isendemic to this area.

Endemic species are those speciesof plants and animals which are foundexclusively in a particular area. They arenot naturally found anywhere else. Aparticular type of animal or plant maybe endemic to a zone, a state or a country.

Madhavji shows sal and wild mango(Fig. 7.3 (a)] as two examples of the

Fig. 7.3 : (a) Wild Mango

Fig. 7.3 : (b) Giant squirrel

endemic flora of the PachmarhiBiosphere Reserve. Bison, Indian giantsquirrel [Fig. 7.3 (b)] and flying squirrelare endemic fauna of this area. Prof.Ahmad explains that the destruction oftheir habitat, increasing population andintroduction of new species may affectthe natural habitat of endemic speciesand endanger their existence.

Species is a group of populationswhich are capable of interbreeding.This means that the members of aspecies can reproduce fertile offspringonly with the members of their ownspecies and not with members ofother species. Members of a specieshave common characteristics.

Activity 7.6

Find out the endemic plants andanimals of the region where you live.

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7.7 Wildlife Sanctuary

Soon Paheli sees a board with ‘PachmarhiWildlife Sanctuary’ written on it.

Prof. Ahmad explains that, likereserve forests, wildlife sanctuariesprovide protection and suitable livingconditions to wild animals. Madhavjifurther explains that sanctuaries areplaces where killing (poaching) orcapturing of animals is strictlyprohibited.

instead of their natural habitat? Inyour opinion, will the animals becomfortable in a zoo or in theirnatural habitat?

7.8 National Park

On the roadside there was anotherboard on which was written ‘SatpuraNational Park’.

Children are now eager to go there.Madhavji tells them that these reservesare large and diverse enough to protectwhole sets of ecosystems. Theypreserve flora, fauna, landscape andhistoric objects of an area. SatpuraNational Park is the first Reserve Forestof India. The finest Indian teak isfound in this forest.

It is a pity that even protected forestsare not safe because people living in theneighbourhood encroach upon themand destroy them.

Children are reminded of their visitto the zoo. They recall that zoos are alsoplaces where animals receive protection.

What is the differencebetween a zoo and awildlife sanctuary?

Some of the threatened wild animalslike black buck, white eyed buck,elephant, golden cat, pink headedduck, gharial, marsh crocodile,python, rhinoceros, etc. are protectedand preserved in our wild lifesanctuaries. Indian sanctuaries haveunique landscapes—broad levelforests, mountain forests and bushlands in deltas of big rivers.

Activity 7.7

Visit a nearby zoo. Observe theconditions provided to the animals.Were they suitable for the animals?Can animals live in artificial setting

Rock shelters are also found insidethe Satpura National Park. These arethe prehistoric evidences of humanlife in these jungles. They give us anidea of the life of the primitive people.Rock paintings are found in theseshelters. A total of 55 rock sheltershave been identified in PachmarhiBiosphere Reserve.Figures of animals and men fighting,hunting, dancing, and playingmusical instruments are depicted inthese paintings. Many tribals stilllive in the area.

As children move ahead, they see aboard with ‘Satpura Tiger Reserve’written on it. Madhavji explains thatProject Tiger was launched by thegovernment to protect the tigers in thecountry. The objective of this projectwas to ensure the survival andmaintenance of the tiger population inthe country.


Fig. 7.4 : Tiger Fig. 7.5 : Wild buffalo Fig. 7.6 : Barasingha

Are tigers still found inthis forest? I hope that I

can see a tiger!

Are only big animalsfacing the extinction?

Tiger (Fig. 7.4) is one of the manyspecies which are slowly disappearingfrom our forests. But, the Satpura TigerReserve is unique in the sense that asignificant increase in the populationof tigers has been seen here. Once upona time, animals like lions, elephants,wild buffaloes (Fig. 7.5) and barasingha(Fig. 7.6) were also found in the SatpuraNational Park. Animals whose numbersare diminishing to a level that they mightface extinction are known as theendangered animals. Boojho isreminded of the dinosaurs whichbecame extinct a long time ago. Survivalof some animals has become difficultbecause of disturbances in their naturalhabitat.

Madhavji tells Paheli that smallanimals are much more in danger ofbecoming extinct than the biggeranimals. At times, we kill snakes,frogs, lizards, bats and owls ruthlesslywithout realising their importance inthe ecosystem. By killing them we areharming ourselves. They might besmall in size but their role in theecosystem cannot be ignored. Theyform part of food chains and foodwebs, about which you learnt inClass VII.

An ecosystem is made of all theplants, animals and microorganisms inan area along with non-livingcomponents such as climate, soil,river deltas, etc.

I wonder if there isany record of the

endangered species!

7.9 Red Data Book

Prof. Ahmad explains about Red DataBook to the children. He tells them thatRed Data Book is the source bookwhich keeps a record of all theendangered animals and plants. There

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are different Red Data Books for plants,animals and other species. (For furtherdetails about Red Data Book, you canlog on to www.wil.gov.in/envis/primates/page102htm/new/nwdc/plants.htm)

7.10 Migration

The excursion party then enters deeperinto the forest under the guidance ofMadhavji. They sit near the TawaReservoir to relax for some time. Paheliobserves some of the birds near theriver. Madhavji tells the children thatthese are the migratory birds. Thesebirds have flown here from other partsof the world.

Migratory birds fly to far away areasevery year during a particular timebecause of climatic changes. They flyfor laying eggs as the weather in theirnatural habitat becomes very cold andinhospitable. Birds who cover longdistances to reach another land areknown as the migratory birds as Pahelilearnt in Class VII.

What would happen if wehad no wood? Is there

any alternativeavailable for wood?

I know that paper is oneof the important products

we get from forests.I wonder whether

there are any alternativesavailable for paper!

Is there any permanentsolution to the problem

of deforestation?

7.11 Recycling of Paper

Prof. Ahmad draws the attention ofthe children to another cause ofdeforestation. He tells them that ittakes 17 full grown trees to make onetonne of paper. Therefore, we shouldsave paper. Prof. Ahmad also tells thatpaper can be recycled five to seventimes for use. If each student saves atleast one sheet of paper in a day, wecan save many trees in a year. Weshould save, reuse used paper andrecycle it. By this we not only savetrees but also save energy and waterneeded for manufacturing paper.Moreover, the amount of harmfulchemicals used in paper making willalso be reduced.

7.12 Reforestation

Prof. Ahmad suggests that the answerto deforestation is reforestation.Reforestation is restocking of thedestroyed forests by planting new trees.The planted trees should generally beof the same species which were foundin that forest. We should plant at leastas many trees as we cut. Reforestationcan take place naturally also. If thedeforested area is left undisturbed, it re-establishes itself. In natural reforestationthere is no role of human beings. We


have already made a tremendousdamage to our forests. If we have toretain our green wealth for generations,plantation of more trees is the onlyoption.

Prof. Ahmad told them that in Indiawe have the Forest (Conservation) Act.This act is aimed at preservation andconservation of natural forests and

meeting the basic needs of the peopleliving in or near the forests.

After some rest Madhavji askschildren to start moving back becauseit is not advisable to stay back in thejungle after sunset. On getting back,Prof. Ahmad and the children thankMadhavji for guiding them through thisexciting experience.

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� Wildlife sanctuary, national park and

biosphere reserve are names given to the

areas meant for conservation and preservation

of forest and wild animals.

� Biodiversity refers to the variety of living

organisms in a specific area.

� Plants and animals of a particular area are

known as the flora and fauna of that area.

� Endemic species are found only in a particular

area.

� Endangered species are those which are facing

the danger of extinction.

� Red Data Book contains a record of the

endangered species.

� Migration is the phenomenon of movement of

a species from its own habitat to some other

habitat for a particular time period every year

for a specific purpose like breeding.

� We should save, reuse and recycle paper to

save trees, energy and water.

� Reforestation is the restocking of destroyed

forests by planting new trees.

Exercises


(a) A place where animals are protected in their natural habitat iscalled .

(b) Species found only in a particular area is known as .

(c) Migratory birds fly to far away places because of changes.

2. Differentiate between the following:

(a) Wildlife sanctuary and biosphere reserve

(b) Zoo and wildlife sanctuary

(c) Endangered and extinct species

(d) Flora and fauna

KEYWORDS

BIODIVERSITY

BIOSPHERE RESERVE

DEFORESTATION

DESERTIFICATION

ECOSYSTEM

ENDANGERED

SPECIES

ENDEMIC SPECIES

EXTINCT

FAUNA

FLORA

MIGRATORY BIRDS

NATIONAL PARK

RED DATA BOOK

REFORESTATION

SANCTUARY


3. Discuss the effects of deforestation on the following:

(a) Wild animals

(b) Environment

(c) Villages (Rural areas)

(d) Cities (Urban areas)

(e) Earth

(f) The next generation

4. What will happen if:

(a) we go on cutting trees.

(b) the habitat of an animal is disturbed.

(c) the top layer of soil is exposed.

5. Answer in brief:

(a) Why should we conserve biodiversity?

(b) Protected forests are also not completely safe for wild animals. Why?

(c) Some tribals depend on the jungle. How?

(d) What are the causes and consequences of deforestation?

(e) What is Red Data Book?

(f) What do you understand by the term migration?

6. In order to meet the ever-increasing demand in factories and for shelter,

trees are being continually cut. Is it justified to cut trees for such projects?

Discuss and prepare a brief report.

7. How can you contribute to the maintenance of green wealth of your locality?

Make a list of actions to be taken by you.

8. Explain how deforestation leads to reduced rainfall.

9. Find out the information about the national parks in your state. Identify

and show their location on the outline map of India.

10. Why should paper be saved? Prepare a list of ways by which you can save

paper.

E X

E R

C I

S E

S

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E X

E R

C I

S E

S

11. Complete the word puzzle:Down1. Species on the verge of extinction.2. A book carrying information about endangered species.5. Consequence of deforestation.Across1. Species which have vanished.3. Species found only in a particular habitat.4. Variety of plants, animals and microorganisms found in an area.

Extended Learning - Activities and Projects

1. Plant at least five different plants in your locality during thisacademic year and ensure their maintenance till they grow.

2. Promise yourself that this year you will gift at least 5 plants to yourfriends and relatives on their achievements, or on occasions likebirthdays. Ask your friends to take proper care of these gifted plantsand encourage them to gift five plants to their friends on suchoccasions. At the end of the year count the plants that have beengifted through this chain.


Did You Know?

1. India has more than half of the world’s wild tigers, 65% of the Asianelephants, 85% of the great one-horned rhinoceros and 100% of theAsian lions.

2. India is sixth on a list of 12 mega-biodiversity countries in the world. Itcontains two of the 13 biodiversity hot spots of the world – North-EastIndia and the Western Ghats. These areas are very rich in biodiversity.

3. One of the most important factors that threatens wildlife today is habitatdestruction due to encroachment.

4. India contains 172 species of animals considered globally threatenedor 2.9% of the worlds total number of threatened species. This includes53 species of mammals, 69 of birds, 23 of reptiles and 3 species ofamphibians. India contains globally important population of some ofAsia’s rarest animals such as the Bengal fox, Marbled cat, Asiatic lion,Indian elephant, Asiatic wild ass, Indian rhinoceros, gaur, Wild asiaticwater buffalo, etc.

For knowing more, you may contact:

� Ministry of Environment and Forests, Govt. of IndiaEnvironment, Forest and Wildlife DepartmentParyavaran Bhavan, CGO Complex, Block – B,Lodhi Road, New Delhi – 110003,Website : http:/envfor.nic.in

� Project Tiger : www.kidsfortigers.org/raisingtigers/projecttiger.php� Biodiversity Hotspots : www.biodiversityhotspots.org

3. Is it justifiable to prevent tribals from staying in the core area of theforest? Discuss the matter in your class and note down the points forand against the motion in your notebook.

4. Study the biodiversity of a park nearby. Prepare a detailed report withphotographs and sketches of flora and fauna.

5. Make a list of the new information you have gathered from this chapter.Which information did you like the most and why?

6. Make a list of various uses of papers. Observe currency notes carefully.Do you find any difference between a currency paper and paper of yournotebook? Find out where currency paper is made.

7. Karnataka Government had launched ‘Project Elephant’ to save Asianelephants in the state. Find out about this and other such campaignslaunched to protect threatened species.

CELL — STRUCTURE AND FUNCTIONSCELL — STRUCTURE AND FUNCTIONS

You have already learnt that thingsaround us are either living ornon-living. Further, you may

recall that all living organisms carry outcertain basic functions. Can you listthese functions?

Different sets of organs perform thevarious functions you have listed. In thischapter, you shall learn about the basicstructural unit of an organ, which is thecell. Cells may be compared to bricks.Bricks are assembled to make a building.Similarly, cells are assembled to makethe body of every organism.

8.1 Discovery of the Cell

Robert Hooke in 1665 observed slices ofcork under a simple magnifying device.Cork is a part of the bark of atree. He took thin slices of cork andobserved them under a microscope. Henoticed partitioned boxes orcompartments in the cork slice (Fig. 8.1).

These boxes appeared like a honey-comb.

He also noticed that one box wasseparated from the other by a wall orpartition. Hooke coined the term ‘cell’for each box. What Hooke observed asboxes or cells in the cork were actuallydead cells.

Cells of living organisms could beobserved only after the discovery ofimproved microscopes. Very little wasknown about the cell for the next 150years after Robert Hooke’s observations.Today, we know a lot about cellstructure and its functions because ofimproved microscopes having highmagnification.

8.2 The Cell

Both, bricks in a building and cells inthe living organisms, are basicstructural units [Fig. 8.2(a), (b)]. Thebuildings, though built of similar bricks,have different designs, shapes and sizes.Similarly, in the living world, organismsdiffer from one another but all are madeup of cells. Cells in the living organismsare complex living structures unlikenon-living bricks.

Fig. 8.1: Cork cells as observed byRobert Hooke

A hen’s egg can be seeneasily. Is it a cell or a

group of cells?

The egg of a hen represents a singlecell and is big enough to be seen by theunaided eye.

8.3 Organisms show Varietyin Cell Number, Shape andSize

How do scientists observe and study theliving cells? They use microscopes whichmagnify objects. Stains (dyes) are usedto colour parts of the cell to study thedetailed structure.

There are millions of livingorganisms. They are of different shapesand sizes. Their organs also vary inshape, size and number of cells. Let usstudy about some of them.

Number of Cells

Can you guess the number of cells in atall tree or in a huge animal like theelephant? The number runs intobillions and trillions. Human body hastrillions of cells which vary in shapesand sizes. Different groups of cellsperform a variety of functions.

Organisms made of more than onecell are called multicellular (multi :many; cellular : cell) organisms. Thenumber of cells being less in smallerorganisms does not, in any way, affectthe functioning of the organisms. Youwill be surprised to know that anorganism with billions of cells beginslife as a single cell which is the fertilisedegg. The fertilised egg cell multiplies andthe number of cells increase asdevelopment proceeds.

Look at Fig 8.3 (a) and (b). Bothorganisms are made up of a single cell.The single-celled organisms are calledunicellular (uni : one; cellular : cell)

Fig. 8.2 : Brick wall and onion peel

(a) Brick wall (b) Onion peel

A billion is a thousand million. Atrillion is a thousand billion.

Fig. 8.3 : (a) Amoeba (b) Paramecium

(a) (b)


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organisms. A single-celled organismperforms all the necessary functionsthat multicellular organisms perform.

A single-celled organism, likeamoeba, captures and digests food,respires, excretes, grows andreproduces. Similar functions inmulticellular organisms are carried outby groups of specialised cells formingdifferent tissues. Tissues, in turn, formorgans.

Activity 8.1

The teacher may show a permanentslide of Amoeba and Parameciumunder a microscope. Alternatively,the teacher can collect pond waterand show these organisms bypreparing the slides.

Shape of Cells

Refer to Fig, 8.3 (a). How do you definethe shape of Amoeba in the figure? Youmay say that the shape appearsirregular. Infact, Amoeba has nodefinite shape, unlike other organisms.It keeps on changing its shape.Observe the projections of varyinglengths protruding out of its body.These are called pseudopodia (pseudo: false; podia : feet), as you learntin Class VII. These projections appearand disappear as Amoeba moves orfeeds.

A white blood cell (WBC) in humanblood is another example of a single cellwhich can change its shape. But whileWBC is a cell, amoeba is a full fledgedorganism capable of independentexistence.

What shape would you expect inorganisms with millions of cells? Fig.8.4 (a, b, c) shows different cells suchas blood, muscle and nerve cells ofhuman beings. The different shapes arerelated to their specific functions.

Generally, cells are round, sphericalor elongated [Fig. 8.4(a)]. Some cells arelong and pointed at both ends. Theyexhibit spindle shape [Fig. 8.4(b)]. Cellssometimes are quite long. Some arebranched like the nerve cell or a neuron[Fig. 8.4(c)]. The nerve cell receives andtransfers messages, thereby helping tocontrol and coordinate the working ofdifferent parts of the body.

The change in shape is due toformation of pseudopodia whichfacilitates movement and help

in capturing food.

Fig. 8.4 :(a) Spherical red blood cellsof humans, (b) Spindle shaped musclecells, (c) Long branched nerve cell

(a)

(b)(c)

What advantage doesAmoeba derive bychanging shape?


in the elephant be much bigger thanthose in a rat. The size of the cell isrelated to its function. For example,nerve cells, both in the elephant andrat, are long and branched. Theyperform the same function, that oftransferring messages.

8.4 Cell Structure andFunction

You have learnt that each livingorganism has many organs. You havestudied in Class VII about the digestiveorgans which together constitute thedigestive system. Each organ in thesystem performs different functionssuch as digestion, assimilation andabsorption. Similarly, different organsof a plant perform specific/specialisedfunctions. For example, roots help in theabsorption of water and minerals.Leaves, as you have learnt in Class VII,are responsible for synthesis of food.

Each organ is further made up ofsmaller parts called tissues. A tissue isa group of similar cells performing aspecific function.

Paheli realised that an organ is madeup of tissues which in turn, are madeup of cells. The cell in a living organismis the basic structural unit.

8.5 Parts of the Cell

Cell Membrane

The basic components of a cell are cellmembrane, cytoplasm and nucleus(Fig. 8.7). The cytoplasm and nucleusare enclosed within the cell membrane,also called the plasma membrane. Themembrane separates cells from oneanother and also the cell from thesurrounding medium. The plasma

Can you guess, which part of the cellgives it shape? Components of the cellare enclosed in a membrane. Thismembrane provides shape to the cellsof plants and animals. Cell wall is anadditional covering over the cellmembrane in plant cells. It gives shapeand rigidity to these cells (Fig. 8.7).Bacterial cell also has a cell wall.

Size of Cells

The size of cells in living organisms maybe as small as a millionth of a metre(micrometre or micron) or may be aslarge as a few centimetres. However, mostof the cells are microscopic in size andare not visible to the unaided eye. Theyneed to be enlarged or magnified by amicroscope. The smallest cell is 0.1 to0.5 micrometre in bacteria. The largestcell measuring 170 mm ×130 mm, isthe egg of an ostrich.

Activity 8.2

Boil a hen’s egg. Remove the shell.What do you observe? A whitematerial surrounds the yellow part.White material is albumin whichsolidifies on boiling. The yellow partis yolk. It is part of the single cell.You can observe this single cellwithout any magnifying device.

Are the cells in anelephant larger than the

cells in a rat?

The size of the cells has no relationwith the size of the body of the animalor plant. It is not necessary that the cells

SCIENCE96

membrane is porous and allows themovement of substances or materialsboth inward and outward.

Activity 8.3

In order to observe the basiccomponents of the cell, take anonion bulb. Remove the dry pinkcoverings (peels). You can easilyseparate these from the fleshy whitelayers of the bulb with the help offorceps or even with your hand. Youcan also break the onion bulb andseparate out thin layers. Place asmall piece of the thin onion peelin a drop of water on a glass slide.The thin layer can be cut intosmaller pieces with the help of ablade or forceps. Add a drop ofmethylene blue solution to the layerand place a coverslip on it. Whileplacing the coverslip ensure thatthere are no air bubbles under thecoverslip. Observe the slide underthe microscope. Draw and label. Youmay compare it with Fig. 8.5.

The boundary of the onion cell is thecell membrane covered by another thickcovering called the cell wall. The centraldense round body in the centre is calledthe nucleus. The jelly-like substance

between the nucleus and the cellmembrane is called cytoplasm.

Fig. 8.5 : Cells observed in an onion peel

I want to know why plantcells need cell walls?

nucleus

cytoplasm

You have learnt earlier that the cellmembrane gives shape to the cell. Inaddition to the cell membrane, there isan outer thick layer in cells of plantscalled cell wall. This additional layersurrounding the cell membrane isrequired by the plants for protection.Plant cells need protection againstvariations in temperature, high windspeed, atmospheric moisture, etc. Theyare exposed to these variations becausethey cannot move. Cells can beobserved in the leaf peel ofTradescantia, Elodea or Rhoeo. Youcan prepare a slide as in the caseof onion.

Paheli asks Boojho if he can alsoobserve animal cells.

Activity 8.4

Take a clean tooth pick, or amatchstick with the tip broken.Scrape inside of your cheek withouthurting it. Place it in a drop of wateron a glass slide. Add a drop of iodineand place a coverslip over it.Alternatively, add 1-2 drops ofmethylene blue solution. Observe itunder the microscope. You maynotice several cells in the scrapedmaterial (Fig. 8.6). You can identifythe cell membrane, the cytoplasmand nucleus. A cell wall is absentin animal cells.


Cytoplasm

It is the jelly-like substance presentbetween the cell membrane and thenucleus. Various other components, ororganelles, of cells are present in thecytoplasm. These are mitochondria, golgibodies, ribosomes, etc. You will learnabout them in later classes.

Nucleus

It is an important component of theliving cell. It is generally spherical andlocated in the centre of the cell. It canbe stained and seen easily with the helpof a microscope. Nucleus is separatedfrom the cytoplasm by a membranecalled the nuclear membrane. Thismembrane is also porous and allows themovement of materials between thecytoplasm and the inside of the nucleus.

With a microscope of highermagnification, we can see a smallerspherical body in the nucleus. It is calledthe nucleolus. In addition, nucleuscontains thread-like structures calledchromosomes. These carry genes andhelp in inheritance or transfer ofcharacters from the parents to theoffspring. The chromosomes can be seenonly when the cell divides.

Nucleus, in addition to its role ininheritance, acts as control centre of theactivities of the cell. The entire contentof a livinhg cell is known as protoplasm.It includes the cytoplasm and thenucleus. Protoplasm is called the livingsubstance of the cell.

Gene

Gene is a unit of inheritance in livingorganisms. It controls the transfer ofa hereditary characteristic fromparents to offspring. This means thatyour parents pass some of theircharacteristics on to you. If your fatherhas brown eyes, you may also havebrown eyes. If your mother has curlyhair, you might also end up havingcurly hair. However, the differentcombination of genes from parentsresult in different characteristics.

Paheli wants to know if thestructure of the nucleus isthe same in cells of plants,

animals and bacteria.

The nucleus of the bacterial cell isnot well organised like the cells ofmulticellular organisms. There is nonuclear membrane. The cells havingnuclear material without nuclearmembrane are termed prokaryoticcells. The organisms with these kindsof cells are called prokaryotes (pro :primitive; karyon : nucleus). Examplesare bacteria and blue green algae. Thecells, like onion cells and cheek cellshaving well organised nucleus witha nuclear membrane are designatedas eukaryotic cells. All organismsother than bacteria and blue greenalgae are called eukaryotes. (eu :true; karyon: nucleus).

cellmembrane

nucleus

cytoplasmnuclearmembrane

Fig. 8.6 : Human cheek cells

SCIENCE98

While observing the onion cellsunder the microscope, did you noticeany blank-looking structures in thecytoplasm? It is called vacuole. It couldbe single and big as in an onion cell.Cheek cells have smaller vacuoles. Largevacuoles are common in plant cells.Vacuoles in animal cells are muchsmaller.

You might have noticed several smallcoloured bodies in the cytoplasm of thecells of Tradescantia leaf. They arescattered in the cytoplasm of the leafcells. These are called plastids. They areof different colours. Some of themcontain green pigment calledchlorophyll. Green coloured plastids are

called chloroplasts. They provide greencolour to the leaves. You may recall thatchlorophyll in the chloroplasts of leaves,is essential for photosynthesis.

8.6 Comparison of Plant andAnimal Cells

If you recall Activities 8.3 and 8.4, youshould be able to compare plant andanimal cells. Observe the plant andanimal cell carefully in Fig. 8.7 (a), (b).

Let us tabulate the similarities anddisinguishing features of plant andanimal cells. Only a few features arementioned. You may add more inTable 8.1.

Table 8.1 : Comparison of Plant Cell and Animal Cell

Sl. No. Part Plant Cell Animal Cell

1. Cell membrane Present Present

2. Cell wall Present Absent

3. Nucleus

4. Nuclear membrane

5. Cytoplasm

6. Plastids

7. Vacuole

Fig. 8.7 : (a) Plant cell (b) Animal cell

(a)(b)

cell wall

chloroplast

cytoplasm

nucleus

vacuole

cell membrane

cell membrane


KEYWORDS

CELL

CELL MEMBRANE

CELL WALL

CHLOROPLAST

CHROMOSOME

CYTOPLASM

EUKARYOTES

GENE

MULTICELLULAR

NUCLEAR MEMBRANE

NUCLEOLUS

NUCLEUS

ORGAN

ORGANELLES

PLASMA MEMBRANE

PLASTID

PROKARYOTES

PSEUDOPODIA

TISSUE

UNICELLULAR

VACUOLE

WHITE BLOOD CELL

(WBC)


� All organisms are made of smaller parts called

organs.

� Organs are made of still smaller parts. The

smallest living part of an organism is a ‘cell’.

� Cells were first observed in cork by Robert

Hooke in 1665.

� Cells exhibit variety of shapes and sizes.

� Number of cells also varies from organism to

organism.

� Some cells are big enough to be seen with the

unaided eye. Hen’s egg is an example.

� Some organisms are single-celled, while others

contain large number of cells.

� The single cell of unicellular organisms

performs all the basic functions performed by

a variety of cells in multicellular organisms.

� The cell has three main parts, (i) the cell

membrane, (ii) cytoplasm which contains

smaller components called organelles, and (iii)

the nucleus.

� Nucleus is separated from cytoplasm by a

nuclear membrane.

� Cells without well organised nucleus, i.e.

lacking nuclear membrane, are called

prokaryotic cells.

� Plant cells differ from animal cells in having

an additional layer around the cell membrane

termed cell wall.

� Coloured bodies called plastids are found in

the plant cells only. Green plastids containing

chlorophyll are called chloroplasts.

� Plant cell has a big central vacuole unlike a

number of small vacuoles in animal cells.

SCIENCE100

Exercises

1. Indicate whether the following statements are True (T) or False (F).

(a) Unicellular organisms have one-celled body. (T/F)

(b) Muscle cells are branched. (T/F)

(c) The basic living unit of an organism is an organ. (T/F)

(d) Amoeba has irregular shape. (T/F)

2. Make a sketch of the human nerve cell. What function do nerve cellsperform?.

3. Write short notes on the following:

(a) Cytoplasm

(b) Nucleus of a cell

4. Which part of the cell contains organelles?

5. Make sketches of animal and plant cells. State three differences betweenthem.

6. State the difference between eukaryotes and prokaryotes.

7. Where are chromosomes found in a cell? State their function.

8. ‘Cells are the basic structural units of living organisms’. Explain.

9. Explain why chloroplasts are found only in plant cells?

10. Complete the crossword with the help of clues given below:

Across

1. This is necessary forphotosynthesis.

3. Term for component presentin the cytoplasm.

6. The living substance in thecell.

8. Units of inheritance presenton the chromosomes.

Down

1. Green plastids.

2. Formed by collection oftissues.

4. It separates the contents ofthe cell from the surroundingmedium.

5. Empty structure in the cytoplasm.

7. A group of cells.



1. Visit a laboratory for senior secondary students in your school orin a neighbouring school. Learn about the functioning of amicroscope in the laboratory. Also observe how a slide is observedunder the microscope.

2. Talk to the senior biology teacher in your school or a neighbouringschool. Find out if there are diseases which are passed on fromparents to the offspring. Find out how these are carried and also ifthese diseases can be treated. For this you can also visit a doctor.

3. Visit an agriculture extension centre in your area. Find out aboutgenetically modified (GM) crops. Prepare a short speech for yourclass on this topic. You may visit www.usc.ernet.in/currsci/sep252001/655.pdf

4. Find out about Bt cotton from an agriculture expert (or fromenvfor.nic.in/divisions/csnrv/btcotton/bgnote.pdf). Prepare a shortnote on its advantages/disadvantages.

For more on cell, visit :

www.enchatedbearning.com/subjects/plants/cell/

Did You Know?The cells in the outermost layer of our skin are dead. An averageadult carries around about 2 kg of dead skin. Billions of tinyfragments of the skin are lost every day. Every time you run yourfinger on a dusty table, you shed a lot of old skin.

REPRODUCTION IN ANIMALSREPRODUCTION IN ANIMALS

Do you recall the processes ofdigestion, circulation andrespiration which you have

studied in your previous classes? Theseprocesses are essential for the survivalof every individual. You have also learntabout the process of reproduction inplants. Reproduction is essential for thecontinuation of a species. Imagine whatwould have happened if organisms hadnot reproduced. You will realise thatreproduction is very important as itensures the continuation of similarkinds of individuals, generation aftergeneration.

You have already learnt in yourprevious class about reproduction inplants. In this chapter, we shall learnhow reproduction takes place inanimals.

9.1 Modes of Reproduction

Have you seen the young ones ofdifferent animals? Try to name some ofthe young ones by completing Table 9.1as shown in examples at S. No. 1and 5.

You must have also seen the youngones of various animals being born.Can you tell how chicks andcaterpillars are born? How are kittensand puppies born? Do you think thatthese young ones looked the samebefore they were born as they do now?Let us find out.

Just as in plants, there are two modesby which animals reproduce. These are:(i) Sexual reproduction, and (ii) Asexualreproduction.

9.2 Sexual Reproduction

Try to recall reproduction in plantswhich you studied in Class VII. You willremember that plants that reproducesexually have male and femalereproductive parts. Can you name theseparts? In animals also, males andfemales have different reproductive partsor organs. Like plants, the reproductiveparts in animals also produce gametesthat fuse to form a zygote. It is the zygotewhich develops into a new individual.This type of reproduction beginningfrom the fusion of male and femalegametes is called sexual reproduction.Let us find out the reproductive parts

Table 9.1

S. No. Animal Young one

1. Human Baby

2. Cat

3. Dog

4. Butterfly

5. Hen Chick

6. Cow

7. Frog

in humans and study the process ofreproduction in them.

Male Reproductive Organs

The male reproductive organs include apair of testes (singular, testis), two spermducts and a penis (Fig. 9.1). The testesproduce the male gametes calledsperms. Millions of sperms are producedby the testes. Look at Fig. 9.2 whichshows the picture of a sperm. Thoughsperms are very small in size, each hasa head, a middle piece and a tail. Doesit appear to be a single cell? Indeed, each

sperm is a single cell with all the usualcell components.

Fig. 9.1: Male reproductiveorgans in humans

What purpose does thetail in a sperm serve?

Fig. 9.3 : Female reproductive organs inhumans

Female Reproductive Organs

The female reproductive organs are a pairof ovaries, oviducts (fallopian tubes) andthe uterus (Fig. 9.3). Ovary produces

Oviduct

OvaryUterus

Head

Middle piece

Tail

Fig. 9.2 : Human sperm


Sperm

Testis

Penis

SCIENCE104

Boojho recalls that the sizeof eggs in animals varies.The egg may be very smallas in humans, much largeras in hens. Ostrich egg is

the largest!

Fertilization

The first step in the process ofreproduction is the fusion of a spermand an ovum. When sperms come incontact with an egg, one of the spermsmay fuse with the egg. Such fusion ofthe egg and the sperm is calledfertilization (Fig. 9.5). Duringfertilization, the nuclei of the sperm andthe egg fuse to form a single nucleus.This results in the formation of afertilized egg or zygote (Fig. 9.6). Did

Fig. 9.5 : Fertilization

Fig. 9.4 : Human Ovum

Nucleus

Sperms

Ovum

Fusingnuclei

you know that the zygote is thebeginning of a new individual?

The process of fertilization is themeeting of an egg cell from the motherand a sperm cell from the father. So,the new individual inherits somecharacteristics from the mother andsome from the father. Look at yourbrother or sister. See if you can recognisesome characters in them similar to thoseof your mother or your father.

Fertilization which takes place insidethe female body is called internalfertilization. Internal fertilizationoccurs in many animals includinghumans, cows, dogs and hens.

Fig. 9.6 : Zygote

female gametes called ova (eggs)(Fig. 9.4). In human beings, a singlematured egg is released into the oviductby one of the ovaries every month.Uterus is the part where development ofthe baby takes place. Like the sperm,an egg is also a single cell.


Why do fish andfrogs lay eggs in

hundreds whereas ahen lays only one

egg at a time?

Have you heard oftest tube babies?

Boojho and Paheli’s teacher once toldthem in the class that in some womenoviducts are blocked. These womenare unable to bear babies becausesperms cannot reach the egg forfertilization. In such cases, doctorscollect freshly released egg andsperms and keep them together for afew hours for IVF or in vitrofertilization (fertilization outside thebody). In case fertilization occurs, thezygote is allowed to develop for abouta week and then it is placed in themother’s uterus. Completedevelopment takes place in the uterusand the baby is born like any otherbaby. Babies born through thistechnique are called test-tubebabies. This term is actuallymisleading because babies cannotgrow in test tubes.

Fig. 9.7 : Eggs of frog

female lays hundreds of eggs. Unlikehen’s egg, frog’s egg is not covered by ashell and it is comparatively verydelicate. A layer of jelly holds the eggstogether and provides protection to theeggs (Fig. 9.7).

You will be surprised to know thatin many animals fertilization takesplace outside the body of the female.In these animals, fertilization takesplace in water. Let us find out howthis happens.

Activity 9.1

Visit some ponds or slow-flowingstreams during spring or rainyseason. Look out for clusters offrog’s eggs floating in water.Write down the colour and size ofthe eggs.

During spring or rainy season, frogsand toads move to ponds and slow-flowing streams. When the male andfemale come together in water, the

As the eggs are laid, the maledeposits sperms over them. Eachsperm swims randomly in water withthe help of its long tail. The spermscome in contact with the eggs. Thisresults in fertilization. This type offertilization in which the fusion of amale and a female gamete takes placeoutside the body of the female is calledexternal fertilization. It is verycommon in aquatic animals such asfish, starfish, etc.

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Development of Embryo

Fertilization results in the formation ofzygote which begins to develop into anembryo [Fig. 9.8(a)]. The zygote dividesrepeatedly to give rise to a ball of cells[Fig. 9.8(b)]. The cells then begin to formgroups that develop into different tissuesand organs of the body. This developingstructure is termed an embryo. Theembryo gets embedded in the wall of theuterus for further development[Fig. 9.8(c)].

The embryo continues to develop inthe uterus. It gradually develops the

Fig. 9.8 : (a) Zygote formation and development of an embryo from the zygote; (b) Ball of cells(enlarged); (c) Embedding of the embryo in the uterus (enlarged)

How could a single cellbecome such a big

individual?

(b)

(a)

(c)

Uterus

Embeddingembryo

Ovary

Ovulation

Zygote

Uteruswall

Developingembryo

Though these animals layhundreds of eggs and release

millions of sperms, all the eggsdo not get fertilized and develop

into new individuals.This is because the eggs andsperms get exposed to watermovement, wind and rainfall.

Also, there are other animals inthe pond which may feedon eggs. Thus, production

of large number of eggs andsperms is necessary to

ensure fertilizationof at least

a few of them.


body parts such as hands, legs, head,eyes, ears, etc. The stage of the embryoin which all the body parts can beidentified is called a foetus (Fig. 9.9).When the development of the foetus iscomplete, the mother gives birth tothe baby.

period? After the chick is completelydeveloped it bursts open the egg shell.

In animals which undergo externalfertilization, development of the embryotakes place outside the female body. Theembryos continue to grow within theiregg coverings. After the embryos develop,the eggs hatch. You must have seennumerous tadpoles swimming in pondsand streams.

Viviparous and Oviparous Animals

We have learnt that some animals givebirth to young ones while some animalslay eggs which later develop into youngones. The animals which give birth toyoung ones are called viviparousanimals. Those animals which lay eggsare called oviparous animals. Thefollowing activity will help youunderstand better and differentiatebetween viviparous and oviparousanimals.

Activity 9.2

Try to collect eggs of the followingorganisms – frog, lizard, butterflyor moth, hen and crow or any otherbird. Were you able to collect eggsof all of them? Make drawings of theeggs that you have collected.

The eggs of a few animals are easyto collect because their mothers laythem outside their bodies. Theseanimals from which you have collectedthe eggs are examples of oviparousanimals. But you would not be able tocollect the eggs of a dog, cow or cat.This is because they do not lay eggs.The mother gives birth to the youngones. These are examples of viviparousanimals.

Fig. 9.9 : Foetus in the uterus

Uterus

Internal fertilization takes place inhens also. But, do hens give birth tobabies like human beings and cows? Youknow that they do not. Then, how arechicks born? Let us find out.

Soon after fertilization, the zygotedivides repeatedly and travels down theoviduct. As it travels down, manyprotective layers are formed around it.The hard shell that you see in a hen’segg is one such protective layer.

After the hard shell is formed aroundthe developing embryo, the hen finallylays the egg. The embryo takes about 3weeks to develop into a chick. You musthave seen the hen sitting on the eggs toprovide sufficient warmth. Did you knowthat development of the chick takesplace inside the egg shell during this

SCIENCE108

Can you now give some moreexamples of viviparous and oviparousanimals?

Young Ones to Adults

The new individuals which are born orhatched from the eggs continue to growtill they become adults. In someanimals, the young ones may look verydifferent from the adults. Recall the lifecycle of the silkworm (egg → larva orcaterpillar → pupa → adult) you studiedin Class VII. Frog is another suchexample (Fig. 9.10).

Observe the different stages of frogstarting from the egg to the adult stage.We find that there are three distinctstages, that is, egg → tadpole (larva) →adult. Don’t the tadpoles look sodifferent from the adults? Can youimagine that these tadpoles would someday become frogs? Similarly, thecaterpillar or the pupa of silkworm looksvery different from the adult moth. The

features that are present in the adult arenot found in these young ones. Then whathappens to the tadpoles or caterpillarsthereafter?

You must have seen a beautiful mothemerging out of the cocoon. In the case oftadpoles, they transform into adultscapable of jumping and swimming. Thetransformation of the larva into an adultthrough drastic changes is calledmetamorphosis. What about the changesthat we observe in our body as we grow?Do you think we too undergometamorphosis? In human beings, bodyparts similar to those present in the adultsare present from the time of the birth.

9.3 Asexual Reproduction

So far, we have learnt about reproductionin some familiar animals. But whatabout very small animals like hydra andmicroscopic organisms like amoeba? Doyou know how they reproduce? Let usfind out.

(a) Eggs

(b) Early tadpole

(c) Late tadpole

(d) Adult frog

Fig. 9.10 : Life cycle of frog


Activity 9.3

Get permanent slides ofhydra, Observe them usinghand lens or a microscope.Look out for any bulgesfrom the parent body.Count the number ofbulges that you see indifferent slides. Also, notethe size of the bulges. Drawthe diagram of hydra, asyou see it. Compare it withthe Fig. 9.11.

Fig. 9.11 : Budding in Hydra

Fig. 9.12 : Binary fission in Amoeba

a

b

c

d

Dividing nucleus

Daughter amoebae

In each hydra, there may be one ormore bulges. These bulges are thedeveloping new individuals and they arecalled buds. Recall the presence of budsin yeast. In hydra too the new individualsdevelop as outgrowths from a singleparent. This type of reproduction inwhich only a single parent is involved iscalled asexual reproduction. Since newindividuals develop from the buds inhydra, this type of asexual reproductionis called budding.

Another method of asexualreproduction is observed in themicroscopic organism, amoeba. Let ussee how this happens.

You have already learnt about thestructure of amoeba. You will recall thatamoeba is a single-celled organisms [Fig.9.12(a)]. It begins the process ofreproduction by the division of itsnucleus into two nuclei [Fig. 9.12(b)].This is followed by division of its bodyinto two, each part receiving a nucleus[Fig. 9.12(c)]. Finally, two amoebae areproduced from one parent amoeba [Fig.9.12(d)]. This type of asexual

SCIENCE110

Story of Dolly, the Clone

Cloning is the production of an exact copy of a cell, any other living part, or acomplete organism. Cloning of an animal was successfully performed for the firsttime by Ian Wilmut and his colleagues at the Roslin Institute in Edinburgh,Scotland. They cloned successfully a sheep named Dolly [Fig. 9.13 (c)]. Dolly wasborn on 5th July 1996 and was the first mammal to be cloned.

(a) Finn Dorsett sheep (b) Scottish blackface ewe (c) Dolly

Fig. 9.13

During the process of cloning Dolly, a cell was collected from the mammary glandof a female Finn Dorsett sheep [Fig. 9.13 (a)]. Simultaneously, an egg was obtainedfrom a Scottish blackface ewe [Fig. 9.13 (b)]. The nucleus was removed from theegg. Then, the nucleus of the mammary gland cell from the Finn Dorsett sheepwas inserted into the egg of the Scottish blackface ewe whose nucleus had beenremoved. The egg thus produced was implanted into the Scottish blackface ewe.Development of this egg followed normally and finally Dolly was born. ThoughDolly was given birth by the Scottish blackface ewe, it was found to be absolutelyidentical to the Finn Dorsett sheep from which the nucleus was taken. Since thenucleus from the egg of the Scottish blackface ewe was removed, Dolly did notshow any character of the Scottish blackface ewe. Dolly was a healthy clone ofthe Finn Dorsett sheep and produced several offspring of her own through normalsexual means. Unfortunately, Dolly died on 14th February 2003 due to a certainlung disease.

Since Dolly, several attempts have been made to produce cloned mammals.However, many die before birth or die soon after birth. The cloned animals aremany-a-time found to be born with severe abnormalities.

reproduction in which an animalreproduces by dividing into twoindividuals is called binary fission.Apart from budding and binary fission,

there are other methods by which asingle parent reproduces the youngones. You will study about these in yourhigher classes.


KEYWORDS

ASEXUAL

REPRODUCTION

BINARY FISSION

BUDDING

EGGS

EMBRYO

EXTERNAL

FERTILIZATION

FERTILIZATION

FOETUS

INTERNAL

FERTILIZATION

METAMORPHOSIS

OVIPAROUS ANIMALS

SEXUAL

REPRODUCTION

SPERMS

VIVIPAROUS ANIMALS

ZYGOTE


� There are two modes by which animalsreproduce. These are: (i) Sexual reproduction,and (ii) Asexual reproduction.

� Reproduction resulting from the fusion of maleand female gametes is called sexualreproduction.

� The reproductive organs in the female includeovaries, oviducts and uterus.

� The reproductive organs in male includetestes, sperm ducts and penis.

� The ovary produces female gametes called ovaand the testes produce male gametes calledsperms.

� The fusion of ovum and sperm is calledfertilization. The fertilized egg is called a zygote.

� Fertilization that takes place inside the femalebody is called internal fertilization. This isobserved in human beings and other animalssuch as hens, cows and dogs.

� Fertilization that takes place outside the femalebody is called external fertilization. This isobserved in frogs, fish, starfish, etc.

� The zygote divides repeatedly to give rise to anembryo.

� The embryo gets embedded in the wall of theuterus for further development.

� The stage of the embryo in which all the bodyparts are identifiable is called foetus.

� Animals such as human beings, cows anddogs which give birth to young ones are calledviviparous animals.

� Animals such as hen, frog, lizard and butterflywhich lay eggs are called oviparous animals.

� The transformation of the larva into adultthrough drastic changes is calledmetamorphosis.

� The type of reproduction in which only a singleparent is involved is called asexualreproduction.

� In hydra, new individuals develop from buds.This method of asexual reproduction is calledbudding.

� Amoeba reproduces by dividing itself into two.This type of asexual reproduction is called

binary fission.

SCIENCE112

Exercises

1. Explain the importance of reproduction in organisms.

2. Describe the the process of fertilization in human beings.

3. Choose the most appropriate answer:

(a) Internal fertilization occurs

(i) in female body.

(ii) outside female body.

(iii) in male body.

(iv) outside male body.

(b) A tadpole develops into an adult frog by the process of

(i) fertilization. (ii) metamorphosis. (iii) embedding. (iv) budding.

(c) The number of nuclei present in a zygote is

(i) none. (ii) one. (iii) two. (iv) four.

4. Indicate whether the following statements are True (T) or False (F):

(a) Oviparous animals give birth to young ones. ( )

(b) Each sperm is a single cell. ( )

(c) External fertilization takes place in frog. ( )

(d) A new human individual develops from a cell called gamete. ( )

(e) Egg laid after fertilization is made up of a single cell. ( )

(f) Amoeba reproduces by budding. ( )

(g) Fertilization is necessary even in asexual reproduction. ( )

(h) Binary fission is a method of asexual reproduction. ( )

(i) A zygote is formed as a result of fertilization. ( )

(j) An embryo is made up of a single cell. ( )

5. Give two difference between a zygote and a foetus.

6. Define asexual reproduction. Describe two methods of asexual reproductionin animals.

7. In which female reproductive organ does the embryo get embedded?

8. What is metamorphosis? Give examples.

9. Differentiate between internal fertilization and external fertilization.


E X

E R

C I

S E

S10. Complete the cross-word puzzle using the hints given below

Across

1. The process of the fusion of the gametes.

6. The type of fertilization in hen.

7. The term used for bulges observed on the sides of the body of Hydra.

8. Eggs are produced here.

Down

2. Sperms are produced in these male reproductive organs.

3. Another term for the fertilized egg.

4. These animals lay eggs.

5. A type of fission in amoeba.


1. Visit a poultry farm. Talk to the manager of the farm and try to findout the answers to the following:

(a) What are layers and broilers in a poultry farm?

(b) Do hens lay unfertilized eggs?

(c) How can you obtain fertilized and unfertilized eggs?

(d) Are the eggs that we get in the stores fertilized or unfertilized?

(e) Can you consume fertilized eggs?

(f) Is there any difference in the nutritional value of the fertilizedand unfertilized eggs?

SCIENCE114

2. Observe live hydra yourself and learn how they reproduce by doingthe following activity:

During the summer months collect water weeds from ponds or ditchesalong with the pond water and put them in a glass jar. After a day orso you may see several hydra clinging to the sides of the jar.

Hydra is transparent, jelly-like and with tentacles. It clings to thejar with the base of its body. If the jar is shaken, the hydra willcontract instantly into a small blob, at the same time drawing itstentacles in.

Now take out few hydras from the jar and put them on a watchglass. Using a hand lens or a binocular or dissection microscope,observe the changes that are taking place in their body. Note downyour observations.

3. The eggs we get from the market are generally the unfertilized ones.In case you wish to observe a developing chick embryo, get a fertilizedegg from the poultry or hatchery which has been incubated for 36hours or more. You may then be able to see a white disc-like structureon the yolk. This is the developing embryo. Sometimes if the heartand blood vessels have developed you may even see a red spot.

4. Talk to a doctor. Find out how twinning occurs. Look for any twinsin your neighbourhood, or among your friends. Find out if the twinsare identical or non-identical. Also find out why identical twins arealways of the same sex? If you know of any story about twins, writeit in your own words. You could visit the following website forinformation on twins: www.keepkidshealthy.com/twins/expecting_twins.html.

For more information on animal reproduction, you can visit :www.saburchill.com/chapters/chap0031.htmlhealthhowstuffworks.com/human-reproduction.htmwww.teenshealth.org/teen/sexual_health

Did You Know?

An interesting organisation is observed in a honey bee hive, a colony of severalthousand bees. Only one bee in the colony lays eggs. This bee is called thequeen bee. All other female bees are worker bees. Their main job is to build thehive, look after the young and feed the queen bee adequately to keep herhealthy so that she could lay eggs. A queen bee lays thousands of eggs. Thefertilized eggs hatch into females, while the unfertilized eggs give rise to males,called drones. It is the job of the worker bees to maintain the temperature ofthe hive around 35ºC to incubate the eggs.

REACHING THE AGE OF ADOLESCENCEREACHING THE AGE OF ADOLESCENCE

In the previous chapter, you havelearnt how animals reproduce. It isonly after ‘growing up’ to a certain

age that human beings and many otheranimals can reproduce. Why canhumans reproduce only after a certainage?

In this chapter, you will learn aboutchanges that take place in the humanbody after which a person becomescapable of reproduction.

In Chapter 9, you have learnt abouthuman reproductive organs. Here, weshall discuss the role that hormones playin bringing about changes that make achild grow into an adult.

10.1 Adolescence and Puberty

Boojho was celebrating his 12thbirthday. After his friends left, Boojhoand Paheli began chatting with theirparents. Paheli studies in an all-girlsschool. She started laughing. Sheremarked that many of Boojho’s schoolfriends, whom she met after a year, hadsuddenly shot up in height. Some ofthem were looking very funny with ahairy line above their lips. Her motherexplained that the boys had grown up.

Growth begins from the day one isborn. But upon crossing the age of 10or 11, there is a sudden spurt in growthwhich becomes noticeable. The changestaking place in the body are part ofgrowing up. They indicate that you are

I wonder how long thisperiod marked by changes

in the body will last!

no longer a child but are on the way tobecoming an adult.

It is a strange period of lifewhen you are neither a child noran adult. I wonder whether thisperiod between childhood andadulthood had a special name!

Growing up is a natural process. Theperiod of life, when the body undergoeschanges, leading to reproductivematurity, is called adolescence.Adolescence begins around the age of11 and lasts upto 18 or 19 years of age.Since this period covers the ‘teens’ (13to 18 or 19 years of age), adolescentsare also called ‘teenagers’. In girls,adolescence may begin a year or twoearlier than in boys. Also, the period ofadolescence varies from person to person.

The human body undergoes severalchanges during adolescence. Thesechanges mark the onset of puberty. Themost important change which markspuberty is that boys and girls becomecapable of reproduction. Puberty endswhen an adolescent reaches reproductivematurity.

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% of full heightBoys Girls

8 72% 77%

9 75% 81%

10 78% 84%

11 81% 88%

12 84% 91%

13 88% 95%

14 92% 98%

15 95% 99%

16 98% 99.5%

17 99% 100%

18 100% 100%

Age inYears

Paheli and Boojhorealised that sudden

increase in height andhairy line above the lips

in boys were signs ofadolescence. They

wanted to know moreabout other changes at

puberty.

10.2 Changes at Puberty

Increase in Height

The most conspicuous change duringpuberty is the sudden increase inheight. At this time the long bones, thatis, the bones of the arms and the legselongate and make a person tall.

Activity 10.1

The following chart gives theaverage rate of growth in height ofboys and girls with age. The figuresin columns 2 and 3, give thepercentage of the height a personhas reached at the age given incolumn 1. For example, by the age11, a boy has reached 81% of hisprobable full height, while a girlhas reached 88% of her fullheight. These figures are onlyrepresentative and there may beindividual variations.

Use the Table for your friendsand work out how tall they are likelyto be. Find out who is likely to be

Calculation for full height (cm)

( )100

Present height cm

% of full height at this age×

(as given in the chart)

Example:

A boy is 9 years old and 120 cmtall. At the end of the growth periodhe is likely to be

×120100

75 cm = 160 cm tall

the tallest and who might be theshortest in your class.

REACHING THE AGE OF ADOLESCENCE 115

There is no need for Paheli to worry.All parts of the body do not grow at thesame rate. Sometimes the arms and legsor hands and feet of adolescents lookoversized and out of proportion with thebody. But soon the other parts catchup and result in a proportionate body.

You must have noticed that heightof an individual is more or less similarto that of some family member. This isbecause height depends on the genesinherited from parents. It is, however,very important to eat the right kind offood during these growing years. Thishelps the bones, muscles and otherparts of the body get adequatenourishment for growth. You will findnutritional needs of adolescentsdiscussed later in the lesson.

Change in Body Shape

Have you noticed that boys in your classhave broader shoulders and wider cheststhan boys in junior classes? This isbecause they have entered the age ofpuberty when shoulders generallybroaden as a result of growth. In girls,the region below the waist becomeswider.

In boys, the muscles of the body growmore prominently than in the girls.Thus, changes occurring in adolescentboys and girls are different.

Voice Change

Did you notice that sometimes the voiceof some of the boys in your class cracks?At puberty, the voice box or the larynxbegins to grow. Boys develop larger voiceboxes. The growing voice box in boys canbe seen as a protruding part of the throat

Activity 10.2

Use the data given in Activity 10.1to draw a graph. Take age on the X-axis and per cent growth in heighton the Y-axis. Highlight the pointrepresenting your age on the graph.Find out the percentage of heightyou have already reached. Calculatethe height you might eventuallyreach. Tally your graph with the onegiven here (Fig. 10.1).

Fig. 10.1 :Fig. 10.1 :Fig. 10.1 :Fig. 10.1 :Fig. 10.1 : Graph showing percentage of heightwith age

I am worried. Though Ihave become taller, my

face looks much smallercompared to my body.

Initially, girls grow faster than boysbut by about 18 years of age, both reachtheir maximum height. The rate ofgrowth in height varies in differentindividuals. Some may grow suddenlyat puberty and then slow down, whileothers may grow gradually.

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and pimples on the face at this timebecause of the increased activity of theseglands in the skin.

Development of Sex Organs

Look up Fig. 9.1 and 9.3 of theprevious lesson which shows sexorgans of humans. At puberty, malesex organs like the testes and penisdevelop completely. The testes alsobegin to produce sperms. In girls, theovaries enlarge and eggs begin tomature. Also ovaries start releasingmature eggs.

Reaching Mental, Intellectual andEmotional Maturity

Adolescence is also a period of changein a person’s way of thinking.Adolescents are more independent thanbefore and are also self conscious.Intellectual development takes place andthey tend to spend considerable timethinking. In fact, it is often the time inone’s life when the brain has thegreatest capacity for learning.Sometimes, however, an adolescent mayfeel insecure while trying to adjust tothe changes in the body and mind. Butas adolescent learners, you should knowthat there is no reason to feel insecure.These changes are a natural part ofgrowing up.

10.3 Secondary SexualCharacters

You have learnt in Chapter 9, thattestes and ovaries are the reproductiveorgans. They produce the gametes,that is, sperms and ova. In girls,breasts begin to develop at puberty

Many of my classmateshave a hoarse voice. Now I

know why?

Fig. 10.2 : Fig. 10.2 : Fig. 10.2 : Fig. 10.2 : Fig. 10.2 : Adam’s apple in a grown up boy

called Adam’s apple (Fig. 10.2). In girls,the larynx is hardly visible from theoutside because of its small size.Generally, girls have a high pitched voice,whereas boys have a deep voice. Inadolescent boys, sometimes, the musclesof the growing voice box go out of controland the voice becomes hoarse. This statemay remain for a few days or weeks afterwhich the voice becomes normal.

A few glands such as sweat glands,oil glands and salivary glands releasetheir secretions through ducts.Endocrine glands release hormonesdirectly into the bloodstream. So, theyare also termed ductless glands.

Increased Activity of Sweat andSebaceous Glands

During puberty the secretion of sweatglands and sebaceous glands (oil glands)increases. Many young people get acne

Adam’sapple


and boys begin to grow facial hair, thatis, moustaches and beard. As thesefeatures help to distinguish the malefrom the female they are calledsecondary sexual characters. Boysalso develop hair on their chest. Inboth, boys and girls, hair grows underthe arms and in the region above thethighs or the pubic region.

Paheli and Boojho have nowunderstood that puberty marks thebeginning of the reproductive periodwhen one becomes capable ofreproduction. But they want to knowif reproductive life, once begun,continues, or it ends some time.

The testes and ovaries secrete sexhormones. You have just learnt thatthese hormones are responsible for themale and female secondary sexualcharacters. Further, the sex hormonesare under the control of hormones fromthe pituitary gland (Fig. 10.3). Thepituitary secretes many hormones, oneof which makes ova mature in theovaries and sperms form in the testes.

Both Boojho and Paheli wish to knowwhat initiates changes at puberty.

Fig. 10.3 :Fig. 10.3 :Fig. 10.3 :Fig. 10.3 :Fig. 10.3 : The onset of puberty is controlled byhormones

The changes which occur atadolescence are controlled byhormones. Hormones are chemicalsubstances. These are secretions fromendocrine glands, or endocrine system.The male hormone or testosteronebegins to be released by the testes atthe onset of puberty. This causeschanges in boys about which you havejust learnt, for example, the growth offacial hair. Once puberty is reached ingirls, ovaries begin to produce the femalehormone or estrogen which makes thebreasts develop. Milk secreting glandsor mammary glands develop inside thebreasts. The production of thesehormones is under the control ofanother hormone secreted from anendocrine gland called pituitary gland.

10.4 Role of Hormones inInitiating ReproductiveFunction

Endocrine glands release hormonesinto the bloodstream to reach aparticular body part called target site.The target site responds to thehormone. There are many endocrineglands or ductless glands in the body.

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10.5 Reproductive Phase ofLife in Humans

Adolescents become capable ofreproduction when their testes andovaries begin to produce gametes. Thecapacity for maturation and productionof gametes lasts for a much longer timein males than in females.

In females, the reproductive phase oflife begins at puberty (10 to 12 years ofage) and generally lasts till the age ofapproximately 45 to 50 years. The ovabegin to mature with the onset ofpuberty. One ovum matures and isreleased by one of the ovaries once inabout 28 to 30 days. During thisperiod, the wall of the uterus becomesthick so as to receive the egg, in caseit is fertilised and begins to develop.This results in pregnancy. Iffertilisation does not occur, thereleased egg, and the thickened liningof the uterus along with its bloodvessels are shed off. This causesbleeding in women which is calledmenstruation. Menstruation occursonce in about 28 to 30 days. The firstmenstrual flow begins at puberty andis termed menarche. At 45 to 50 yearsof age, the menstrual cycle stops.Stoppage of menstruation is termedmenopause. Initially, menstrual cyclemay be irregular. It take some time tobecome regular.

Menstrual cycle is controlled byhormones. The cycle includes thematuration of the egg, its release,thickening of uterine wall and itsbreakdown if pregnancy does notoccur. In case the egg is fertilised itbegins to divide and then getsembedded in the uterus for furtherdevelopment as you have learnt inChapter 9 (Fig. 9.8).

10.6 How is the Sex of theBaby Determined?

I heard my mother and myaunt talking about my cousinwho is going to have a baby.

They were discussing whethershe would give birth to a boy

or a girl. I wonder what makesthe fertilised egg developeither into a boy or a girl!

Paheli says that thereproductive life of a

woman lasts frommenarche to menopause.

Is she right?

Boy or Girl?

Inside the fertilised egg or zygote is theinstruction for determining the sex ofthe baby. This instruction is present inthe thread-like structures, calledchromosomes in the fertilised egg.Recall from Chapter 8, thatchromosomes are present inside thenucleus of every cell. All human beingshave 23 pairs of chromosomes in thenuclei of their cells. Two chromosomesout of these are the sex chromosomes,named X and Y. A female has two Xchromosomes, while a male has one Xand one Y chromosome. The gametes(egg and sperm) have only one set ofchromosomes. The unfertilised eggalways has one X chromosome. But


sperms are of two kinds. One kind hasan X chromosome, and the other kindhas a Y chromosome.

See Fig. 10.4. When a spermcontaining X chromosome fertilises theegg, the zygote would have two Xchromosomes and develop into a femalechild. If the sperm contributes a Ychromosome to the egg (ovum) atfertilisation, the zygote would developinto a male child.

and ovaries to produce their hormones.You have already learnt that thepituitary gland is an endocrine gland.It is attached to the brain.

Apart from the pituitary, the testesand the ovaries, there are otherendocrine glands in the body such asthyroid, pancreas and adrenals(Fig. 10.5).

Fig. 10.4 : Fig. 10.4 : Fig. 10.4 : Fig. 10.4 : Fig. 10.4 : Sex determination in humans

boy girl

sperm

eggs

Now you know that the sexchromosomes of the father determine thesex of an unborn baby. The belief thatthe mother is responsible for the sex ofher baby is completely wrong and toblame her for this is totally unjustified.

10.7 Hormones other thanSex Hormones

Look at Fig. 10.3 again. The hormonessecreted by the pituitary stimulate testes

Fig. 10.5 Fig. 10.5 Fig. 10.5 Fig. 10.5 Fig. 10.5 : Position of endocrine glands in thehuman body

pituitarygland

thyroid gland

adrenalgland

pancreas

testis

position ofthe ovary inthe female

Boojho and Paheli had once visitedtheir aunt who was a doctor andremembered that a boy named Kaka hada very big and bulging throat. Their aunthad told them that Kaka was sufferingfrom ‘goitre’, a disease of the thyroidgland. Kaka’s thyroid gland was notproducing the hormone thyroxine.

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Their aunt also told them that theiruncle was suffering from ‘diabetes’because his pancreas was not producingthe hormone insulin in sufficientquantities. Boojho and Paheli thenasked their aunt about the adrenalglands, which are also shown in thechart hung on the wall of her clinic. Theaunt told them that adrenal glandssecrete hormones which maintain thecorrect salt balance in the blood.Adrenals also produce the hormoneadrenalin. It helps the body to adjustto stress when one is very angry,embarrassed or worried.

Thyroid and adrenals secrete theirhormones when they receive orders fromthe pituitary through its hormones.Pituitary also secretes growth hormonewhich is necessary for the normalgrowth of a person.

This change from larva to adult iscalled metamorphosis (Fig. 9.10).Metamorphosis in insects is controlledby insect hormones. In a frog, it iscontrolled by thyroxine, the hormoneproduced by thyroid. Thyroxineproduction requires the presence ofiodine in water. If the water in whichthe tadpoles are growing does notcontain sufficient iodine, the tadpolescannot become adults.

Are there hormones inother animals also? Havethey any role to play in

reproduction?

If people do not haveenough iodine in their diet,will they get goitre caused

by lack of thyroxine?

10.8 Role of Hormones incompleting the LifeHistory of Insects andFrogs

You have already learnt about the lifehistory of the silk moth and the frog.The caterpillar has to pass throughvarious stages to become an adult moth.Recall from Class VII the stages of thelife history of the silk moth. Similarly,the tadpole passes through certainstages to become a frog (Chapter 9).

Activity 10.3

Collect information from magazinesor from doctors and prepare a noteon the importance of consumingiodised salt. You can also look forthis information on the internet.

10.9 Reproductive Health

The physical and mental well being ofan individual is regarded as anindividual’s health. To keep the bodyhealthy, every human being, at any age,needs to have a balanced diet. Theperson must also observe personalhygiene and undertake adequatephysical exercise.

During adolescence, however, thesebecome even more essential as the bodyis growing.

Nutritional Needs of the Adolescents

Adolescence is a stage of rapid growthand development. Hence the diet for an


adolescent has to be carefully planned.You have already learnt what a balanceddiet is. Recall that a balanced diet meansthat the meals include proteins,carbohydrates, fats and vitamins inrequisite proportions. Our Indian mealof roti/rice, dal (pulses) and vegetablesis a balanced meal. Milk is a balancedfood in itself. Fruits also providenourishment. For infants, mother’s milkprovides all the nourishment that theyneed.

Iron builds blood and iron-rich foodsuch as leafy vegetables, jaggery, meat,citrus, Indian gooseberry (amla) are goodfor adolescents.

Check items for lunch and dinner inyour meal. Is the meal balanced andnutritious? Does it include cerealswhich give energy and milk, meat, nutsand pulses which provide proteins forgrowth? Also, does it include fats andsugar that give energy? What aboutfruits and vegetables which areprotective foods? Chips and packed ortinned snacks, though very tastyshould never replace regular meals asthey do not have adequate nutritionalvalue.

Activity 10.4

Make a group with your friends.Write down the items of food in yourbreakfast, lunch and dinner youhad on the previous day. Identifythe items responsible for propergrowth. Also identified the junkfood that you consumed theprevious day.

Personal Hygiene

Everyone should have a bath at leastonce everyday. It is more necessary forteenagers because the increased activityof sweat glands sometimes makes thebody smelly. All parts of the body shouldbe washed and cleaned everyday. If

meat

vegetables

fruits

milk andeggs

grains

Fig. 10.6 Fig. 10.6 Fig. 10.6 Fig. 10.6 Fig. 10.6 : Nutritious items of food

Activity 10.5

Get ideas from the pictures given inFig.10.6. Prepare charts or postersand paste them in the class so thatyou are aware of the diet foradolescents. You may use yourcreative ideas and present it like anadvertisement. You may evenorganise a competition on this topic.

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cleanliness is not maintained there arechances of catching bacterial infection.Girls should take special care ofcleanliness during the time of menstrualflow. They should keep track of theirmenstrual cycle and be prepared for theonset of menstruation.

Physical exercise

Walking and playing in fresh air keepsthe body fit and healthy. All young boysand girls should take walks, exerciseand play outdoor games.

Activity 10.6

Collect data on the number ofchildren in your class who exerciseregularly and who do not exerciseregularly. Did you notice anydifference in their fitness andhealth? Prepare a report on thebenefits of regular exercise.

Say “NO” to Drugs

Adolescence is a period of muchactivity in the body and mind whichis a normal part of growing up. So donot feel confused or insecure. Ifanybody suggests that you will getrelief if you take some drugs, just say‘No’ unless prescribed by the doctor.Drugs are addictive. If you take themonce, you feel like taking them againand again. They harm the body in thelong run. They ruin health andhappiness.

You must have heard about AIDSwhich is caused by a dangerous virus,HIV. This virus can pass on to anormal person from an infectedperson by sharing the syringes usedfor injecting drugs. It can also betransmitted to an infant from theinfected mother through her milk. Thevirus can also be transmitted throughsexual contact with a person infectedwith HIV.

Adolescent Pregnancy

You might be knowing that in our country, the legal age for marriage is 18 yearsfor girls and 21 years for boys. This is because teenage mothers are not preparedmentally or physically for motherhood. Early marriage and motherhood causehealth problems in the mother and the child. It also curtails employmentopportunities for the young woman and may cause mental agony as she is notready for responsibilities of motherhood.

Myths, Taboos, Do’s and Don’ts

You have learnt here and fromChapter 9 the scientific facts relatedto human reproduction. There aremany wrong notions which you shouldnow be able to discard as informedadolescents. For example, there aremyths and taboos regarding bodilychanges that adolescents experience.Some of these are given below andyou can now argue why these aremyths and not facts.1. A girl becomes pregnant if

she looks at boys duringmenstruation.

2. The mother is responsible for thesex of her child.

3. A girl should not be allowed towork in the kitchen duringmenstruation.

You may come across many other mythsand taboos. Discard them.


KEYWORDS

ADAM’S APPLE

ADOLESCENCE

ADRENALIN

BALANCED DIET

ENDOCRINE GLANDS

ESTROGEN

HORMONES

INSULIN

PITUITARY GLAND

PUBERTY

REPRODUCTIVE

HEALTH

SECONDARY SEXUAL

CHARACTERS

SEX CHROMOSOMES

TARGET SITE

TESTOSTERONE

THYROXINE

VOICE BOX


� Humans become capable of reproduction af-

ter puberty sets in. Between the ages of 11

years and 19 years children are called ado-

lescents.

� The onset of puberty brings about growth of

the reproductive organs. Hair grow at various

places on the body. Breasts develop in girls

and facial hair (moustache and beard) appear

in boys. Voice of boys becomes hoarse as voice

box enlarges during adolescence.

� Children gain height during adolescence.

� The onset of puberty and maturity of

reproductive parts are controlled by hormones.

� Hormones are secretions of endocrine glands

which pour them directly into the blood stream.

� Pituitary gland secretes hormones which in-

clude growth hormone and hormones that

make other glands such as the testes, ovaries,

thyroids and adrenals, secrete hormones.

Pancreas secretes insulin, thyroid produces

thyroxine and adrenals produce adrenalin.

� Testosterone is the male hormone and

estrogen, the female hormone. The uterine wall

in females prepares itself to receive the

developing fertilised egg. In case there is no

fertilisation, the thickened lining of the uterine

wall breaks down and goes out of the body

along with blood. This is called menstruation.

� Sex of the unborn child depends on whether

the zygote has XX or XY chromosomes.

� It is important to eat balanced food and

maintain personal hygiene during

adolescence.

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Exercises

1. What is the term used for secretions of endocrine glands responsible forchanges taking place in the body?

2. Define adolescence.

3. What is menstruation? Explain.

4. List changes in the body that take place at puberty.

5. Prepare a Table having two columns depicting names of endocrine glandsand hormones secreted by them.

6. What are sex hormones? Why are they named so? State their function.

7. Choose the correct option.

(a) Adolescents should be careful about what they eat, because

(i) proper diet develops their brains.

(ii) proper diet is needed for the rapid growth taking place in theirbody.

(iii) adolescents feel hungry all the time.

(iv) taste buds are well developed in teenagers.

(b) Reproductive age in women starts when their

(i) menstruation starts.

(ii) breasts start developing.

(iii) body weight increases.

(iv) height increases.

(c) The right meal for adolescents consists of

(i) chips, noodles, coke.

(ii) chapati, dal, vegetables.

(iii) rice, noodles and burger.

(iv) vegetable cutlets, chips and lemon drink.

8. Write notes on—

(a) Adam’s apple.

(b) Secondary sexual characters.

(c) Sex determination in the unborn baby.


9. Word game : Use the clues to work out the words.

Across

3. Protruding voice box in boys

4. Glands without ducts

7. Endocrine gland attached to brain

8. Secretion of endocrine glands

9. Pancreatic hormone

10. Female hormone

Down

1. Male hormone

2. Secretes thyroxine

3. Another term for teenage

5. Hormone reaches here through blood stream

6. Voice box

7. Term for changes at adolescence

E X

E R

C I

S E

S

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10. The table below shows the data on likely heights of boys and girls as theygrow in age. Draw graphs showing height and age for both boys andgirls on the same graph paper. What conclusions can be drawn fromthese graphs?

Height (cm)Boys Girls

0 53 53

4 96 92

8 114 110

12 129 133

16 150 150

20 173 165

Age(Years)


1. Find out from your elder relatives about their awareness of thelegal status of early marriage. You yourself may get information onit from your teacher, parents, a doctor or the internet. Write a two-minute speech explaining why early marriage is not good for thecouple.

2. Collect newspaper cuttings and information in magazines about HIV/AIDS. Write a one page article of 15 to 20 sentences on HIV/AIDS.

3. In our country, according to a census, there are 882 adolescentfemales for every 1000 males. Find out.

(a) the concerns of the community regarding this low ratio.Remember that the chance of having a boy or a girl is equal.

(b) what amniocentesis is and how useful this technique is. Whyis its use for identification of sex of the unborn child bannedin India?

4. Put your ideas together and write a short note on the importance ofknowing facts about reproduction.

For more information, visit :� www.teenshealth.org/teen/sexual_health/� www.ama_assn.org/ama/pub/category/1947.html� www.adolescenthealth.com

FORCE AND PRESSUREFORCE AND PRESSURE

In Class VII, you have learnt howobjects move. Do you recall how wecan decide whether an object is

moving faster than the other? What doesthe distance moved by an object in unittime indicate? You also know that amoving object like a ball rolling on theground slows down. Sometimes it maychange its direction of motion. It is alsopossible that the ball may slow downand also change its direction. Did youever wonder what makes an object toslow down or go faster, or change itsdirection of motion?

Let us recall some of our everydayexperiences. What do you do to makea football move? What do you do tomake a moving ball move faster? Howdoes a goalkeeper stop a ball? Howdo f ielders stop a ball hit by abatsman? A hockey player changesthe direction of the moving ball with

a flick of the stick (Fig. 11.1). In allthese situations the ball is eithermade to move faster or slower or itsdirection of motion is changed.

We often say that a force has beenapplied on a ball when it is kicked,pushed, thrown or flicked. What is aforce? What can it do to bodies on whichit is applied? We shall seek answers tosuch questions in this chapter.

11.1 Force – A Push or a Pull

Actions l ike picking, opening,shutting, kicking, hitting, lifting,flicking, pushing, pulling are oftenused to describe certain tasks. Eachof these actions usually results insome kind of change in the motion ofan object. Can these terms bereplaced with one or more terms? Letus find out.

(a) (b) (c)

Fig. 11.1 : (a) A goal keeper saving a goal (b) A hockey player flicking a ball(c) A fielder stopping a ball

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Activity 11.1

Table 11.1 gives some examples of familiar situations involving motion of objects.You can add more such situations or replace those given here. Try to identifyaction involved in each case as a push and/or a pull and record your observations.One example has been given to help you.

S .No

Description ofthe situation

Action : (pushing/ pulling/picking/hitting/lifting/ lowering/flying/kicking/ throwing/shutting/flicking )

Action can begrouped as a

Push Pull

1. Moving a bookplaced on a table

2. Opening orshutting a door

3. Drawing a bucketof water from awell

4. A football playertaking a penaltykick

5. A cricket ball hitby a batsman

6. Moving a loadedcart

7. Opening adrawer

Pushing Pulling Lifting — Yes Yes

Table 11.1 : Identifying Actions as Push or Pull

I learnt in Class VI that amagnet attracts a piece of

iron towards it. Is attractionalso a pull? What aboutrepulsion between similar

poles of two magnets? Is it apull or a push?

Do you notice that each of the actionscan be grouped as a pull or a push orboth? Can we infer from this, that to movean object, it has to be pushed or pulled?

In science, a push or a pull on anobject is called a force. Thus, we cansay that the motion imparted to objectswas due to the action of a force. Whendoes a force come into play? Let usfind out.


11.2 Forces are due to anInteraction

Suppose a man is standing behind astationary car (Fig.11.2). Will the carmove due to his presence? Suppose theman now begins to push the car, thatis, he applies a force on it. The car maybegin to move in the direction of theapplied force. Note that the man has topush the car to make it move.

Fig. 11.3 shows three situations thatmay be familiar to you. Can you decidewho is pulling and who is pushing inthese cases? In Fig. 11.3 (a), both thegirls appear to push each other whilethe pair of girls in Fig. 11.3 (b) are tryingto pull each other. Similarly, the cowand the man in Fig. 11. 3(c) appear topull each other. The girls in the twosituations shown here are applying force

Fig 11.3 (c) : Who is pulling whom?

Fig.11.2 : A car being pushed by a man

Fig11.3 (a) : Who is pushing whom? Fig 11.3 (b) : Who is pulling whom ?

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on each other. Is it also true for the manand the cow?

From these examples, we can infer thatat least two objects must interact for a forceto come into play. Thus, an interaction ofone object with another object results in aforce between the two objects.

11.3 Exploring Forces

Let us try to learn more about forces.

Have you ever seen a game of tug-ofwar? In this game two teams pull at arope in opposite directions (Fig. 11.5).Members of both the teams try to pullthe rope in their direction. Sometimesthe rope simply does not move. Is it notsimilar to the situation shown in Fig.11.3 (b)? The team that pulls harder,that is, applies a larger force, finally winsthe game.

Activity 11.2

Choose a heavy object like a table or a box,which you can move only by pushinghard. Try to push it all by yourself. Canyou move it? Now ask one of your friendsto help you in pushing it in thesame direction [Fig.11.4(a)]. Is iteasier to move it now? Can youexplain why?Next push the same object, but askyour friend to push it from theopposite side [Fig.11.4 (b)]. Does theobject move? If it does, note thedirection in which it moves. Can youguess which one of you is applying alarger force?

Fig. 11.4 : Two friends pushing a heavy load(a) in the same direction, (b) in opposite direction

Fig. 11.5 : The rope may not move if the two teams pull at it with equal force

(a)

(b)


What do these examples suggestabout the nature of force?

Forces applied on an object in thesame direction add to one another. Nowrecall what happened when you andyour friend pushed the heavy box in thesame direction in Activity 11.2.

If the two forces act in the oppositedirections on an object, the net forceacting on it is the difference betweenthe two forces. What did you observein Activity 11.2 when both of you werepushing the heavy box from oppositedirections?

Activity 11.3

Take a rubber ball and place it on alevel surface such as a table top ora concrete floor. Now, gently pushthe ball along the level surface(Fig. 11.6). Does the ball begin tomove? Push the ball again while itis still moving. Is there any changein its speed? Does it increase ordecrease?Next, place your palm in front of themoving ball. Remove your palm assoon as the moving ball touches it.Does your palm apply a force on theball? What happens to the speed ofthe ball now? Does it increase ordecrease? What would happen if youlet your palm hold the moving ball?

Does it mean that the netforce on an object is zero if the

two forces acting on it inopposite directions are equal?

Recall that in the tug-of-war whentwo teams pull equally hard, the ropedoes not move in any direction.

So, we learn that a force could belarger or smaller than the other. Thestrength of a force is usually expressedby its magnitude. We have also tospecify the direction in which a forceacts. Also, if the direction or themagnitude of the applied force changes,its effect also changes.

In general, more than one force maybe acting on an object. However, theeffect on the object is due to the netforce acting on it.

11.4 A Force can Change theState of Motion

Let us now find out what happens whena force acts on an object.

Fig. 11.6 : A ball at rest begins to movewhen a force is applied on it

You might recall similar situations.For example, while taking a penalty kickin football, the player applies a force onthe ball. Before being hit, the ball wasat rest and so its speed was zero. Theapplied force makes the ball movetowards the goal. Suppose, thegoalkeeper dives or jumps up to savethe goal. By his action the goalkeepertries to apply a force on the moving ball.The force applied by him can stop ordeflect the ball, saving a goal beingscored. If the goalkeeper succeeds in

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stopping the ball, its speed decreases tozero.

These observations suggest that aforce applied on an object may changeits speed. If the force applied on theobject is in the direction of its motion,the speed of the object increases. If theforce is applied in the direction oppositeto the direction of motion, then it resultsin a decrease in the speed of the object.

Activity 11.4

Take a ball and place it on a levelsurface as you did in Activity 11.3.Make the ball move by giving it apush. Now place the ruler from yourgeometry box in its path as shownin Fig. 11.8. In doing so, you wouldapply a force on the moving ball.Does the ball continue to move inthe same direction after it strikesthe ruler? Repeat the activity andtry to obstruct the moving ball byplacing the ruler such that it makesdifferent angles to its path. In eachcase note your observations aboutthe direction of motion of the ballafter it strikes the ruler.

I have seenchildren competing with one

another in moving a rubber tyreor a ring by pushing it

(Fig. 11.7). I now understandwhy the speed of the tyreincreases whenever it is

pushed.

Fig. 11.7 : To move a tyre faster it has to bepushed repeatedly

Paheli is curious to know whetherapplication of a force can only changethe speed of an object. Let us find out.

Fig. 11.8 : (a) A ball set in motion by pushing italong a level surface and (b) thedirection of motion of the ball after itstrikes the ruler placed in its path

(a)

(b)


Let us consider some moreexamples. In a game of volleyball,players often push the moving ball totheir teammates to make a winningmove. Sometimes the ball is returnedto the other side of the court bypushing or smashing it. In cricket, abatsman plays his or her shot byapplying a force on the ball with thebat. Is there any change in thedirection of motion of the ball in thesecases? In all these examples the speedand the direction of the moving ballchange due to the application of aforce. Can you give a few moreexamples of this kind?

A change in either the speed of anobject, or its direction of motion, or both,is described as a change in its state ofmotion. Thus, a force may bring achange in the state of motion of anobject.

11.5 Force can Change theShape of an Object

Activity 11.5

Some situations have been given inColumn 1 of Table 11.2 on page 134in which objects are not free to move.Column 2 of the Table suggests themanner in which a force can beapplied on each object while Column3 shows a diagram of the action. Tryto observe the effect of force in asmany situations as possible. You canalso add similar situations usingavailable material from yourenvironment. Note your observationsin Columns 4 and 5 of the Table.

What do you conclude from theobservations noted in Table 11.2?What happens when you apply a forceon an inflated balloon by pressing itbetween your palms? What happensto the shape of a ball of dough when itis rolled to make a chapati? Whathappens when you press a rubber ballplaced on a table? In all theseexamples you saw that the applicationof force on an object may change itsshape.

Having performed all the aboveactivities, you would have realised thata force

� may make an object move from rest.� may change the speed of an object

if it is moving.� may change the direction of motion

of an object.� may bring about a change in the

shape of an object.� may cause some or all of these

effects.

State of Motion

The state of motion of an object isdescribed by its speed and thedirection of motion. The state of restis considered to be the state of zerospeed. An object may be at rest orin motion; both are its states ofmotion.

Does it mean that the application ofa force would always result in a changein the state of motion of the object? Letus find out.

It is common experience that manya time application of force does not resultin a change in the state of motion. Forexample, a heavy box may not move atall even if you apply the maximum forcethat you can exert. Again, no effect offorce is observed when you try to pusha wall.

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While a force may cause one or moreof these effects, it is important toremember that none of these actions cantake place without the action of a force.Thus, an object cannot move by itself,it cannot change speed by itself, itcannot change direction by itself andits shape cannot change by itself.

11.6 Contact Forces

Muscular Force

Can you push or lift a book lying on atable without touching it? Can you lifta bucket of water without holding it?

Generally, to apply a force on an object,your body has to be in contact with the

It is the muscular force that enablesus to perform all activities involvingmovement or bending of our body. InClass VII you have learnt that in theprocess of digestion the food getspushed through the alimentary canal.Could it be a muscular force that doesit? You also know that lungs expandand contract while we inhale andexhale air during breathing. Whereare these muscles located whichmake breathing possible? Can you lista few more examples of the forceexerted by the muscles in our body?

Description ofsituation

How to applyforce

Diagram Action of force

Change instate ofmotion

Change inshape

Yes No Yes No

A lump of dough ona plate

Spring fixed to theseat of a bicycle

A rubber bandsuspended from ahook/nail fixed on awall

A plastic or metalscale placedbetween two bricks

Pressing it downwith your hands

By sitting on theseat

By hanging aweight or bypulling its free end

By putting a weightat the centre of thescale

Table 11.2 : Studying the Effect of Force on Objects


object. The contact may also be with thehelp of a stick or a piece of rope. Whenwe push an object like a school bag orlift a bucket of water, where does theforce come from? This force is causedby the action of muscles in our body.The force resulting due to the action ofmuscles is known as the muscularforce.

Animals also make use of muscularforce to carry out their physical activitiesand other tasks. Animals like bullocks,horses, donkeys and camels are used toperform various tasks for us. Inperforming these tasks they usemuscular force (Fig. 11.9).

Since muscular force can be appliedonly when it is in contact with an object,it is also called a contact force. Arethere other types of contact forces? Letus find out.

Friction

Recall some of your experiences. A ballrolling along the ground gradually slowsdown and finally comes to rest. When

we stop pedalling a bicycle, it graduallyslows down and finally comes to a stop.A car or a scooter also comes to rest onceits engine is switched off. Similarly, aboat comes to rest if we stop rowing it.Can you add some more suchexperiences?

In all these situations no forceappears to be acting on the objects, yettheir speed gradually decreases andthey come to rest after some time. Whatcauses a change in their state of motion?Could some force be acting on them!Can you guess the direction in whichthe force must be acting in each case?

The force responsible for changing thestate of motion of objects in all theseexamples is the force of friction. It is theforce of friction between the surface ofthe ball and the ground that brings themoving ball to rest. Similarly, frictionbetween water and the boat brings it toa stop once you stop rowing.

The force of friction always acts onall the moving objects and its directionis always opposite to the direction ofmotion. Since the force of friction arises

Fig.11.9 : Muscular force of animals is used to carry out many difficult tasks

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Does the magnet on the rollers beginto move when the other magnet is broughtnear it? Does it always move in thedirection of the approaching magnet?What do these observations suggest? Doesit mean that some force must be actingbetween the two magnets?

You have learnt in Class VI that likepoles of two magnets repel each other andunlike poles attract each other. Attractionor repulsion between objects can also beseen as another form of pull or push. Doyou have to bring the magnets in contactfor observing the force between them? Amagnet can exert a force on anothermagnet without being in contact with it.The force exerted by a magnet is anexample of a non-contact force.

Similarly, the force exerted by amagnet on a piece of iron is also a non-contact force.

Electrostatic Force

Activity 11.7

Take a plastic straw and cut it intonearly two equal pieces. Suspend oneof the pieces from the edge of a tablewith the help of a piece of thread (Fig.11.11). Now hold the other piece ofstraw in your hand and rub its freeend with a sheet of paper. Bring therubbed end of the straw near thesuspended straw. Make sure that thetwo pieces do not touch each other.What do you observe?

Next, rub the free end of thesuspended piece of straw with asheet of paper. Again, bring the pieceof straw that was rubbed earlier withpaper near the free end of thesuspended straw. What do youobserve now?

due to contact between surfaces, it isalso an example of a contact force. Youwill learn more about this force inChapter 12.

You may be wondering whether it isessential for the agent applying a forceon an object to be always in contact withit. Let us find out.

11.7 Non-contact Forces

Magnetic Force

Activity 11.6

Take a pair of bar magnets. Placethe longer side of one of the magnetsover three round shaped pencils orwooden rollers as shown inFig.11.10. Now bring one end of theother magnet near the end of themagnet placed on the rollers. Makesure that the two magnets do nottouch each other. Observe whathappens. Next, bring the other endof the magnet near the same end ofthe magnet placed on the rollers(Fig.11.10). Note what happens tothe magnet placed on the rollersevery time another magnet isbrought near it.

Fig.11.10 : Observing attraction and repulsionbetween two magnets


A straw is said to have acquiredelectrostatic charge after it has beenrubbed with a sheet of paper. Such astraw is an example of a charged body.

The force exerted by a charged bodyon another charged or uncharged bodyis known as electrostatic force. Thisforce comes into play even when thebodies are not in contact. Theelectrostatic force, therefore, is anotherexample of a non-contact force. You willlearn more about electric charges inChapter 15.

C. Gravitational Force

You know that a coin or a pen falls to theground when it slips off your hand. Theleaves or fruits also fall to the groundwhen they get detached from the plant.Have you ever wondered why it is so?

When the coin is held in your hand itis at rest. As soon as it is released, itbegins to move downwards. It is clear thatthe state of motion of the coin undergoesa change. Can this happen without aforce acting on it? Which is this force?

Objects or things fall towards theearth because it pulls them. This forceis called the force of gravity, or justgravity. This is an attractive force. Theforce of gravity acts on all objects. Theforce of gravity acts on all of us all thetime without our being aware of it. Waterbegins to flow towards the ground assoon as we open a tap. Water in riversflows downward due to the force ofgravity.

Gravity is not a property of the earthalone. In fact, every object in theuniverse, whether small or large,exerts a force on every other object.This force is known as thegravitational force.

11.8 Pressure

You have learnt in Class VII that strongwinds during a storm or a cyclone canblow away even the roof-tops. You alsolearnt that winds and cyclones arecaused by the differences in airpressure. Is there any relation betweenpressure and force? Let us find out.

Try to push a nail into a woodenplank by its head. Did you succeed? Trynow to push the nail by the pointed end(Fig. 11.12). Could you do it this time?

Fig. 11.12 : Pushing a nail into a wooden plank

Fig.11.11 : A straw rubbed with paper attractsanother straw but repels it if it hasalso been rubbed with a sheet ofpaper.

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Try cutting vegetables with a blunt anda sharp knife. Which is easier?

Do you get the feeling that the areaover which the force is applied (forexample, the pointed end of the nail)plays a role is making these tasks easier?

The force acting on a unit area of asurface is called pressure.

pressure = force / area on which it acts

At this stage we consider only thoseforces which act perpendicular to thesurface on which the pressure is tobe computed.

Note that the area is in thedenominator in the above expression.So, the smaller the area, larger thepressure on a surface for the same force.The area of the pointed end of the nailis much smaller than that of its head.The same force, therefore, produces apressure sufficient to push the pointedend of the nail into the wooden plank.

Can you explain now why shoulderbags are provided with broad straps andnot thin strap? And, why the toolsmeant for cutting and piercing alwayshave sharp edges?

Do liquids and gases also exertpressure? Does it also depend on thearea on which the force acts? Let usfind out.

11.9 Pressure exerted byLiquids and Gases

Activity 11.8

Take a transparent glass tube or aplastic pipe. The length of the pipe/tube should be about 15 cm and itsdiameter should be 5-7.5 cm. Alsotake a piece of thin sheet of a goodquality rubber, say, a rubberballoon. Stretch the rubber sheettightly over one end of the pipe.Hold the pipe at the middle, keepingit in a vertical position (Fig.11.14).Ask one of your friends to pour somewater in the pipe. Does the rubbersheet bulge out? Note also theheight of the water column in thepipe. Pour some more water.

Fig. 11.13 : A porter carrying a heavy load

I now understand whyporters place on their headsa round piece of cloth, when

they have to carry heavyloads (Fig. 11.13). By thisthey increase the area ofcontact of the load with

their head. So, the pressureon their head is reducedand they find it easier to

carry the load.


sheet fixed to the glass tube bulgethis time? Pour some more water inthe bottle. Is there any change inthe bulge of the rubber sheet?

Fig.11.15 :A liquid exerts pressure on the wallsof the container

Observe again the bulge in therubber sheet and the height of thewater column in the pipe. Repeatthis process a few more times. Canyou see any relation between theamount of the bulge in the rubbersheet and the height of the watercolumn in the pipe?

Fig.11.14 :Pressure exerted by water at thebottom of the container depends onthe height of its column

Activity 11.9

Take a plastic bottle. You can takea discarded water or soft drinkbottle. Fix a cylindrical glass tube,a few cm long near its bottom asshown in Fig. 11.15. You can do soby slightly heating one end of theglass tube and then quicklyinserting it near the bottom of thebottle. Make sure that the waterdoes not leak from the joint. If thereis any leakage, seal it with moltenwax. Cover the mouth of the glasstube with a thin rubber sheet as youdid in Activity 11.8. Now fill thebottle upto half with water. What doyou observe? Why does the rubber

Note that the rubber sheet has beenfixed on the side of the container andnot at the bottom. Does the bulging ofthe rubber sheet in this case indicatethat water exerts pressure on the sidesof the container as well? Let usinvestigate further.

Activity 11.10

Take an empty plastic bottle or acylindrical container. You can takea used tin of talcum powder or aplastic bottle. Drill four holes allaround near the bottom of thebottle. Make sure that the holes areat the same height from the bottom(Fig. 11.16). Now fill the bottle withwater. What do you observe?Does the water coming out of theholes falls at the same distance fromthe bottle? What does this indicate?

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Unit area

Do you recall what happens to theair in the bicycle tube when it has apuncture? Do these observationssuggest that air exerts pressure on theinner walls of an inflated balloon or atube? So, we find that gases, too, exertpressure on the walls of theircontainer.

11.10 Atmospheric Pressure

You know that there is air all aroundus. This envelop of air is known as theatmosphere. The atmospheric airextends up to many kilometres abovethe surface of the earth. The pressureexerted by this air is known asatmospheric pressure. We know thatpressure is force per unit area. If weimagine a unit area and a very longcylinder standing on it filled with air,then the weight of the air in this cylinderis the atmospheric pressure (Fig. 11.17).

But, how large or small is theatmospheric pressure? Let us get anidea about its magnitude.

I have seen fountains of watercoming out of the leaking joints

or holes in pipes supplyingwater. Is it not due to the

pressure exerted by water on thewalls of the pipes?

Fig. 11.16 :Liquids exert equal pressure at thesame depth

Fig. 11.17 : Atmospheric pressure is the weightof air in a column of unit area

When you inflate a balloon, why doyou have to close its mouth? Whathappens when you open the mouth ofan inflated balloon? Suppose you havea balloon which has holes. Would yoube able to inflate it? If not, why? Can wesay that air exerts pressure in alldirections?

Can you now say that liquids exertpressure on the walls of the container?

Do gases also exert pressure? Dothey also exert pressure on the walls oftheir containers? Let us find out.


If the area of my headwere 10 cm × 10 cm, how

much weight of air would Ibe carrying on my head?

When you press the sucker, most ofthe air between its cup and the surfaceescapes out. The sucker sticks to thesurface because the pressure ofatmosphere acts on it. To pull the suckeroff the surface, the applied force shouldbe large enough to overcome theatmospheric pressure. This activitymight give you an idea about themagnitude of atmospheric pressure. Infact, it would not be possible for anyhuman being to pull the sucker off thesurface if there were no air at all betweenthe sucker and the surface. Does it giveyou an idea how large the atmosphericpressure is?

Fig.11.18 : A rubber sucker pressed on a sur-face.

Activity 11.11

Take a good quality rubber sucker.It looks like a small rubber cup (Fig.11.18). Press it hard on a smoothplane surface. Does it stick to thesurface? Now try to pull it off thesurface. Can you do it?

Fig. 11.19 :Pressure of atmosphere on yourhead

The weight of air in a column of theheight of the atmosphere and area10 cm × 10 cm (Fig. 11.19) is as largeas 1000 kg. The reason we are notcrushed under this weight is that thepressure inside our bodies is also equalto the atmospheric pressure and cancelsthe pressure from outside.

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Did you know?

Otto von Guericke, a German scientist of 17th century, invented a pump toextract air out of the vessel. With the help of this pump, he demonstrateddramatically the force of the air pressure. He joined two metallic hemispheresof 51 cm diameter each and pumped air out of them. Then he employed eighthorses on each hemisphere to pull them apart (Fig. 11.20). So great is theforce of air pressure that the hemispheres could not be pulled apart.

Fig. 11.20 : Horses pulling the hemispheres


KEYWORDS

ATMOSPHERIC

PRESSURE

CONTACT FORCE

ELECTROSTATIC

FORCE

FORCE

FRICTION

GRAVITATIONAL

FORCE

GRAVITY

MAGNETIC FORCE

MUSCULAR FORCE

NON-CONTACT

FORCE

PRESSURE

PULL

PUSH


� Force could be a push or a pull.

� A force arises due to the interaction between

two objects.

� Force has magnitude as well as direction.

� A change in the speed of an object or the

direction of its motion or both implies a change

in its state of motion.

� Force acting on an object may cause a change

in its state of motion or a change in its shape.

� A force can act on an object with or without

being in contact with it.

� Force per unit area is called pressure.

� Liquids and gases exert pressure on the walls

of their containers.

� The pressure exerted by air around us is

known as atmospheric pressure.

Exercises

1. Give two examples each of situations in which you push or pull to changethe state of motion of objects.

2. Give two examples of situations in which applied force causes a change inthe shape of an object.

3. Fill in the blanks in the following statements:

(a) To draw water from a well we have to ——— at the rope.

(b) A charged body ———— an uncharged body towards it.

(c) To move a loaded trolley we have to ———— it.

(d) The north pole of a magnet ———— the north pole of another magnet.

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4. An archer stretches her bow while taking aim at the target. She thenreleases the arrow, which begins to move towards the target. Based onthis information fill up the gaps in the following statements using thefollowing terms:

muscular, contact, non-contact, gravity, friction, shape, attraction(a) To stretch the bow, the archer applies a force that causes a change

in its ——.(b) The force applied by the archer to stretch the bow is an example of —

—— force.(c) The type of force responsible for a change in the state of motion of the

arrow is an example of a ———— force.(d) While the arrow moves towards its target, the forces acting on it are

due to ———— and that due to ———— of air.

5. In the following situations identify the agent exerting the force and theobject on which it acts. State the effect of the force in each case.(a) Squeezing a piece of lemon between the fingers to extract its juice.(b) Taking out paste from a toothpaste tube.(c) A load suspended from a spring while its other end is on a hook fixed

to a wall.(d) An athlete making a high jump to clear the bar at a certain height.

6. A blacksmith hammers a hot piece of iron while making a tool. How doesthe force due to hammering affect the piece of iron?

7. An inflated balloon was pressed against a wall after it has been rubbedwith a piece of synthetic cloth. It was found that the balloon sticks to thewall. What force might be responsible for the attraction between the balloonand the wall?

8. Name the forces acting on a plastic bucket containing water held aboveground level in your hand. Discuss why the forces acting on the bucket donot bring a change in its state of motion.

9. A rocket has been fired upwards to launch a satellite in its orbit. Namethe two forces acting on the rocket immediately after leaving the launchingpad.

10. When we press the bulb of a dropper with its nozzle kept in water, air inthe dropper is seen to escape in the form of bubbles. Once we release thepressure on the bulb, water gets filled in the dropper. The rise of water inthe dropper is due to

(a) pressure of water

(b) gravity of the earth

(c) shape of rubber bulb

(d) atmospheric pressure

E X

E R

C I

S E

S



1. Make a 50 cm × 50 cm bed of dry sand about 10 cm in thickness.Make sure that its top surface is levelled. Take a wooden or a plasticstool. Cut two strips of graph paper each with a width of 1 cm.Paste them vertically on any leg of the stool - one at the bottom andthe other from the top. Now gently put the stool on the sand bedwith its legs resting on the sand. Increase the size of sand bed ifrequired. Now put a load, say a school bag full of books, on the seatof the stool. Mark the level of sand on the graph strip. This wouldgive you the depth, if any, to which the legs of stool sink in sand.Next, turn the stool upside down so that now it rests on its seat onthe sand bed. Note the depth to which the stool sinks now. Next,put the same load on the stool and note the depth to which it sinksin the sand. Compare the pressure exerted by the stool in the twosituations.

2. Take a tumbler and fill it with water. Cover the mouth of the tumblerwith a thick card similar to that of a postcard. Hold the tumblerwith one hand while keeping the card pressed to its mouth withyour other hand. Turn the tumbler upside down while keeping thecard pressed to its mouth. Make sure that the tumbler is heldvertical. Gently remove the hand pressing the card. What do youobserve? Does the card get detached allowing the water to spill?With a little practice you will find that the card continues to holdwater in the tumbler even after it is not supported by your hand.Also try this activity by using a piece of cloth to hold the tumbler inan upside down position (Fig. 11.21).

Fig. 11.21

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3. Take 4-5 plastic bottles of different shapes and sizes. Join themtogether with small pieces of glass or rubber tube as shown inFig. 11.22. Keep this arrangement on a level surface. Now pourwater in any one of the bottles. Note whether the bottle in whichwater is poured gets filled first or all the bottles get filled upsimultaneously. Note the level of water in all the bottles from timeto time. Try to explain your observations.

Fig. 11.22

For more information on force and pressure visit:� www.glenbrook.k12.il.us/gbssci/phys/class/newtlauws/

u2l2a.html� www.hatesville.k12.in.us/physics/phyNet/Mechanics/

Newton2/Pressure.html� kids.earth.nasa.gov/archive/air_pressure/

FRICTIONFRICTION

You might have seen a driver of acar or a truck slowing down thevehicle at a traffic signal. You, too,

slow down your bicycle wheneverneeded by applying brakes. Have youever thought why a vehicle slowsdown when brakes are applied? Notonly vehicles, any object, moving overthe surface of another object slowsdown when no external force isapplied on it. Finally it stops. Haveyou not seen a moving ball on theground stopping after some time?Why do we slip when we step on abanana peel (Fig. 12.1)? Why is itdifficult to walk on a smooth and wetfloor?

Fig. 12.1 : A boy falls down when he steps on abanana peel

12.1 Force of Friction

Activity 12.1

Gently push a book on a table [Fig.12.2(a)]. You observe that it stopsafter moving for some distance.Repeat this activity pushing thebook from the opposite direction[Fig. 12.2, (b)]. Does the book stopthis time, too? Can you think of anexplanation? Can we say that a forcemust be acting on the bookopposing its motion? This force iscalled the force of friction.

Fig. 12.2 (a), (b) : Friction opposes relative motionbetween the surfaces of thebook and the table

(a)

(b)

You will find the answers to suchquestions in this chapter.

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You saw that if you apply the forcealong the left, friction acts along theright. If you apply the force along theright, the friction acts along the leftdirection. In both cases the force opposesthe motion of the book. The force offriction always opposes the applied force.

In the above activity, the force offriction acts between the surface of thebook and the surface of the table.

Is the friction the same for all thesurfaces? Does it depend on thesmoothness of the surfaces? Let us findout.

12.2 Factors affectingFriction

Activity 12.2

Tie a string around a brick. Pull thebrick by a spring balance (Fig.12.3). You need to apply some force.Note down the reading on thespring balance when the brick justbegins to move. It gives you ameasure of the of force of frictionbetween the surface of the brick andthe floor.

for this difference? Repeat thisactivity by wrapping a piece of jutebag around the brick. What do youobserve?

Fig. 12.3 : A brick is being pulled by springbalance.

Now wrap a piece of polythenearound the brick and repeat theactivity. Do you observe anydifference in the readings of thespring balance in the above twocases? What might be the reason

Spring Balance

Spring balance is a device used formeasuring the force acting on anobject. It consists of a coiled springwhich gets stretched when a force isapplied to it. Stretching of the springis measured by a pointer moving ona graduated scale. The reading on thescale gives the magnitude of the force.

Activity 12.3

Make an inclined plane on a smoothfloor, or on a table. You may use awooden board supported by bricks,or books. [Fig. 12.4 (a)]. Put a markwith a pen at any point A on theinclined plane. Now let a pencil cellmove down from this point. How fardoes it move on the table beforecoming to rest? Note down thedistance. Now spread a piece of cloth

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Fig. 12.5 : Surface irregularities

over the table. Make sure that thereare no wrinkles in the cloth. Try theactivity again [Fig. 12.4 (b)].

Does the distance covered depend onthe nature of the surface on which thecell moves?

Could the smoothness of the surfaceof the cell also affect the distancetravelled by it?

(a)

(b)

Repeat this activity by spreadinga thin layer of sand over the table.Maintain the same slopethroughout the activity.

In which case is the distance coveredthe minimum? Why is the distancecovered by the pencil cell different everytime. Try to reason why? Discuss theresult.

Fig. 12.4 : The pencil cell covers differentdistances on different surfaces

I shall try the activityby wrapping a piece of

sandpaper aroundthe cell.

Friction is caused by theirregularities on the two surfaces incontact. Even those surfaces whichappear very smooth have a large numberof minute irregularities on them (Fig.12.5). Irregularities on the two surfaceslock into one another. When we attemptto move any surface, we have to apply aforce to overcome interlocking. Onrough surfaces, there are a largernumber of irregularities. So the force offriction is greater if a rough surface isinvolved.

Mark A

Mark A

SCIENCE150

We see that the friction is caused bythe interlocking of irregularities in thetwo surfaces. It is obvious that the forceof friction will increase if the twosurfaces are pressed harder. You canexperience it by dragging a mat whennobody is sitting on it, and when apreson is sitting on it.

Recall your experience when last timeyou moved a heavy box from one placeto another. If you have no suchexperience, get that experience now.What is easier — to move the box fromrest, or to move it when it is already inmotion?

The force required to overcomefriction at the instant an object startsmoving from rest is a measure of staticfriction. On the other hand, the forcerequired to keep the object moving withthe same speed is a measure of slidingfriction.

friction and you find it somewhat easierto move the box already in motion thanto get it started.

12.3 Friction : A NecessaryEvil

Recall now some of your experiences. Isit easier to hold a kulhar (earthen pot)or a glass tumbler? Suppose the outersurface of the tumbler is greasy, or hasa film of cooking oil on it; would itbecome easier or more difficult to holdit? Just think : would it be possible tohold the glass at all if there is nofriction?

Recall also how difficult it is to moveon a wet muddy track, or wet marblefloor. Can you imagine being able to walkat all if there were no friction?

When the box starts sliding, thecontact points on its surface, do not getenough time to lock into the contactpoints on the floor. So, the slidingfriction is slightly smaller than the static

Fig. 12.7 : A nail is fixed in the wall due tofriction

You could not write with pen orpencil if there were no friction. Whenyour teacher is writing with chalk onthe blackboard, its rough surface rubsoff some chalk particles which stick to

Fig. 12.6 : You have to push on the box to keep itmoving

FRICTION 151

the black board. Could it happen if therewere no friction between the chalk andthe board?

If an object started moving, it wouldnever stop if there were no friction. Hadthere been no friction between the tyresof the automobiles and the road, theycould not be started or stoped or turnedto change the direction of motion. Youcould not fix a nail in the wall (Fig. 12.7)or tie a knot. Without friction no buildingcould be constructed.

Fig. 12.8 : Soles of shoes wear out due tofriction

Fig. 12.10 : Stricking a match stick produces fireby friction

produces heat. In fact, when a machineis operated, heat generated causes muchwastage of energy. We shall discuss theways of minimising friction in thefollowing section.

Fig. 12.9 : Rubbing of your palms makes you feelwarm

On the other hand, friction is an evil,too. It wears out the materials whetherthey are screws, ball bearings or solesof shoes (Fig. 12.8). You must have seenworn-out steps of foot over-bridges atrailway stations.

Friction can also produce heat.Vigorously rub your palms together fora few minutes (Fig. 12.9). How do youfeel? When you strike a matchstickagainst the rough surface, it catches fire(Fig. 12.10).

You might have observed that the jarof a mixer becomes hot when it is run

for a few minutes. You can cite variousother examples in which friction

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12.4 Increasing and ReducingFriction

As you have seen in the previous section,friction is desirable in some situations.

Have you ever thought why the soleof your shoes is grooved [Fig. 12.11 (a)]?It is done to provide the shoes bettergrip on the floor, so that you can movesafely. Similarly, the treaded tyres ofcars,trucks and bulldozers provide bettergrip with the ground.

In some situations, however, frictionis undesirable and we would want tominimize it.

Why do you sprinkle fine powder onthe carrom board? You might havenoticed that when a few drops of oil arepoured on the hinges of a door, the doormoves smoothly. A bicycle and a motormechanic uses grease between themoving parts of these machines. In allthe above cases, we want to reduce

Fig. 12.12 : Powder is sprinkled on the carromboard to reduce friction

friction in order to increase efficiency.When oil, grease or graphite is appliedbetween the moving part of a machine,a thin layer is formed there and movingsurfaces do not directly rub against eachother (Fig. 12.13). Interlocking ofirregularities is avoided to a great extent.Movement becomes smooth. Thesubstances which reduce friction arecalled lubricants. In some machines, itmay not be advisable to use oil aslubricant. An air cushion between themoving parts is used to reduce friction.

We deliberately increase friction byusing brake pads in the brake systemof bicycles and automobiles. When youare riding a bicycle, the brake pads donot touch the wheels. But when youpress the brake lever, these pads arrestthe motion of the rim due to friction.The wheel stops moving. You might haveseen that kabaddi players rub theirhands with soil for a better grip of theiropponents. Gymnasts apply some coarsesubstance on their hands to increasefriction for better grip.

(a) (b)

Fig. 12.11 : Soles of shoes and tyres are treadedto increase friction

Fig. 12.13 : Action of lubricant

Lubricant

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Activity 12.4

Take a few pencils which arecylindrical in shape. Place themparallel to each other on a table.Place a thick book over it (Fig.12.15). Now push the book. Youobserve the pencils rolling as thebook moves. Do you feel it easier tomove the book in this way than toslide it? Do you think thatresistance to the motion of the bookhas been reduced? Have you seenheavy machinary being moved byplacing logs under it?

12.5 Wheels Reduce Friction

You must have seen attaches and otherpieces of luggage fitted with rollers. Evena child can pull such pieces of luggage(Fig. 12.14). Why is it so? Let us findout.

Can we reduce friction tozero by polishing surfacesor using large amount of

lubricants?

Friction can never beentirely eleminated. No

surface is perfectly smooth.Some irregularities are

always there.

Fig. 12.15 : Motion of the book on rollers

When one body rolls over the surfaceof another body, the resistance to itsmotion is called the rolling friction.Rolling reduces friction. It is alwayseasier to roll than to slide a body overanother. That is the reason it isconvenient to pull the luggages fittedwith rollers. Can you now understandwhy wheel is said to be one of thegreatest inventions of mankind?

Since the rolling friction is smallerthan the sliding friction, sliding isreplaced in most machines by rollingby the use of ball bearings. Commonexamples are the use of ball bearingsbetween hubs and the axles of ceilingfans and bicycles (Fig. 12.16).Fig. 12.14 : Rolling reduces friction

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12.6 Fluid Friction

You know that air is very light and thin.Yet it exerts frictional force on objectsmoving through it. Similarly, water andother liquids exert force of friction whenobjects move through them. In science,the common name of gases and liquids

is fluids. So we can say that fluids exertforce of friction on objects in motionthrough them.

The frictional force exerted by fluidsis also called drag.

The frictional force on an object in afluid depends on its speed with respectto the fluid. The frictional force alsodepends on the shape of the object andthe nature of the fluid.

It is obvious that when objects movethrough fluids, they have to overcomefriction acting on them. In this processthey lose energy. Efforts are, therefore,made to minimize friction. So, objectsare given special shapes. Where do youthink the scientists get hints for thesespecial shapes? From nature, of course.Birds and fishes have to move about influids all the time. Their bodies musthave evolved to shapes which wouldmake them not to lose much energy inovercoming friction. You read aboutthese shapes in Class VI. Look carefullyat the shape of an aeroplanes (Fig.12.17). Do you find any similarity in itsshape and that of a bird? In fact, allvehicles are designed to have shapeswhich reduce fluid friction.

Fig. 12.17 : Similarity in shapes of an aeroplane and a bird

Fig. 12.16 : Ball bearings reduce friction

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KEYWORDS

BALL BEARING

DRAG

FLUID FRICTION

FRICTION

INTERLOCKING

LUBRICANTS

ROLLING FRICTION

SLIDING FRICTION

STATIC FRICTION


� Friction opposes the relative motion between

two surfaces in contact. It acts on both the

surfaces.

� Friction depends on the nature of surfaces in

contact.

� For a given pair of surfaces friction depends

upon the state of smoothness of those

surfaces.

� Friction depends on how hard the two surfaces

press together.

� Static friction comes into play when we try to

move an object at rest.

� Sliding friction comes with play when an

object is sliding over another.

� Sliding friction is smaller than static friction.

� Friction is important for many of our activities.

� Friction can be increased by making a surface

rough.

� The sole of the shoes and the tyres of the

vehicle are treaded to increase friction.

� The friction is sometimes undesirable.

� Friction can be reduced by using lubricants.

� When one body rolls over another body, rolling

friction comes into play. Rolling friction is

smaller than the sliding friction.

� In many machines, friction is reduced by

using ball bearings.

� Fluid friction can be minimised by giving

suitable shapes to bodies moving in fluids.

Exercises


(a) Friction opposes the _____________ between the surfaces in contactwith each other.

(b) Friction depends on the _____________ of surfaces.

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(c) Friction produces __________.

(d) Sprinkling of powder on the carrom board ________ friction.

(e) Sliding friction is ___________ than the static friction.

2. Four children were asked to arrange forces due to rolling, static and slidingfrictions in a decreasing order. Their arrangements are given below.Choose the correct arrangement.

(a) Rolling, static, sliding

(b) Rolling, sliding, static

(c) Static, sliding, rolling

(d) Sliding, static, rolling

3. Alida runs her toy car on dry marble floor, wet marble floor, newspaperand towel spread on the floor. The force of friction acting on the car ondifferent surfaces in increasing order will be :

(a) Wet marble floor, dry marble floor, newspaper and towel.

(b) Newspaper, towel, dry marble floor, wet marble floor.

(c) Towel, newspaper, dry marble floor, wet marble floor.

(d) Wet marble floor, dry marble floor, towel, newspaper

4. Suppose your writing desk is tilted a little. A book kept on it starts slidingdown. Show the direction of frictional force acting on it.

5. You spill a bucket of soapy water on a marble floor accidently. Would itmake it easier or more difficult for you to walk on the floor? Why?

6. Explain why sportsmen use shoes with spikes.

7. Iqbal has to push a lighter box and Seema has to push a similar heavierbox on the same floor. Who will have to apply a larger force and why?

8. Explain why the sliding friction is less than the static friction.

9. Give examples to show that friction is both a friend and a foe.

10. Explain why objects moving in fluids must have special shapes.

E X

E R

C I

S E

S

FRICTION 157


1. What role does friction play in the sport of your choice? Collectsome pictures of that sport in action where friction is eithersupporting it or opposing it. Display these pictures with propercaptions on the bulletin board of your classroom.

2. Imagine that friction suddenly vanishes. How would the life beaffected. List ten such situations.

3. Visit a shop which sells sports shoes. Observe the soles of shoesmeant for various sports. Describe your deservations.

4. A toy to play with:

Take an empty match box. Take out its tray. Cut a used refill of a ballpen of the same width as the tray as shown in the figure below. Fixthe refill with two pins on the top of the tray as shown in Fig. 12.18.Make two holes on the opposite sides of the tray. Make sure that theholes are large enough to allow a thread to pass through them easily.Take a thread about a metre long and pass it through the holes asshown. Fix beads at the two ends of the thread so that it does notcome out. Insert the tray in the outer cover of the matchbox.

Suspend the match box by the thread. Leave the thread loose.The match box will start falling down due to gravity. Tighten thethread now and observe what happens.

Explain your observation. Can you relate it to friction?

Fig. 12.18

You can read more on the related topic on the following websites :� http://www.school-for-champions.com/science/friction.htm� http//hyperphysics.phy-astr.gsu.edu/hbase/firct2.html

SCIENCE158

A riddle for you

In some situations, I oppose the motion

in other situations, I facilitate the motion

but, I always oppose

the relative motion

between two moving surfaces.

Put some lubricant, and

I become small there.

Make the moving surfaces rough

I make the movement tough.

I may be—

static, sliding or rolling

but whenever two surfaces are in motion

I am always there,

Tell me who I am!

SOUNDSOUND

How do you come to know that a‘period’ is over in your school?You come to know easily that

someone is at your door when he knocksor you hear the sound of the doorbell.Most of the time you can make out thatsomeone is approaching you by justhearing the foot steps.

You might have played a game calledhide and seek. In this game a person isblind-folded and has to catch theremaining players. How is the blind-folded person able to guess which playeris closest to her?

Sound plays an important role in ourlife. It helps us to communicate with oneanother. We hear a variety of sounds inour surroundings.

Make a list of sounds you hear inyour surroundings.

In the music room of your school youhear the sounds made by musicalinstruments like flute, tabla,harmonium etc. (Fig 13.1).

How is sound produced? How does ittravel from one place to another? Howdo we hear sound? Why are some soundslouder than others? We shall discusssuch questions in this chapter.

Fig. 13.1 : Some musical instruments

Tabla

Harmonium

Sitar

Flute

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13.1 Sound is Produced by aVibrating Body

Touch the school bell when not in use.What do you feel? Again touch it whenproducing sound. Can you feel itvibrating?

Activity 13.1

Take a metal plate (or a shallowfrying pan). Hang it at a convenientplace in such a way that it does nottouch any wall. Now strike it with astick (Fig.13.2). Touch the plate orpan gently with your finger. Do youfeel the vibrations?

Fig. 13.2 : Striking a frying pan

Again strike the plate with the stickand hold it tightly with your handsimmediately after striking. Do youstill hear the sound? Touch theplate after it stops producing sound.Can you feel the vibrations now?

The to and fro or back and forthmotion of an object is termed asvibration as you learnt in Class VII.When a tightly stretched band isplucked, it vibrates and producessound. When it stops vibrating, it doesnot produce any sound.

Activity 13.3

Take a metal dish. Pour water in it.Strike it at its edge by a spoon (Fig.13.4). Do you hear a sound? Againstrike the plate and then touch it.Can you feel the dish vibrating?Strike the dish again. Look at thesurface of water. Do you see anywaves there? Now hold the dish.What change do you observe on thesurface of water? Can you explainthe change? Is there a hint toconnect sound with the vibrationsof a body?

Activity 13.2

Take a rubber band. Put it aroundthe longer side of a pencil box(Fig. 13.3). Insert two pencilsbetween the box and the stretchedrubber. Now, pluck the rubber bandsomewhere in the middle. Do youhear any sound? Does the bandvibrate?

Fig. 13.3 : Plucking the rubber band

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We see that a vibrating objectproduces sound. In some cases, thevibrations are easily visible to us. Butin most cases, their amplitude is so smallthat we cannot see them. However, wecan feel them.

Activity 13.4

Take a hollow coconut shell andmake a musical instrument Ektara.You can also make it with the helpof an earthen pot (Fig. 13.5). Playthis instrument and identify itsvibrating part.

Table 13.1 : Musical instruments andtheir Vibrating Parts.

S.No. Musical Vibrating partInstrument producing sound

1. Veena Stretched string

2. Tabla Stretched

membrane

3. Flute Air -column

4. _________ _________

5 . _________ _________

6 . _________ _________

7 . _________ _________

Many of you might have seen themanjira (cymbals), the ghatam, and thenoot (mudpots) and the kartal. Theseinstruments are commonly used inmany parts of our country. Thesemusical instruments are simply beatenor struck. Can you name a few othermusical instruments of this type?

You, too can make a musicalinstrument.

Fig. 13.4 : Vibrating plate produces wavesin water

Fig. 13.6 : A few more musical instruments

Ghatam

Make a list of familiar musicalinstruments and identify their vibratingparts. A few examples are given in Table13.1. Complete the rest of the Table.

Fig. 13.5 : Ektara

Manjira

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13.2 Sound Produced byHumans

Speak loudly for a while or sing a song,or buzz like a bee. Put your hand onyour throat as shown in Fig. 13.8. Doyou feel any vibrations?

In humans, the sound is producedby the voice box or the larynx. Put yourfingers on the throat and find a hardbump that seems to move when youswallow. This part of the body is knownas the voice box. It is at the upper endof the windpipe. Two vocal cords, arestretched across the voice box or larynxin such a way that it leaves a narrowslit between them for the passage of air(Fig.13.8).

Fig. 13.7 : Jaltrang

When we pluck the string of aninstrument, like the sitar, the soundthat we hear is not only that of thestring. The whole instrument is forcedto vibrate, and it is the sound of thevibration of the instrument that wehear. Similarly, when we strike themembrane of a mridangam, the soundthat we hear is not only that of themembrane but of the whole body ofthe instrument.

When we speak, doesany part of our body

vibrate?

Fig.13.8 : Voice box in humans

When the lungs force air through theslit, the vocal cords vibrate, producingsound. Muscles attached to the vocalcords can make the cords tight or loose.When the vocal cords are tight and thin,the type or quality of voice is different

Activity 13.5

Take 6-8 metal bowls or tumblers.Fill them with water up to differentlevels, increasing gradually fromone end to the other. Now takea pencil and strike the bowls gently.Strike all of them in succession. Youwill hear a pleasant sound. This isyour Jaltrang (Fig.13.7).

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from that when they are loose andthick. Let us see how the vocal cordsfunction.

Activity 13.6

Take two rubber strips of the samesize. Place these two pieces oneabove the other and stretch themtight. Now blow air through the gapbetween them [Fig. 13.9(a)]. As theair blows through the stretchedrubber strips, a sound is produced.You can also take a piece of paperwith a narrow slit and hold itbetween your fingers as shown inFig. 13.9 (b). Now blow through theslit and listen to the sound. Ourvocal cords produce sound in asimilar manner.

(a)

(b)

Fig. 13.9 (a), (b) : Working of vocal cords

The vocal cords in men are about20mm long. In women these are about5mm shorter. Children have veryshort vocal cords. This is the reasonwhy the voices of men, women andchildren are different.

13.3 Sound Needs a Mediumfor Propagation

When you call up your friend who isstanding at a distance, your friend isable to hear your voice. How does thesound travel to her?

Activity 13.7

Take a metal glass tumbler. Makesure that it is dry. Place a cell phonein it. (Remember that the cell phonemust not be kept in water.) Ask yourfriend to give a ring on this cellphone from another cell phone.Listen to the ring carefully.Now, surround the rim of thetumbler with your hands (Fig.13.10). Put your mouth on the

Fig. 13.10 : Sound needs a medium to travel

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Place your ear gently on the watersurface (Fig. 13.11). (Be careful :water should not enter in your ear).Can you hear the sound of the bell?Does it indicate that sound cantravel through liquids?

opening between your hands.Indicate to your friend to give a ringagain. Listen to the ring whilesucking air from the tumbler.

Does the sound become fainteras you suck?

Remove the tumbler from yourmouth. Does the sound becomeloud again?

Can you think of an explanation? Isit possible that the decreasing amountof air in the tumbler had something todo with decreasing loudness of the ring?

Indeed, if you had been able to suckall the air in the tumbler, the sound wouldstop completely. Actually, sound needs amedium to travel. When air has beenremoved completely from a vessel, it is saidthat there is vacuum in the vessel. Thesound cannot travel through vacuum.

Does sound travel in liquids? Let usfind out.

Activity 13.8

Take a bucket or a bathtub. Fill itwith clean water. Take a small bellin one hand. Shake this bell insidethe water to produce sound. Makesure that the bell does not touchthe body of the bucket or the tub.

Oh ! That is how whalesand dolphins might becommunicating under

water.

Fig. 13.11 : Sound travelling through water

Fig. 13.12 : Sound travelling through a me-tre scale

Can you hear the sound of thescratch? Ask your friends aroundyou if they were able to hear thesame sound?

Let us find out if sound can travelthrough solids also.

Activity 13.9

Take a metre scale or a long metalrod and hold its one end to yourear. Ask your friend to gentlyscratch or tap the other end of thescale (Fig. 13.12).

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You can also perform the aboveactivity by placing your ear at one endof a long wooden or metallic table andasking your friend to gently scratch theother end of the table (Fig. 13.13).

Fig. 13.13 : Sound can travel through solids

We find that sound can travelthrough wood or metal. In fact, soundcan travel through any solid. You canperform interesting activities to showthat sound can also travel throughstrings. Recall if you made a toytelephone (Fig. 13.14). Can you say thatthe sound can travel through strings?

Fig. 13.14 : A toy telephone

We have learnt so far that vibratingobjects produce sound and it is carriedin all directions in a medium. How dowe hear it?

13.4 We hear Sound throughOur Ears

The shape of the outer part of the ear islike a funnel. When sound enters in it,it travels down a canal at the end ofwhich a thin membrane is stretchedtightly. It is called the eardrum. Itperforms an important function. Tounderstand what the eardrum does, letus build a tin can model of the eardrum.

Activity 13.10

Take a tin can. Cut its ends. Stretcha piece of rubber balloon across oneend of the can and fasten it with arubber band. Put four or five grainsof dry cereal on the stretched rubber.Now ask your friend to speak“Hurrey, Hurrey” from the open end(Fig.13.15). Observe what happensto the grain. Why do the grainsjump up and down?

Fig. 13.15 : Tin can eardrum

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The eardrum is like a stretchedrubber sheet. Sound vibrations makethe eardrum vibrate (Fig. 13.16). Theeardrum sends vibrations to the innerear. From there, the signal goes to thebrain. That is how we hear.

You have already learnt in earlier classesabout the oscillatory motion and itstime period.

The number of oscillations persecond is called the frequency ofoscillation. Frequency is expressed inhertz. Its symbol is Hz. A frequency of1 Hz is one oscillation per second. If anobject oscillates 20 times in one second,what would be its frequency?

You can recognise many familiarsounds without seeing the objectsproducing them. How is it possible?These sounds must be different toenable you to recognise them. Have youever thought what factors make themdifferent? Amplitude and frequency aretwo important properties of any sound.Can we differentiate sounds on the basisof their amplitudes and frequencies?

Loudness and Pitch

Activity 13.11

Take a metallic tumbler and atablespoon. Strike the tablespoongently at the brim of the tumbler.

Fig. 13.16 : Human ear

Fig. 13.17 : Thermocole ball touching thevibrating glass tumbler

We must NEVER put a sharp,pointed or hard thing into our

ear. It can damage theeardrum. The damaged

eardrum can impair hearing.

13.5 Amplitude, Time PeriodAnd Frequency Of AVibration

We have learnt that the to and fro motionof an object is known as vibration. Thismotion is also called oscillatory motion.

Eardrum

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Hear the sound produced. Now bangthe spoon on the tumbler and hearthe sound produced again. Is thesound louder when the tumbler isstruck hard?

Now suspend a small thermocoleball touching the rim of the tumbler(Fig. 13.17). Vibrate the tumbler bystriking it. See how far the ball isdisplaced. The displacement of theball is a measure of the amplitudeof vibration of the tumbler.

Now, strike the tumbler gentlyand then with some force. Comparethe amplitudes of vibrations of thetumbler in the two cases. In whichcase is the amplitude larger?

Loudness of sound is proportional tothe square of the amplitude of thevibration producing the sound. Forexample, if the amplitude becomestwice, the loudness increases by afactor of 4. The loudness is expressedin a unit called decibel (dB). Thefollowing table gives some idea of theloudness of sound coming fromvarious sources.

Normal breathing 10 dB

Soft whisper (at 5m) 30 dB

Normal conversation 60 dB

Busy traffic 70 dB

Average factory 80 dB

Above 80 dB the noise becomesphysically painful.

I wonder why my voiceis different from that of

my teacher.

Compare the sound of a baby withthat of an adult. Is there any difference?Even if two sounds are equally loud,they differ in some way. Let us see how.

Fig. 13.18 : Frequency determines thepitch of a sound

The loudness of sound depends onits amplitude. When the amplitude ofvibration is large, the sound producedis loud. When the amplitude is small,the sound produced is feeble.

The frequency determines theshrillness or pitch of a sound. If thefrequency of vibration is higher we saythat the sound is shrill and has a higherpitch. If the frequency of vibration islower, we say that the sound has a lowerpitch. For example, a drum vibrates witha low frequency. Therefore, it produces

SCIENCE168

a low-pitched sound. On the otherhand, a whistle has a high frequencyand therefore, produces a sound ofhigher pitch (Fig. 13.18). A bird makesa high-pitched sound whereas a lionmakes a low-pitched roar. However, theroar of a lion is very loud while thesound of the bird is quite feeble.

Every day you hear the voices ofchildren and adults. Do you find anydifference in their voices? Can you saythat the frequency of the voice of a childis higher than that of an adult? Usuallythe voice of a woman has a higherfrequency and is shriller than that of aman.

13.6 Audible and InaudibleSounds

We know that we need a vibrating bodyfor the production of sound. Can wehear the sound of all vibrating bodies?

The fact is that sounds of frequenciesless than about 20 vibrations per second(20 Hz) cannot be detected by the humanear. Such sounds are called inaudible.On the higher side, sounds of frequencieshigher than about 20,000 vibrations persecond (20 kHz) are also not audible tothe human ear. Thus, for human ear,the range of audible frequencies isroughly from 20 to 20,000 Hz.

Some animals can hear sounds offrequencies higher than 20,000 Hz.Dogs have this ability. The police usehigh frequency whistles which dogs canhear but humans cannot.

The ultrasound equipment, familiarto us for investigating and trackingmany medical problems, works atfrequencies higher than 20,000 Hz.

13.7 Noise and Music

We hear different types of soundsaround us. Is the sound alwayspleasing? Does a sound sometimescause discomfort to you? Some soundsare pleasant to the ear, whereas someare not.

Suppose construction work is goingon in your neighbourhood. Are thesounds coming from the constructionsite pleasing? Do you enjoy the soundsproduced by horns of buses and trucks?Such unpleasant sounds are callednoise. In a classroom, if all the studentsspeak together, what would the soundproduced be called?

On the other hand you enjoy soundsfrom musical instruments. Musicalsound is one which is pleasing to theear. Sound produced by a harmoniumis a musical sound. The string of a sitaralso gives out a musical sound. But, ifa musical sound becomes too loud,would it remain melodious?

13.8 Noise Pollution

You already know about air pollution.Presence of unwanted gases andparticles in air is called air pollution.Similarly, presence of excessive orunwanted sounds in the environmentis called noise pollution. Can you listsome sources of noise pollution? Majorcauses of noise pollution are sounds ofvehicles, explosions including burstingof crackers, machines, loudspeakers etc.What sources in the home may lead tonoise? Television and transistor radioat high volumes, some kitchenappliances, desert coolers, airconditioners, all contribute to noisepollution.

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Hearing Impairment

The total hearing imparment, which is rare, is usually from birth itself. Partialdisability is generally the result of a disease, injury or age. Children with impairedhearing need special care. By learning sign language, such children cancommunicate effectively. Because speech develops as the direct result of hearing,a child with a hearing loss may have defective speech also. Technological devicesfor the hearing-impared have made it possible for such persons to improve theirquality of life. Society can do much to improve the living environment for thehearing-impaired and help them live normal lives.

What are the harms of noisepollution?

Do you know that presence ofexcessive noise in the surroundings maycause many health related problems.Lack of sleep, hypertension (high blood-pressure), anxiety and many more healthdisorders may be caused by noisepollution. A person who is exposed to aloud sound continuously may gettemporary or even permanentimpairment of hearing.

Measures to Limit Noise Pollution

To control noise, we must control thesources of noise. How can this beachieved? For this, silencing devices

must be installed in air craft engines,transport vehic les, industr ialmachines and home appliances.

How can the noise pollution becontrolled in a residential area?

The noisy operations must beconducted away from any residentialarea. Noise producing industriesshould be set up away from suchareas. Use of automobile hornsshould be minimised. TV and musicsystems should be run at lowvolumes. Trees must be planted alongthe roads and around buildings tocut down on the sounds reaching theresidents, thus reducing the harmfuleffects of noise pollution.

SCIENCE170

KEYWORDS

AMPLITUDE

AUDIBLE

EARDRUM

HERTZ (Hz)

LARYNX

LOUDNESS

NOISE

OSCILLATION

PITCH

SHRILLNESS

TIME PERIOD

VIBRATION

VOICE BOX

WIND PIPE


� Sound is produced by vibrating objects.

� In human beings, the vibration of the vocal

cords produces sound.

� Sound travels through a medium (gas, liquid

or solid). It cannot travel in vacuum.

� The eardrum senses the vibrations of sound,

It sends the signals to the brain. This process

is called hearing.

� The number of oscillations or vibrations per

second is called the frequency of oscillation.

� The frequency is expressed in hertz (Hz)

� Larger the amplitude of vibration, louder is

the sound.

� Higher the frequency of vibration, the higher

is the pitch, and shriller is the sound.

� Unpleasant sounds are called noise.

� Excessive or unwanted sounds lead to noise

pollution. Noise pollution may pose health

problems for human beings.

� Attempts should be made to minimise noise

pollution.

� Plantation on the roadside and elsewhere can

reduce noise pollution.

Exercises

1. Choose the correct answer :Sound can travel through

(a) gases only (b) solids only(c) liquids only (d) solids, liquids and gases.

2. Which of the following voices is likely to have minimum frequency?(a) Baby girl (b) Baby boy(c) A man (d) A woman

SOUND 171

3. In the following statements, tick T against those which are true, and Fagainst those which are false:

(a) Sound cannot travel in vacuum. (T/F)

(b) The number of oscillations per second of a vibrating object is calledits time period. (T/F)

(c) If the amplitude of vibration is large, sound is feeble. (T/F)

(d) For human ears, the audible range is 20 Hz to 20,000 Hz. (T/F)

(e) The lower the frequency of vibration, the higher is the pitch. (T/F)

(f) Unwanted or unpleasant sound is termed as music. (T/F)

(g) Noise pollution may cause partial hearing impairment. (T/F)

4. Fill in the blanks with suitable words.

(a) Time taken by an object to complete one oscillation is called................... .

(b) Loudness is determined by the .........................of vibration.

(c) The unit of frequency is..........................

(d) Unwanted sound is called ......................... .

(e) Shrillness of a sound is determined by the ................. of vibration.

5. A pendulum oscillates 40 times in 4 seconds. Find its time period andfrequency.

6. The sound from a mosquito is produced when it vibrates its wings at anaverage rate of 500 vibrations per second. What is the time period of thevibration?

7. Identify the part which vibrates to produce sound in the followinginstruments:

(a) Dholak (b) Sitar (c) Flute

8. What is the difference between noise and music? Can music become noisesometimes?

9. List sources of noise pollution in your surroundings.

10. Explain in what way noise pollution is harmful to humans.

11. Your parents are going to buy a house. They have been offered one on theroadside and another three lanes away from the roadside. Which housewould you suggest your parents should buy? Explain your answer.

12. Sketch larynx and explain its function in your own words.

13. Lightning and thunder take place in the sky at the same time and at thesame distance from us. Lightning is seen earlier and thunder is heardlater. Can you explain?

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1. Visit the music room of your school. You may also visit musiciansin your locality. Make a list of musical instruments. Note down theparts of these instruments that vibrate to produce sound.

2. If you play a musical instrument, bring it to the class anddemonstrate how you play it.

3. Prepare a list of famous Indian musicians and the instrumentsthey play.

4. Take a long thread. Place your hands over your ears and get someone to place this thread round your head and hands. Ask her tomake the thread taut and hold its ends in one hand. Now ask herto draw her finger and thumb tightly along the thread (Fig. 13.19).Can you hear a rolling sound like that of a thunder? Now repeatthe activity while another friend stands near both of you. Can hehear any sound?

Fig. 13.19

5. Make two toy telephones. Use them as shown in Fig. 13.20. Makesure that the two strings are taut and touch each other. Let one ofyou speak. Can the remaining three persons hear? See how manymore friends you can engage in this way. Explain your observations.

Fig. 13.20

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6. Identify the sources of noise pollution in your locality. Discuss withyour parents, friends and neighbours. Suggest how to control noisepollution. Prepare a brief report and present it in the class.

You can read more on the related topics on the following websites:

� www.physicsclassroom.com/Class/sound/soundtoc.html� health.howstuffworks.com/hearing.htm� www.jaltarang.com for jaltarang� www.tempro/com/articles/hearing.html� www.cartage.org.lb/en/themes/sciences/physics/mainpage.htm

Golconda fort

Did You Know

Golconda fort, near Hyderabad, is one of the most magnificientforts in India. It is famous for many engineering and architecturalmarvels. One of the marvels is the water supply system. But,perhaps, more astonishing is a dome near the entrance to the fort.A hand-clap at a particular point under the dome reverberatesand can be heard at the highest point of the fort, about a kilometreaway. This was devised as a warning system. If a guard saw asuspicious movement outside the fort, he clapped at the particularpoint under the dome, and the army inside the fort was alerted tothe danger of the approaching enemy.

CHEMICAL EFFECTS OF ELECTRIC

CURRENT

CHEMICAL EFFECTS OF ELECTRIC

CURRENT

Your elders might have cautionedyou against touching an electricalappliance with wet hands. But

do you know why it is dangerous totouch an electrical appliance with wethands?

We have learnt earlier that thematerials, which allow electric currentto pass through them, are goodconductors of electricity. On the otherhand, materials, which do not allowelectric current to pass through themeasily, are poor conductors of electricity.

In Class VI, we made a tester(Fig.14.1) to test whether a particularmaterial allows the electric current topass through it or not. Do you recallhow the tester helped us in decidingthat?

We found that metals such as copperand aluminium conduct electricitywhereas materials such as rubber,

plastic and wood do not conductelectricity. However, so far we have usedour tester to test materials which werein solid state. But what about liquids?Do liquids also conduct electricity? Letus find out.

Paheli and Boojho want toremind you that one

should not experimentwith the electric supply

from the mains or agenerator or an inverter.Use only electric cells for

all the activities suggestedhere.

Fig.14.1 : A tester

14.1 Do Liquids ConductElectricity?

To test whether a liquid allows electriccurrent to pass through it or not, wecan use the same tester (Fig. 14.1) which

we made in Class VI. However, replacethe cell by a battery. Also, before usingthe tester we should check whether it isworking or not.

Activity 14.1

Join the free ends of the testertogether for a moment. Thiscompletes the circuit of the testerand the bulb should glow. However,if the bulb does not glow, it meansthat the tester is not working. Canyou think of the possible reasons?Is it possible that the connectionsare loose? Or, the bulb is fused? Or,your cells are used up? Check thatall the connections are tight. If theyare, then replace the bulb withanother bulb. Now test if the testeris working or not. If it is still notworking then replace the cells withfresh cells.

Now that our tester is working, letus use it to test the various liquids.

(Caution: While checking your tester,do not join its free ends for more thana few seconds. Otherwise the cells ofthe battery will drain very quickly.)

Activity 14.2

Collect a few small plastic or rubbercaps of discarded bottles and cleanthem. Pour one teaspoon of lemonjuice or vinegar in one cap. Bringyour tester over this cap and let theends of the tester dip into lemon juiceor vinegar as shown in Fig.14.2. Takecare that the ends are not more than1 cm apart but at the same time donot touch each other. Does the bulb

Fig. 14.2 : Testing conduction of electricity inlemon juice or vinegar

of the tester glow? Does lemon juiceor vinegar conduct electricity? Howwould you classify lemon juice orvinegar— a good conductor or a poorconductor?

When the liquid between the twoends of the tester allows the electriccurrent to pass, the circuit of the testerbecomes complete. The current flows inthe circuit and the bulb glows. Whenthe liquid does not allow the electriccurrent to pass, the circuit of the testeris not complete and the bulb does notglow.

In some situations even though theliquid is conducting, the bulb may notglow. It may have happened in Activity14.2. What can be the reason?

Do you remember why the bulbglows when the electric current passesthrough it? Due to the heating effectof current, the filament of the bulb getsheated to a high temperature and itstarts glowing. However, if the currentthrough a circuit is too weak, thefilament does not get heated


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Activity 14.3

Take the tray from inside a discardedmatchbox. Wrap an electric wire a fewtimes around the tray. Place a smallcompass needle inside it. Nowconnect one free end of the wire tothe terminal of a battery. Leave theother end free. Take another piece ofwire and connect it to the otherterminal of the battery (Fig. 14.4).

Fig 14.4 : Another tester

sufficiently and it does not glow. Andwhy is the current in the circuit weak?Well, though a material may conductelectricity, it may not conduct it aseasily as a metal. As a result, thecircuit of the tester may be completeand yet the current through it may betoo weak to make the bulb glow. Canwe make another tester which candetect a weak current?

You may use an LED (Fig. 14.3) inplace of the electric bulb in the testerof Fig. 14.2. LED glows even when aweak electric current flows throughit.

There are two wires (called leads)attached to an LED. One lead isslightly longer than the other.Remember that while connecting toa circuit, the longer lead is alwaysconnected to the positive terminal ofthe battery and the shorter lead isconnected to the negative terminal ofthe battery.

Fig. 14.3 : LEDs

We can use another effect of an electriccurrent to make another kind of tester.Do you recall that electric currentproduces a magnetic effect? Whathappens to a compass needle kept nearbywhen current flows in a wire? Even ifthe current is small, the deflection of themagnetic needle can be seen. Can wemake a tester using the magnetic effectof currents? Let us find out.

Join the free ends of two wiresmomentarily. The compass needleshould show deflection. Your testerwith two free ends of the wire is ready.

Now repeat Activity 14.2 using thistester. Do you find a deflection in thecompass needle the moment you dipthe free ends of the tester in lemonjuice?

Take out the ends of the testerfrom the lemon juice, dip them inwater and then wipe them dry.Repeat the activity with other liquidssuch as tap water, vegetable oil, milk,honey. (Remember to wash and wipedry the ends of tester after testing eachliquid). In each case observe whetherthe magnetic needle shows deflectionor not. Record your observations inTable 14.1.


Table 14.1 : Good/Poor Conducting Liquids

S.No Material Compass Needle Shows Good Conductor/Deflection Yes/No Poor Conductor

1. Lemon juice Yes Good Conductor

2. Vinegar

3. Tap Water

4. Vegetable oil

5. Milk

6. Honey

7.

8.

9.

10.

When the free ends of thetester do not touch each

other, there is an air gapbetween them. Paheli knows

that air is a poor conductor ofelectricity. But she has also readthat during lightning, an electriccurrent passes through air. Shewonders if air is indeed a poorconductor under all conditions.This makes Boojho ask whether

other materials classified aspoor conductors also allowelectricity to pass under

certain conditions.

From Table 14.1, we find that someliquids are good conductors of electricityand some are poor conductors.

Actually, under certain conditionsmost materials can conduct. That is whyit is preferable to classify materials asgood conductors and poor conductorsinstead of classifying as conductors andinsulators.

We have tested the conduction ofelectricity through tap water. Let us nowtest the conduction of electricity throughdistilled water.

Activity 14.4

Take about two teaspoonfuls ofdistilled water in a clean and dryplastic or rubber cap of a bottle. (Youmay obtain distilled water from yourschool science lab. You may also getdistilled water from a medical storeor a doctor or a nurse). Use the testerto test whether distilled waterconducts electricity or not. What doyou find? Does distilled waterconduct electricity? Now dissolve apinch of common salt in distilledwater. Again test. What do youconclude this time?

When salt is dissolved in distilledwater, we obtain salt solution. This is aconductor of electricity.

The water that we get from sourcessuch as taps, hand pumps, wells andponds is not pure. It may contain several

SCIENCE178

salts dissolved in it. Small amounts ofmineral salts are naturally present in it.This water is thus a good conductor ofelectricity. On the other hand, distilledwater is free of salts and is a poorconductor.

Most liquids that conduct electricityare solutions of acids, bases and salts.

When an electric current flowsthrough a conducting solution, does itproduce an effect on the solution?

14.2 Chemical Effects ofElectric Current

In Class VII, we have learnt some effectsof electric current. Can you list theseeffects? What effect does the currentproduce when it flows through aconducting solution? Let us find out.

Activity 14.6

Take out carbon rods carefully fromtwo discarded cells. Clean their metalcaps with sand paper. Wrap copperwires around the metal caps of thecarbon rods and join them to abattery (Fig. 14.5). We call these tworods electrodes. (Instead of carbon

Small amounts of mineral saltspresent naturally in water arebeneficial for human health.

However, these salts make waterconducting. So, we should never

handle electrical applianceswith wet hands or whilestanding on a wet floor.

We have found that common salt, whendissolved in distilled water, makes it a goodconductor. What are the other substanceswhich, when dissolved in distilled water,make it conducting? Let us find out.

Caution: Do the next activity under thesupervision of your teacher/parent orsome elderly person, because the use ofacid is involved in it.

Activity 14.5

Take three clean plastic or rubber capsof bottles. Pour about two teaspoonfulsof distilled water in each of them. Adda few drops of lemon juice or dilutehydrochloric acid to distilled water inone cap. Now in the second capcontaining distilled water, add a fewdrops of a base such as caustic sodaor potassium iodide. Add a little sugarto the distilled water in the third capand dissolve it. Test which solutionsconduct electricity and which do not.What results do you obtain? Fig.14.5 : Passing current through water

Metal Cap

Carbonrod

Water

Carbonrod


Boojho decided to testwhether some fruits andvegetables also conductelectricity or not. He cut

a potato into two halves andinserted the copper wires of a testerinto it. Just then his mother calledhim and he forgot to take out thewires of the tester inserted into thepotato. When he came back afterhalf an hour, he noticed that therewas a greenish blue spot on thepotato around one wire whereasthere was no such spot around theother wire (Fig. 14.6).

rods, you may take two iron nailsabout 6 cm long ). Pour a cupful ofwater in a glass/plastic bowl. Add ateaspoonful of salt or a few drops oflemon juice to water to make it moreconducting. Now immerse theelectrodes in this solution. Make surethat the metal caps of the carbon rodsare outside the water. Wait for 3-4minutes. Observe the electrodescarefully. Do you notice any gasbubbles near the electrodes? Can wecall the change taking place in thesolution a chemical change? Recallthe definition of a chemical changethat you learnt in Class VII.

In 1800, a British chemist, WilliamNicholson (1753–1815), had shown

that if electrodeswere immersedin water, and acurrent was passed,bubbles of oxygenand hydrogen wereproduced. Oxygenbubbles formedon the electrode

connected to the positive terminal ofthe battery and hydrogen bubblesformed on the other electrode.

The passage of an electric currentthrough a conducting solution causeschemical reactions. As a result, bubblesof a gas may be formed on the electrodes.Deposits of metal may be seen onelectrodes. Changes of colour ofsolutions may occur. The reaction woulddepend on what solution and electrodesare used. These are some of the chemicaleffects of the electric current.

Fig. 14.6 : Testing potato

He was surprised with thisobservation and along with Pahelirepeated this activity many times.They found that it was always the wireconnected to the positive terminal,which had greenish blue spot aroundit. They felt that this discovery wasvery useful because it could be usedfor identifying the positive terminalof a cell or a battery concealed in abox. They decided to report theirfinding to a children’s magazine.

Remember that Boojho set out totest whether potato conducted

SCIENCE180

Allow the current to pass forabout 15 minutes. Now remove theelectrodes from the solution andlook at them carefully. Do you findany difference in any one of them?Do you find a coating over it? Whatcolour is the coating? Note down theterminal of the battery with whichthis electrode is connected.

electricity or not. What he found wasthat current produced a chemicaleffect in the potato. To him this wasvery exciting. In fact, this is howscience sometimes works. You arelooking for something and youdiscover something else. Manyimportant discoveries have beenmade in this manner.

14.3 Electroplating

Recall that a brand new bicycle has shinyhandlebar and wheel rims. However, ifthese are accidentally scratched, theshiny coating comes off revealing a notso shiny surface beneath. You mighthave also seen women using ornaments,which appear to be made of gold.However, with repeated use, the goldcoating wears off, revealing silver or someother metal beneath.

In both these cases, a metal has acoating of another metal. Do you wonderhow a layer of one metal can bedeposited on top of another? Well, letus try doing it ourselves.

Activity 14.7

We will need copper sulphate andtwo copper plates of size around 10cm × 4 cm. Take 250 mL of distilledwater in a clean and dry beaker.Dissolve two teaspoonfuls of coppersulphate in it. Add a few drops ofdilute sulphuric acid to coppersulphate solution to make it moreconducting. Clean copper plates withsand paper. Now rinse them withwater and dry them. Connect thecopper plates to the terminals of abattery and immerse them in coppersulphate solution (Fig. 14.7).

When electric current is passedthrough the copper sulphate solution,copper sulphate dissociates intocopper and sulphate. The free coppergets drawn to the electrode connectedto the negative terminal of the batteryand gets deposited on it. But whatabout the loss of copper from thesolution?

From the other electrode, a copperplate, an equal amount of copper getsdissolved in the solution. Thus, the

After doing the electroplatingactivity, Paheli interchangedthe electrodes and repeated

the activity. What do you thinkshe would observe this time?

Fig.14.7 : A simple circuit showing electroplating

Copper plate

Coppersulphatesolution

Copper plate


loss of copper from the solution isrestored and the process keeps going.This means that copper getstransferred from one electrode to theother.

In the electroplating factories thedisposal of the used conductingsolution is a major concern. It is apolluting waste and there are specificdisposal guidelines to protect theenvironment.

Fig. 14.8 : Some electroplated objects

silver and gold on less expensive metals.These ornaments have the appearanceof silver or gold but are much lessexpensive.

Tin cans, used for storing food, aremade by electroplating tin onto iron. Tinis less reactive than iron. Thus, fooddoes not come into contact with iron andis protected from getting spoilt.

Iron is used in bridges andautomobiles to provide strength.However, iron tends to corrode and rust.So, a coating of zinc is deposited on ironto protect it from corrosion andformation of rust.

Boojho could get only onecopper plate. So he

performed Activity 14.7 byconnecting a carbon rod inplace of the copper platewhich was connected tothe negative terminal of

the battery. He succeededin obtaining a coating of

copper on carbon rod.

The process of depositing a layer ofany desired metal on another materialby means of electricity is calledelectroplating. It is one of the mostcommon applications of chemical effectsof electric current.

Electroplating is a very usefulprocess. It is widely used in industryfor coating metal objects with a thinlayer of a different metal (Fig.14.8). Thelayer of metal deposited has somedesired property, which the metal of theobject lacks. For example, chromiumplating is done on many objects suchas car parts, bath taps, kitchen gasburners, bicycle handlebars, wheel rimsand many others.

Chromium has a shiny appearance.It does not corrode. It resists scratches.However, chromium is expensive and itmay not be economical to make thewhole object out of chromium. So theobject is made from a cheaper metal andonly a coating of chromium over it isdeposited. Jewellery makers electroplate

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KEYWORDS

ELECTRODE

ELECTROPLATING

GOOD CONDUCTOR

LED

POOR CONDUCTOR


� Some liquids are good conductors of electricity

and some are poor conductors.

� Most liquids that conduct electricity are

solutions of acids, bases and salts.

� The passage of an electric current through a

conducting liquid causes chemical reactions.

The resulting effects are called chemical effects

of currents.

� The process of depositing a layer of any desired

metal on another material, by means of

electricity, is called electroplating.

Exercises

1. Fill in the blanks

(a) Most liquids that conduct electricity are solutions of ,______________ and ______________.

(b) The passage of an electric current through a solution causes______________ effects.

(c) If you pass current through copper sulphate solution, copper getsdeposited on the plate connected to the terminal of thebattery.

(d) The process ofdepositing a layer ofany desired metal onanother material bymeans of electricity iscalled .

2. When the free ends of a testerare dipped into a solution,the magnetic needle showsdeflection. Can you explainthe reason?

3. Name three liquids, whichwhen tested in the mannershown in Fig.14.9, maycause the magnetic needleto deflect.

Fig. 14.9


4. The bulb does not glow in the setup shown in Fig.14.10. List the possiblereasons. Explain your answer.

Fig. 14.10

5. A tester is used to check the conduction of electricity through two liquids,labeled A and B. It is found that the bulb of the tester glows brightly forliquid A while it glows very dimly for liquid B. You would conclude that(i) liquid A is a better conductor than liquid B.(ii) liquid B is a better conductor than liquid A.(iii) both liquids are equally conducting.(iv) conducting properties of liquid cannot be compared in this manner.

6. Does pure water conduct electricity? If not, what can we do to make itconducting?

7. In case of a fire, before the firemen use the water hoses, they shut off themain electrical supply for the area. Explain why they do this.

8. A child staying in a coastal region tests the drinking water and also theseawater with his tester. He finds that the compass needle deflects morein the case of seawater. Can you explain the reason?

9. Is it safe for the electrician to carry out electrical repairs outdoors duringheavy downpour? Explain.

10. Paheli had heard that rainwater is as good as distilled water. So shecollected some rainwater in a clean glass tumbler and tested it using atester. To her surprise she found that the compass needle showeddeflection. What could be the reasons?

11. Prepare a list of objects around you that are electroplated.

12. The process that you saw in Activity 14.7 is used for purification of copper.A thin plate of pure copper and a thick rod of impure copper are used aselectrodes. Copper from impure rod is sought to be transfered to the thincopper plate. Which electrode should be attached to the positive terminalof battery and why?

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1. Test the conduction of electricity through various fruits andvegetables. Display your result in a tabular form.

2. Repeat the Activity 14.7 with a zinc plate in place of the copperplate connected to the negative terminal of the battery. Now replacezinc plate with some other metallic object and again repeat theactivity. Which metal gets deposited over which other metal? Discussyour findings with your friends.

3. Find out if there is a commercial electroplating unit in your town.What objects are electroplated there and for what purpose? (Theprocess of electroplating in a commercial unit is much more complexthan what we did in Activity 14.7). Find out how they dispose off thechemicals they discard.

4. Imagine that you are an ‘entrepreneur’ and have been provided aloan by a bank to set up a small electroplating unit. What objectyou would like to electroplate and for what purpose? (Look up themeaning of ‘entrepreneur’ in a dictionary).

5. Find out the health concerns associated with chromiumelectroplating. How are people trying to resolve them?

6. You can make a fun pen for yourself. Take a conducting metal plateand spread a moist paste of Potassium Iodide and starch. Connectthe plate to a battery as shown in Fig. 14.11. Now using the freeend of the wire, write a few letters on the paste. What do you see?

Fig. 14.11


Did You Know?

LEDs (Light Emitting Diodes) are available inmany colours such as red, green, yellow, blue,white and are increasingly being used for manyapplications, for example in traffic signal lights.LEDs are increasingly being used for lighting. Acluster of white LEDs grouped together forms aLED light source. LED light sources consume lesselectricity and have longer lifetime than lightbulbs and fluorescent tubes. But LED lightsources are expensive, so CFLs are currently thebest choice. However, CFLs contain mercurywhich is toxic. Therefore, used or broken CFLsneed to be disposed off safely. Once thetechnological advances reduce the cost of LEDs,they will become the preferred lighting source.

For more information on this topic visit:� www.tutorvista.com/content/physics/physics-iv/thermal-

chemical-currents/chemical-effects-current.php� www.physchem.co.za/Redox/Electrolysis.htm� electronics.howstuffworks.com/led.htm

SOME NATURAL PHENOMENASOME NATURAL PHENOMENA

In Class VII you read about winds,storms and cyclones. You learnt thatcyclones can cause a lot of damage tohuman life and property. You alsolearnt that we can protect ourselves fromthese destructive phenomena to someextent. In this chapter we shall discusstwo other destructive naturalphenomena. These are lightning andearthquakes. We shall also discuss whatsteps we can take to minimisedestruction caused by thesephenomena.

15.1Lightning

You might have seen sparks on aelectric pole when wires become loose.This phenomenon is quite commonwhen a wind is blowing and shackingthe wires. You might also have seensparks when a plug is loose in itssocket. Lightning is also an electricspark, but on a huge scale.

In ancient times people did notunderstand the cause of these sparks.They were, therefore, afraid of lightningand thought that the wrath of gods wasvisiting them. Now, of course, weunderstand that lightning is caused bythe accumulation of charges in theclouds. We need not be afraid oflightning, but we have to take

precautions to protect ourselves fromthe deadly sparks.

The Sparks that the Greeks KnewAbout

The ancient Greeks knew as early as600 B.C. that when amber (amber is akind of resin) was rubbed with fur, itattracted light objects such as hair. Youmight have seen that when you takeoff woollen or polyester clothes, yourhair stands on ends. If you take offthese clothes in the dark, you see evena spark and hear crackling sound. In1752 Benjamin Franklin, an Americanscientist, showed that lightning and thespark from your clothes are essentiallythe same phenomena. However, thisrealisation took 2000 years.

Scientific discoveries are aresult of hardwork by many

people. It can sometime takesa long time.

I wonder why they took somany years to realise the

similarity.

We shall now study some propertiesof electric charges. We shall also see howthey are related to the lightning in thesky.

Let us perform some activities tounderstand the nature of electriccharges. But recall first what you mighthave played as a game. When you ruba plastic scale on your dry hair, thescale can attract very small pieces ofpaper.

15.2 Charging by rubbing

Activity 15.1

Take a used ballpen refill and rubit vigorously with a piece ofpolythene. Bring it close to smallpieces of paper. Take care not totouch the rubbed end of the refillwith your hand or with a metallicobject. Repeat the activity with smallpieces of dry leaf, husk and mustardseeds. Record your observations.

When a plastic refill is rubbed withpolythene, it acquires a small electriccharge. Similarly, when a plastic combis rubbed with dry hair, it acquires asmall charge. These objects are calledcharged objects. In the process ofcharging the refill and the plastic comb,polythene and hair also get charged.

Let’s try to charge some otherobjects that are familiar to you.

Activity 15.2

Collect the objects and thematerials listed in Table 15.1. Tryto charge each by rubbing with thematerials mentioned in the Table.Record your findings.You can add more items to theTable.

Table 15.1

Objects rubbed Materials Attracts/does not Chargedused for rubbing attract pieces /not charged

of paper

Refill Polythene,woollen cloth

Balloon Polythene, woollencloth, dry hair

Eraser Wool

Steel spoon Polythene,

woollen cloth


SCIENCE188

15.3 Types of Charges andtheir Interaction

We will select some objects from Table15.1 for the next activity.

Activity 15.3

a) Inflate two balloons. Hang themin such a way that they do not toucheach other (Fig. 15.1). Rub both theballoons with a woollen cloth andrelease them. What do you observe?

refill also with polythene. Bring itclose to the charged refill. Be carefulnot to touch the charged end withyour hand. Is there any effect onthe refill in the tumbler? Do the twoattract each other, or repel eachother?In this activity we have brought closetogether the charged objects thatwere made of the same material.What happens if two charged objectsmade of different materials arebrought close to each other? Let’sfind out.b) Rub a refill and place it gently ina glass tumbler as before (Fig. 15.3).Bring an inflated charged balloonnear the refill and observe.

Fig. 15.1 : Like charges repel each other

Fig. 15.2 : Interaction between like charges

Now let us repeat this activity withthe used pen refills. Rub one refillwith polythene. Place it carefully ina glass tumbler using the tumbleras a stand (Fig. 15.2). Rub the other

Fig. 15.3 : Unlike charges attract each other

Let’s summarise the observations:A charged balloon repelled a charged

balloon.A charged refill repelled a charged

refill.But a charged balloon attracted a

charged refill.Does it indicate that the charge on

the balloon is of a different kind fromthe charge on the refill? Can we saythen, that there are two kinds of


charges? Can we also say that thecharges of the same kind repel eachother, while charges of different kindattract each other?

It is a convention to call the chargeacquired by a glass rod when it isrubbed with silk as positive. Theother kind of charge is said to benegative.

It is observed that when a chargedglass rod is brought near a chargedplastic straw rubbed with polythenethere is attraction between the two.

What do you think would be the kindof charge on the plastic straw? Yourguess, that the plastic straw would carrya negative charge is correct.

The electrical charges generated byrubbing are static. They do not move bythemselves. When charges move, theyconstitute an electric current. You havebeen reading about electric current sinceClass VI. The current in a circuit whichmakes a bulb glow, or the current thatmakes a wire hot, is nothing but amotion of charges.

15.4Transfer of Charge

Activity 15.4

Take an empty jam bottle. Take apiece of cardboard slightly bigger insize than the mouth of the bottle.Pierce a hole in it so that a metalpaper clip could be inserted. Openout paper clip as shown in Fig. 15.4.Cut two strips of aluminium foilabout 4 cm × 1 cm each. Hang themon the paper clip as shown. Insertthe paper clip in the cardboard lidso that it is perpendicular to it (Fig.15.4). Charge a refill and touch it

Fig 15.4 : A simple electroscope

with the end of the paper clip.Observe what happens. Is there anyeffect on the foil strips? Do theyrepel each other or attract eachother? Touch now, other chargedbodies with the end of the paper clip.Do foil strips behave in the sameway in all cases? Can this apparatusbe used to detect whether a body ischarged or not? Can you explainwhy the foil strips repel each other?

The aluminium foil strips receive thesame charge from the charged refillthrough the paper clip (remember thatmetals are good conductors ofelectricity). The strips carrying similarcharges repel each other and theybecome wide open. Such a device canbe used to test whether an object iscarrying charge or not. This device isknown as electroscope.

Thus, we find that electrical chargecan be transferred from a charged objectto another through a metal conductor.

Touch the end of the paper clip gentlywith hand and you will find a change in

SCIENCE190

the foil strips. They come back to theiroriginal state. Repeat charging of foilstrips and touching the paper clip. Everytime you will find that the foil stripscollapse as soon as you touch thepaperclip with hand. Why does ithappen? The reason is that the foil stripslose charge to the earth through yourbody. We say that the foil strips aredischarged. The process of transferingof charge from a charged object to theearth is called earthing.

Earthing is provided in buildings toprotect us from electrical shocks dueto any leakage of electrical current.

15.5 The Story of Lightning

It is now possible to explain lightningin terms of the charges produced byrubbing.

You learnt in Class VII that duringthe development of a thunderstorm, theair currents move upward while thewater droplets move downward. Thesevigorous movements cause separationof charges. By a process, not yetcompletely understood, the positivecharges collect near the upper edges ofthe clouds and the negative chargesaccumulate near the lower edges. Thereis accumulation of positive charges nearthe ground also. When the magnitudeof the accumulated charges becomes verylarge, the air which is normally a poorconductor of electricity, is no longer ableto resist their flow. Negative and positivecharges meet, producing streaks ofbright light and sound. We see streaksas lightning (Fig. 15.5). The process iscalled an electric discharge.

Fig 15.5 : Accumulation of charges leading to lightning.


The process of electric discharge canoccur between two or more clouds, orbetween clouds and the earth. Today weneed not get frightened by lightning likethe ancient people did. Now weunderstand the basic phenomenon.Scientists are trying hard to improve ourunderstanding. However, lightningstrike could destroy life and property. Itis, therefore, necessary to take measuresto protect ourselves.

15.6 Lightning Safety

During lightning and thunderstorm noopen place is safe.� Hearing thunder is an alert to rush

to a safer place.� After hearing the last thunder, wait

for some time before coming out ofthe safe place.

Finding a safe place

A house or a building is a safe place.If you are travelling by car or by bus,

you are safe inside with windows anddoors of the vehicle shut.

Do’s and Don’ts during a Thunder-storm

OutsideOpen vehicles, like motorbikes, tractors,construction machinery, open cars arenot safe. Open fields, tall trees, sheltersin parks, elevated places do not protectus from lightning strokes.

Carrying umbrella is not a good ideaat all during thunderstorms.

If in a forest, take shelter undershorter trees.

If no shelter is available and you arein an open field, stay far away from alltrees. Stay away from poles or other

metal objects. Do not lie on the ground.Instead, squat low on the ground. Placeyour hands on your knees with yourhead between the hands (Fig. 15.6). Thisposition will make you the smallesttarget to be struck.

Fig. 15.6 : Safe position during lightning

Inside the houseLightning can strike telephone cords,electrical wires and metal pipes (Do youremember, lightning is an electricaldischarge?). During a thunderstormcontact with these should be avoided. Itis safer to use mobile phones andcordless phones. However, it is not wiseto call up a person who is receiving yourphone through a wired phone.

Bathing should be avoided duringthunderstorms to avoid contact withrunning water.

Electrical appliances like computers,TVs, etc., should be unplugged. Electricallights can remain on. They do not causeany harm.

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Lightning Conductors

Lightning Conductor is a device usedto protect buildings from the effect oflightning. A metallic rod, taller than thebuilding, is installed in the walls of thebuilding during its construction. Oneend of the rod is kept out in the air andthe other is buried deep in the ground(Fig. 15.7). The rod provides easy routefor the transfer of electric charge to theground.

The metal columns used duringconstruction, electrical wires and waterpipes in the buildings also protect us toan extent. But do not touch them duringa thunderstorm.

15.7Earthquakes

You just learnt about thunderstorm andlightning. In Class VII you learnt aboutcyclones. These natural phenomena cancause large scale destruction of humanlife and property. Fortunately, thesephenomena can be predicted to someextent. The weather department canwarn about a thunderstorm developingin some area.

If a thunderstorm occurs there isalways a possibility of lightning andcyclones accompanying it. So, we gettime to take measures to protectourselves from the damage caused bythese phenomena.

Fig. 15.7 : Lightning conductor

lightningconductor

copperplate


There is, however, one naturalphenomenon which we are not yet ableto predict. It is an earthquake. It cancause damage to human life andproperty on a huge scale.

A major earthquake occurred in Indiaon 8th October 2005 in Uri andTangdhar towns of North Kashmir (Fig.15.8). Before that a major earthquakeoccurred on 26th January 2001 in BhujDistrict of Gujarat.

Activity 15.5

Ask your parents about the hugedamages to life and property causedby these earthquakes. Collect a fewpictures showing the damage

caused by these earthquakes fromnewspapers and magazines of thosedays. Prepare a short report on thesuffering of the people during theearthquakes.

What is an earthquake? Whathappens when it occurs? What can wedo to minimise its effects? These aresome of the questions which we shalldiscuss below.

What is an Earthquake?

An earthquake is a sudden shaking ortrembling of the earth lasting for a veryshort time. It is caused by a disturbancedeep inside the earth’s crust.

Fig. 15.8 : Kashmir earthquake

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My grandmother told methat the earth is balancedon the horn of a bull andwhen the bull shifts it to

the other horn, anearthquake takes place.

How could it be true?

Earthquakes occur all the time, all overthe earth. They are not even noticed.Major earthquakes are much lessfrequent. They can cause immensedamage to buildings, bridges, dams andpeople. There can be a great loss to lifeand property. The earthquakes cancause floods, landslides and tsunamis.A major tsunami occurred in the IndianOcean on 26th December 2004. All thecoastal areas around the ocean sufferedhuge losses.

Activity 15.6

Take an outline map of the world.Locate the eastern coast andAndaman and Nicobar Islands inIndia. Mark other countries aroundthe Indian Ocean which could havesuffered damage. Collect accountsof the devastation caused by thetsunami in India from your parents,or other elders in the family or inthe neighbourhood.

What Causes an Earthquake?

Fig. 15.10 : Earth plates

What could cause adisturbance inside

the earth?

Fig. 15.9 : Structure of the earth

Now we know that the tremors arecaused by the disturbance deep downinside the uppermost layer of the earthcalled the crust (Fig. 15.9).

crust

mantle

outercore

innercore

In ancient times, people did not knowthe true cause of earthquakes. Theirideas were, therefore, expressed inmythical stories such as the one told byBoojho’s grandmother. Similar mythswere prevalent in other parts of theworld.

The outermost layer of the earth isnot in one piece. It is fragmented. Eachfragment is called a plate (Fig. 15.10).These plates are in continual motion.When they brush past one another, or

ArabianPlate

EurasianPlate


a plate goes under another due tocollision (Fig. 15.11), they causedisturbance in the earth’s crust. It isthis disturbance that shows up as anearthquake on the surface of the earth.

carried out. However, most earthquakesare caused by the movement of earth’splates.

Since earthquakes are caused by themovement of plates, the boundaries ofthe plates are the weak zones whereearthquakes are more likely to occur.The weak zones are also known asseismic or fault zones. In India, theareas most threatened are Kashmir,Western and Central Himalayas, thewhole of North-East, Rann of Kutch,Rajasthan and the Indo – GangeticPlane. Some areas of South India alsofall in the danger zone (Fig. 15.12).

If scientists know so muchabout earthquakes, can

they also predict when andwhere the next one will

strike?

I read somewhere thatunderground explosions

could also causetremors.

Fig. 15.11 : Movements of earth’s plates

Fig. 15.12 : Movements of Indian earth’s plate

The power of an earthquake isexpressed in terms of a magnitude on ascale called Richter scale. Reallydestructive earthquakes havemagnitudes higher than 7 on theRichter scale. Both Bhuj and Kashmirearthquakes had magnitudes greaterthan 7.5.

Although, we know for sure whatcauses an earthquake, it is not yetpossible to predict when and where thenext earthquake might occur.

Tremors on the earth can also becaused when a volcano erupts, or ameteor hits the earth, or anunderground nuclear explosion is

Colliding

Brushingpast

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The tremors produce waves on the surface of the earth. These are called seismicwaves. The waves are recorded by an instrument called the seismograph(Fig. 15.13). The instrument is simply a vibrating rod, or a pendulum, whichstarts vibrating when tremors occur. A pen is attached to the vibrating system.The pen records the seismic waves on a paper which moves under it. By studyingthese waves, scientists can construct a complete map of the earthquake, as shown

in Fig. 15.14. They can also estimateits power to cause destruction.

Fig. 15.13 : A seismograph

A typical seismograph record

Fig. 15.14 : Map of the earthquake

Epicentre

Seismicwaves

Focus

Inside the earth

Earth’sSurface

Like many other scales in science (decibel is another example), Richter scale isnot linear. This means that an earthquake of magnitude 6 does not have one andhalf times the destructive energy of an earthquake of magnitude 4. In fact, anincrease of 2 in magnitude means 1000 times more destructive energy. Forexample, an earthquake of magnitude 6 has thousand times more destructiveenergy than an earthquake of magnitude 4.

Protection against Earthquakes

We know from the earlier pages thatearthquakes cannot be predicted. Wehave also seen that they can be highlydestructive. It is, therefore, importantthat we take necessary precautions toprotect ourselves all the time. Peopleliving in seismic zones, where theearthquakes are more likely to occur,

have to be specially prepared. First ofall, the buildings in these zones shouldbe designed so that they can withstandmajor tremors. Modern buildingtechnology can make it possible.

It is advisable to make the structuresimple so that it is ‘Quake Safe’.� Consult qualified architects and

structural engineers.


� In highly seismic areas, the use ofmud or timber is better than theheavy construction material. Keeproofs as light as possible. In casethe structure falls, the damage willnot be heavy.

� It is better if the cupboards andshelves are fixed to the walls, so thatthey do not fall easily.

� Be careful where you hang wallclocks, photo-frames, water heatersetc., so that in the event of anearthquake, they do not fall onpeople.

� Since some buildings may catch firedue to an earthquake, it is necessarythat all buildings, especially tallbuildings, have fire fightingequipment in working order.The Central Building Research

Institute, Roorkee, has developedknowhow to make quake proof houses.

In the event that an earthquake doesstrike, take the following steps to protectyourself:

1. If you are at home:� Take shelter under a table and stay

there till shaking stops.� Stay away from tall and heavy objects

that may fall on you.� If you are in bed, do not get up.

Protect your head with a pillow.

2. If you are outdoors:

� Find a clear spot, away frombuildings, trees and overhead powerlines. Drop to the ground.

� If you are in a car or a bus, do notcome out. Ask the driver to driveslowly to a clear spot. Do not comeout till the tremors stop.

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KEYWORDS

CRUST

DISCHARGE

EARTH’S PLATES

EARTHQUAKE

ELECTROSCOPE

LIGHTNING

LIGHTNING

CONDUCTOR

NEGATIVE CHARGE

POSITIVE CHARGE

RICHTER SCALE

SEISMOGRAPH

THUNDER

THUNDERSTORM

TRANSFER OF

CHARGE

TSUNAMI

TREMOR


� Some objects can be charged by rubbing with

other objects.

� There are two kinds of charges — positive

charge and negative charge

� Like charges repel and unlike charges attract

each other.

� The electrical charges produced by rubbing

are called static charges.

� When charges move, they constitute an

electric current.

� An electroscope may be used to detect

whether a body is charged or not.

� The process of transfer of charge from a

charged object to the earth is called earthing.

� The process of electric discharge between

clouds and the earth or between different

clouds causes lightning.

� Lightning strike could destroy life and

property.

� Lightning conductors can protect buildings

from the effects of lightning.

� An earthquake is a sudden shaking or

trembling of the earth.

� Earthquake is caused by a disturbance deep

inside the earth’s crust.

� It is not possible to predict the occurrence of

an earthquake.

� Earthquakes tend to occur at the boundaries

of earth’s plates. These boundaries are known

as fault zones.

� Destructive energy of an earthquake is

measured on the Richter scale. The

earthquake measuring 7 or more on Richter

scale can cause severe damage to life and

property.

� We should take necessary precautions to

protect ourselves from earthquakes.


Exercises

Select the correct option in Questions 1 and 2.1. Which of the following cannot be charged easily by friction?

(a) A plastic scale(b) A copper rod(c) An inflated balloon(d) A woollen cloth.

2. When a glass rod is rubbed with a piece of silk cloth the rod(a) and the cloth both acquire positive charge.(b) becomes positively charged while the cloth has a negative charge.(c) and the cloth both acquire negative charge.(d) becomes negatively charged while the cloth has a positive charge.

3. Write T against true and F against false in the following statements:(a) Like charges attract each other (T/F)(b) A charged glass rod attract a charged plastic straw (T/F)(c) Lightning conductor cannot protect a building from lightning (T/F)(d) Earthquakes can be predicted in advance (T/F)

4. Sometime, a crackling sound is heard while taking off sweater duringwinters. Explain.

5. Explain why a charged body loses its charge if we touch it with our hand.

6. Name the scale on which the destructive energy of an earthquake ismeasured. An earthquake measures 3 on this scale. Would it be recordedby a seismograph? Is it likely to cause much damage?

7. Suggest three measures to protect ourselves from lightning.

8. Explain why a charged balloon is repelled by another charged balloonwhereas an uncharged balloon is attracted by another charged balloon?

9. Describe with the help of a diagram an instrument which can be used todetect a charged body.

10. List three states in India where earthquakes are more likely to strike.

11. Suppose you are outside your home and an earthquake strikes. Whatprecaution would you take to protect yourself?

12. The weather department has predicted that a thunderstorm is likely tooccur on a certain day. Suppose you have to go out on that day. Would youcarry an umbrella? Explain.

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1. Open a water tap. Adjust the flow so that it forms a thin stream.Charge a refill. Bring it near the water stream. Observe whathappens. Write a short report on the activity.

2. Make your own charge detector. Take a paper strip roughly 10 cm ×3 cm. Give it a shape as shown in Fig. 15.15. Balance it on aneedle. Bring a charged body near it. Observe what happens. Writea brief report, explaining its working.

3. This activity should be performed at night. Go to a room wherethere is a fluorescent tube light. Charge a balloon. Switch off thetube light so that the room is completely dark. Bring the chargedballoon near the tubelight. You should see a faint glow. Move theballoon along the length of the tube and observe how glow changes.

Caution : Do not touch the metal parts of the tube or the wiresconnecting the tube with the mains.

4. Find out if there is an organisation in your area which providesrelief to those suffering from natural disaster. Enquire about thetype of help they render to the victims of earthquakes. Prepare abrief report on the problems of the earthquake victims.

For more information on these topics, visit:� science.howstuffworks.com/lightning.htm� science.howstuffworks.com/earthquake.htm� www.enchantedlearning.com/subjects/astronomy/planets/

earth/continents.shtml

Fig. 15.15

Paper 8.0cm

Fold along

Cut along

Needle

Thermocole

LIGHTLIGHT

The world is largely known throughthe senses. The sense of sightis one of the most important

senses. Through it we see mountains,rivers, trees, plants, chairs, people andso many other things around us. We alsosee clouds, rainbows and birds flyingin the sky. At night we see the moonand the stars. You are able to see thewords and sentences printed on thispage. How is seeing made possible?

16.1 What makes ThingsVisible

Have you ever thought how we see thevarious objects? You may say that eyessee the objects. But, can you see anobject in the dark? It means that eyesalone cannot see any object. It is onlywhen light from an object enters oureyes that we see the object. The lightmay have been emitted by the object, ormay have been reflected by it.

You learnt in Class VII that a polishedor a shiny surface can act as a mirror. Amirror changes the direction of lightthat falls on it. Can you tell in whichdirection the light falling on a surfacewill be reflected? Let us find out.

16.2 Laws of Reflection

Activity 16.1

Fix a white sheet of paper on adrawing board or a table. Take a

comb and close all its openingsexcept one in the middle. You canuse a strip of black paper forthis purpose. Hold the combperpendicular to the sheet of paper.Throw light from a torch throughthe opening of the comb from oneside (Fig. 16.1). With slightadjustment of the torch and thecomb you will see a ray of light alongthe paper on the other side of thecomb. Keep the comb and the torchsteady. Place a strip of plane mirrorin the path of the light ray (Fig.16.1). What do you observe?

After striking the mirror, the ray oflight is reflected in another direction.The light ray, which strikes any surface,is called the incident ray. The ray thatcomes back from the surface afterreflection is known as the reflected ray.

Fig. 16.1 : Arrangement for showing reflection

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A ray of light is an idealization. Inreality, we have a narrow beam oflight which is made up of severalrays. For simplicity, we use the termray for a narrow beam of light.

Draw lines showing the position ofthe plane mirror, the incident ray andthe reflected ray on the paper with thehelp of your friends. Remove the mirrorand the comb. Draw a line making anangle of 90º to the line representing themirror at the point where the incidentray strikes the mirror. This line is knownas the normal to the reflecting surfaceat that point (Fig. 16.2). The angle

Table 16.1 : Angles of Incidenceand Reflection

S. Angle of Angle ofNo. incidence (∠∠∠∠∠i) reflection (∠∠∠∠∠r)

1.

2.

3.

4.

5.

Do you see any relation between theangle of incidence and the angle ofreflection. Are they approximately equal?If the experiment is carried out carefully,it is seen that the angle of incidence isalways equal to the angle of reflection.This is known as the law of reflection.Let us perform another activity onreflection.

What would happen if Ithrew the light on the

mirror along the normal.

Activity 16.2

Perform Activity 16.1 again. Thistime use a sheet of stiff paper or achart paper. Let the sheet project alittle beyond the edge of the Table(Fig. 16.4). Cut the projectingportion of the sheet in the middle.Look at the reflected ray. Make surethat the reflected ray extends to theprojected portion of the paper. Bendthat part of the projected portion onwhich the reflected ray falls. Canyou still see the reflected ray? Bringthe paper back to the original

Fig. 16.2 : Drawing the normal

Fig. 16.3 : Angle of incidence and angle ofreflection

Reflectedray

Normal

Incidentray

between the normal and incident ray iscalled the angle of incidence (∠∠∠∠∠i). Theangle between the normal and thereflected ray is known as the angle ofreflection (∠∠∠∠∠r) (Fig. 16.3). Measure theangle of incidence and the angle ofreflection. Repeat the activity severaltimes by changing the angle ofincidence. Enter the data in Table 16.1.

LIGHT 203

position. Can you see the reflectedray again? What do you infer?

with the sun as the source of light insteadof a torch. You, too, can use the sun asthe source of light.

These activities can also be performedby making use of the Ray StreakApparatus (available in the kitprepared by NCERT).

Bhoojo remembered that he hadstudied in Class VII some features of theimage of an object formed by a planemirror. Paheli asked him to recall thosefeatures:

(i) Was the image erect or upsidedown?

(ii) Was it of the same size as theobject?

(iii) Did the image appear at the samedistance behind the mirror as theobject was in front of it?

(iv) Could it be obtained on a screen?Let us understand a little more about

the formation of image by a plane mirrorin the following way:

Activity 16.3

A source of light O is placed in frontof a plane mirror PQ. Two rays OAand OC are incident on it (Fig. 16.5).Can you find out the direction ofthe reflected rays?Draw normals to the surface of themirror PQ, at the points A and C.Then draw the reflected rays at thepoints A and C. How would youdraw these rays? Call the reflectedrays AB and CD, respectively.Extend them further. Do they meet?Extend them backwards. Do theymeet now? If they meet, mark thispoint as I. For a viewer’s eye at E(Fig. 16.5), do the reflected rays

(b)Fig. 16.4 (a), (b) : Incident ray, reflected ray

and the normal at the pointof incidence lie in the sameplane

When the whole sheet of paper isspread on the table, it represents oneplane. The incident ray, the normal atthe point of incidence and the reflectedray are all in this plane. When you bendthe paper you create a plane differentfrom the plane in which the incidentray and the normal lie. Then you do notsee the reflected ray. What does itindicate? It indicates that the incidentray, the normal at the point ofincidence and the reflected ray all liein the same plane. This is another lawof reflection.

Paheli and Bhoojo performed theabove activities outside the classroom

(a)

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appear to come from the point I.Since the reflected rays do notactually meet at I, but only appearto do so, we say that a virtual imageof the point O is formed at I. As youhave learnt already in Class VII,such an image cannot be obtainedon a screen.

You may recall that in an imageformed by a mirror the left of the objectappears on the right and the rightappears on the left. This is known aslateral inversion.

16.3 Regular and DiffusedReflection

Activity 16.4

Imagine that parallel rays areincident on an irregular surface asshown in Fig. 16.6. Remember thatthe laws of reflection are valid ateach point of the surface. Use theselaws to construct reflected rays atvarious points. Are they parallel toone another? You will find thatthese rays are reflected in differentdirections. (Fig. 16.7)

When all the parallel rays reflectedfrom a plane surface are not parallel,the reflection is known as diffused orirregular reflection. Remember that thediffused reflection is not due to thefailure of the laws of reflection. It iscaused by the irregularities in thereflecting surface, like that of acardboard.

On the other hand reflection froma smooth surface like that of a mirroris called regular reflection (Fig. 16.8).Images are formed by regularreflection.

Fig. 16.5 : Image formation in a plane mirror

Fig. 16.6 : Parallel rays incident onan irregular surface

Fig. 16.7 : Rays reflected from irregularsurface

Fig. 16.8 : Regular reflection

LIGHT 205

Let us find out.

16.4 Reflected Light Can beReflected Again

Recall the last time you visited a hairdresser. She/he makes you sit in front ofa mirror. After your hair cut is complete,she/he places a mirror at your back toshow you how the hair has been cut(Fig. 16.9). Can you think how you couldsee the hair at the back of your head?

Paheli recalls having constructed aperiscope as an Extended Activity inClass VI. The periscope makes use oftwo plane mirrors. Can you explainhow reflection from the two mirrorsenables you to see objects which arenot visible directly? Periscopes areused in submarines, tanks and alsoby soldiers in bunkers to see thingsoutside.

16.5 Multiple Images

You are aware that a plane mirror formsonly a single image of an object. Whathappens if two plane mirrors incombination are used? Let us see.

Do We See all Objects due to Reflected Light?

Nearly everything you see around is seen due to reflected light. Moon, for example,receives light from the sun and reflects it. That’s how we see the moon. The objectswhich shine in the light of other objects are called illuminated objects. Can youname some other such objects?

There are other objects, which give their own light, such as the sun, fire, flameof a candle and an electric lamp. Their light falls on our eyes. That is how we seethem. The objects which emit their own light are known as luminous objects.

Fig. 16.9 : Mirror at the hair dresser shop

I have a question. Can thereflected rays be further reflected

if incident on another mirror?

SCIENCE206

Activity 16.5

Take two plane mirrors. Set themat right angles to each other withtheir edges touching (Fig. 16.10). Tohinge them you can use adhesivetape. Place a coin in between themirrors. How many images of thecoin do you see (Fig. 16.10)?

Now hinge the mirrors using theadhesive tape at different angles, say45º, 60º, 120º, 180º etc. Place someobject (say a candle) in betweenthem. Note down the number ofimages of the object in each case.

Finally, set the two mirrorsparallel to each other. Find out howmany images of a candle placedbetween them are formed (Fig.16.11).

Fig. 16.10 : Images in plane mirror at rightangle to each other

Fig. 16.11 : Image in plane mirror parallelto each other

mirrors

a

b c

Can you now explain how you cansee the back of your head at the hairdresser’s shop?

This idea of number of images formedby mirrors placed at an angle to oneanother is used in a kaleidoscope tomake numerous beautiful patterns. Youcan also make a kaleidoscope yourself.

Kaleidoscope

Activity 16.6

To make a kaleidoscope, get threerectangular mirror strips about 15cm long and 4 cm wide each. Jointhem together to form a prism asshown in Fig. 16.12(a). Fix themin a circular cardboard tube or tubeof a thick chart paper. Make surethat the tube is slightly longer thanthe mirror strips. Close one end ofthe tube by a cardboard dischaving a hole in the centre,through which you can see [Fig.16.12(b)]. To make the disc durable,paste a piece of transparent plasticsheet under the cardboard disc. At

Fig. 16.12 : Making a kaleidoscope

LIGHT 207

the other end, touching themirrors, fix a circular plane glassplate [Fig. 16.12(c)]. Place on thisglass plate several small pieces ofcoloured glass (broken pieces ofcoloured bangles). Close this endof the tube by a ground glass plate.Allow enough space for the colourpieces to move around.

Your kaleidoscope is ready. Whenyou peep through the hole, you will beable to see a variety of patterns in thetube. Interesting feature of akaleidoscope is that you will never seethe same pattern again. Designers ofwallpapers and fabrics and artists usekaleidoscopes to get ideas for newpatterns. To make your toy attractive,you can wrap the kaleidoscope in acoloured paper.

16.6 Sunlight — White orColoured

In Class VII, you learnt that the sunlightis referred to as white light. You alsolearnt that it consists of seven colours.Here is another activity (Activity 16.7)showing that sunlight consists of severalcolours.

16.7 What is inside Our Eyes?We see things only when light comingfrom them enters our eyes. Eye is oneof our most important sense organs. Itis, therefore, important to understandits structure and working.

The eye has a roughly sphericalshape. Outer coat of the eye is white. Itis tough so that it can protect theinterior of the eye from accidents. Itstransparent front part is called cornea

Activity 16.7

Fig. 16.13 : Dispersion of light

Get a plane mirror of suitable size.Place it in a bowl (Katori) as shownin Fig. 16.13. Fill the bowl with water.Put this arrangement near a windowsuch that direct sunlight falls on themirror. Adjust the position of the bowlsuch that thereflected light fromthe mirror falls ona wall. If the wall isnot white, fix a sheetof white paper on it.Reflected light willbe seen to havemany colours. Howcan you explainthis? The mirrorand water form aprism. This breaks

up the light into its colours, as youlearnt in Class VII. Splitting of lightinto its colours is known asdispersion of light. Rainbow is anatural phenomenon showingdispersion.

SCIENCE208

Fig. 16.15 : Demonstration of blind spot

Caution : For this activity, never usea laser torch.

Activity 16.8

Look into your friend’s eye. Observethe size of the pupil. Throw light onher eye with a torch. Observe thepupil now. Switch off the torch, andobserve her pupil once again. Doyou notice any change in the sizeof the pupil? In which case was thepupil larger? Why do you think itwas so?

In which case do you need toallow more light in the eye, whenthe light is dim or bright?

Behind the pupil of the eye is a lenswhich is thicker in the centre. Whatkind of lens is thicker at the centre?Recall what you learnt about lenses in

(Fig. 16.14). Behind the cornea, we finda dark muscular structure called iris.In the iris, there is a small openingcalled the pupil. The size of the pupilis controlled by the iris. The iris is thepart of that eye which gives it itsdistinctive colour. When we say that aperson has green eyes, we refer actuallyto the colour of the iris. The iris controlsthe amount of light entering into theeye. Let us see how.

Iris

Lens

Ciliarymuscle

Cornea

Retina

OpticNerve

Fig. 16.14 : Human eye

Class VII. The lens focuses light on theback of the eye, on a layer called retina(Fig. 16.14). Retina contains severalnerve cells. Sensations felt by the nervecells are then transmitted to the brainthrough the optic nerve.There are two kinds of cells(i) cones, which are sensitive to bright

light and(ii) rods, which are sensitive to dim light.

Besides, cones sense colour. At thejunction of the optic nerve and theretina, there are no sensory cells, so novision is possible at that spot. This iscalled the blind spot. Its existence canbe demonstrated as follows:

Activity 16.9

Make a round mark and a cross ona sheet of paper with the spot to theright of the cross (Fig. 16.15). Thedistance between two marks may be6-8 cm. Hold the sheet of paper atarms length from the eye. Closeyour left eye. Look continuously atthe cross. Move the sheet slowlytowards you, keeping your eye onthe cross. What do you find? Doesthe round mark disappear at somepoint? Now close your right eye.Look at the round mark now andrepeat the activity. Does the crossdisappear? The disappearance of thecross or the round mark shows thatthere is a point on the retina whichcannot send messages to the brainwhen light falls on it.

LIGHT 209

Nature has provided eyes witheyelids to protect from any objectentering the eye. Eyelids also shut outlight when not required.

Eye is such a wonderful instrumentthat it (normal) can see distant objectsas well near objects clearly. Theminimum distance at which the eye cansee objects distinctly varies with age. Themost comfortable distance at which onecan read with a normal eye is about25 cm.

Some persons can see near objectsclearly but cannot see distant objectsso clearly. On the other hand, somepersons cannot see near objects clearlybut they can see distant objects quitewell. With suitable corrective lenses,these defects of the eye can becorrected.

Sometimes, particularly in old age,eyesight becomes foggy. It is due to theeye lens becoming cloudy. When ithappens, persons are said to havecataract. There is a loss of vision,sometimes extremely severe. It ispossible to treat this defect. The opaquelens is removed and a new artificiallens is inserted. Modern technologyhas made this procedure simplerand safer.

16.8 Care of Eyes

It is necessary that you take proper careof your eyes. If there is any problemyou should go to an eye specialist. Havea regular checkup.

� If advised, use suitable spectacles.

� Too little or too much light is badfor eyes. Insufficient light causeseyestrain and headaches. Too muchlight, like that of the sun, a powerful

The movies that we see are actuallya number of separate pictures inproper sequence. They are made tomove across the eye usually at the rateof 24 pictures per second (faster than16 per second). So, we see a movingpicture.

Front side ofcardboard

Reverse sideof cardboard

The impression of an image does notvanish immediately from the retina. Itpersists there for about 1/16th of asecond. So, if still images of a movingobject are flashed on the eye at a ratefaster than 16 per second, then the eyeperceives this object as moving.

Activity 16.10

Get a square piece of cardboard ofside 6-8 cm. Make two holes asshown in Fig. 16.16. Thread astring through the two holes. Draw/paste a cage on one side of thecardboard and a bird on the otherside. Twist the string and make thecard twirl rapidly. Do you see thebird in the cage?

Fig. 16.16 : Bird in cage

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lamp or a laser torch can injure theretina.

� Do not look at the sun or a powerfullight directly.

� Never rub your eyes. If particles ofdust go into your eyes, wash youreyes with clean water. If there is noimprovement go to a doctor.

� Wash your eyes frequently withclean water.

� Always read at the normal distancefor vision. Do not read by bringingyour book too close to your eyes orkeeping it too far.You learnt about balanced diet in

Class VI. If food is deficient in somecomponents, eye may also suffer. Lackof vitamin A in foodstuff is responsiblefor many eye troubles. Most commonamongst them is night blindness.

One should, therefore, include in thediet components which have vitamin A.

Did you know?

Animals have eyes shaped in different ways. Eyes of a crab are quite small butthey enable the crab to look all around. So, the crab can sense even if the enemyapproaches from behind. Butterfly has large eyes that seem to be made up ofthousands of little eyes (Fig.16.17). It can see not only in thefront and the sides but the backas well.A night bird (owl) can see very wellin the night but not during theday. On the other hand, day lightbirds (kite, eagle) can see wellduring the day but not in thenight. The Owl has a large corneaand a large pupil to allow morelight in its eye. Also, it has on itsretina a large number of rods andonly a few cones. The day birdson the other hand, have morecones and fewer rods.

Fig. 16.17 : Eyes of butterfly

Eyes

Raw carrots, broccoli and greenvegetables (such as spinach) and codliver oil are rich in vitamin A. Eggs,milk, curd, cheese, butter and fruitssuch as papaya and mango are also richin vitamin A.

16.9 Visually ChallengedPersons Can Read andWrite

Some persons, including children, canbe visually handicapped. They have verylimited vision to see things. Somepersons cannot see at all since birth.Some persons may lose their eyesightbecause of a disease. Such persons tryto identify things by touching andlistening to voices more carefully. Theydevelop their other senses more sharply.However, additional resources canenable them to develop their capabilitiesfurther.

LIGHT 211

16.10 What is a BrailleSystem?

The most popular resource for visuallychallenged persons is known as Braille.

Louis Braille

Louis Braille, himself a visuallychallenged person, developed asystem for visually challenged personsand published it in 1821.

Resources can be of two types : Non-optical aids and optical aids.Non-optical aids include visual aids, tactual aids (using the sense of touch),auditory aids (using the sense of hearing) and electronic aids. Visual aids, canmagnify words, can provide suitable intensity of light and material at properdistances. Tactual aids, including Braille writer slate and stylus, help the visuallychallenged persons in taking notes, reading and writing. Auditory aids includecassettes, tape recorders, talking books and other such devices. Electronic aids,such as talking calculators, are also available for performing many computationaltasks. Closed circuit television, also an electronic aid, enlarges printed materialwith suitable contrast and illumination. Nowadays, use of audio CDs and voiceboxes with computers are also very helpful for listening to and writing the desiredtext.Optical aids include bifocal lenses, contact lenses, tinted lenses, magnifiers andtelescopic aids. While the lens combinations are used to rectify visual limitations,telescopic aids are available to view chalkboard and class demonstrations.

Fig. 16.18 : Example of dot patterns usedin Braille System

Braille system has 63 dot patterns orcharacters. Each characterrepresents a letter, a combination ofletters, a common word or agrammatical sign. Dots are arrangedin cells of two vertical rows of threedots each.

The present system was adopted in1932. There is Braille code for commonlanguages, mathematics and scientificnotation. Many Indian languages canbe read using the Braille system.

Patterns of dots to represent someEnglish alphabets and some commonwords are shown below.

These patterns when embossed onBraille sheets help visuallychallenged to recognise words bytouching. To make them easier totouch, the dots are raised slightly.

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Some visually challenged Indians have great achievements to theircredit. Diwakar, a child prodigy has given amazing performancesas a singer.

Mr. Ravindra Jain, born completely visually challenged,obtained his Sangeet Prabhakar degree from Allahabad. He hasshown his excellence as a lyricist, singer and music composer.

Mr. Lal Advani, himself visually challenged, established anAssociation for special education and rehabilitation of disabledin India. Besides, he represented India on Braille problems toUNESCO.

Helen A Keller, an American author and lecturer, is perhaps the most well-known and inspiring visually challenged person. She lost her sight when she wasonly 18 months old. But because of her resolve and courage she could completeher graduation from a university. She wrote a number of books including TheStory of my Life (1903).

Helen A Keller

Visually challenged people learn theBraille system by beginning withletters, then special characters andletter combinations. Methods dependupon recognition by touching. Each

character has to be memorised. Brailletexts can be produced by hand or bymachine. Type writer - like devices andprinting machines have now beendeveloped.

LIGHT 213

KEYWORDS

ANGLE OF

INCIDENCE

ANGLE OF

REFLECTION

BLIND SPOT

BRAILLE

CONES

CORNEA

DIFFUSED/

IRREGULAR

REFLECTION

DISPERSION

INCIDENT RAYS

IRIS

KALEIDOSCOPE

LATERAL INVERSION

LAWS OF

REFLECTION

PUPIL

REFLECTED RAYS

REFLECTION

REGULAR

REFLECTION

RETINA

RODS


� Light is reflected from all surfaces.

� Regular reflection takes place when light is

incident on smooth, polished and regular

surfaces.

� Diffused/irregular reflection takes place from

rough surfaces.

� Two laws of reflection are

(i) The angle of incidence is equal to the angle

of reflection.

(ii) Incident ray, reflected ray and the normal

drawn at the point of incidence to the

reflecting surface, lie in the same plane.

� Image formed in a plane mirror undergoes

lateral inversion.

� Two mirrors inclined to each other give multiple

images.

� Beautiful patterns are formed in a

kaleidoscope because of multiple reflections.

� Sunlight, called white light, consists of seven

colours.

� Splitting of light into its constituent colours is

known as dispersion.

� Important parts of the eye are cornea, iris,

pupil, lens, retina and optic nerve.

� A normal eye can see nearby and distant

objects clearly.

� Visually challenged persons can read and

write using Braille system.

� Visually challenged persons develop their

other senses more sharply to improve their

interaction with their environment.

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Exercises

1. Suppose you are in a dark room. Can you see objects in the room? Canyou see objects outside the room. Explain.

2. Differentiate between regular and diffused reflection. Does diffusedreflection mean the failure of the laws of reflection?

3. Mention against each of the following whether regular or diffused reflectionwill take place when a beam of light strikes. Justify your answer in eachcase.

(a) Polished wooden table (b) Chalk powder(c) Cardboard surface (d) Marble floor with water

spread over it(e) Mirror (f) Piece of paper

4. State the laws of reflection.

5. Describe an activity to show that the incident ray, the reflected ray andthe normal at the point of incidence lie in the same plane.

6. Fill in the blanks in the following :

(a) A person 1 m in front of a plane mirror seems to be _______________ mfrom his image.

(b) If you touch your ____________ ear with right hand in front of a planemirror it will be seen in the mirror that your right ear is touched with____________

(c) The size of the pupil becomes ____________ when you see in dim light.

(d) Night birds have ____________ cones than rods in their eyes..

Choose the correct option in Questions 7 – 8

7. Angle of incidence is equal to the angle of reflection

(a) Always (b) Sometimes(c) Under special conditions (d) Never

8. Image formed by a plane mirror is

(a) virtual, behind the mirror and enlarged

(b) virtual, behind the mirror and of the same size as the object

(c) real at the surface of the mirror and enlarged

(d) real, behind the mirror and of the same size as the object.

9. Describe the construction of a kaleidoscope.

10. Draw a labeled sketch of the human eye.

LIGHT 215

11. Gurmit wanted to perform Activity 16.8 using a laser torch. Her teacheradvised her not to do so. Can you explain the basis of the teachers advise?

12. Explain how you can take care of your eyes.

13. What is the angle of incidence of a ray if the reflected ray is at an angle of90° to the incident ray?

14. How many images of a candle will be formed if it is placed between twoparallel plane mirrors separated by 40 cm?

15. Two mirrors meet at right angles. A ray of light is incident on one at anangle of 30° as shown in Fig. 16.19. Draw the reflected ray from the secondmirror.

Fig. 16.19

16. Boojho stands at A just on the side of a plane mirror as shown in Fig.16.20. Can he see himself in the mirror? Also can he see the image ofobjects situated at P, Q and R?

Fig. 16.20

17. (a) Find out the position of the image of an object situated at A in theplane mirror (Fig. 16.21).

(b) Can Paheli at B see this image?

(c) Can Boojho at C see this image?

(d) When Paheli moves from B to C, where does the image of A move?

Fig. 16.21

E X

E R

C I

S E

S

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Extended Learning — Activities and Project

1. Make your own mirror. Take a glass strip or glass slab. Clean it andput it on a white sheet of paper. See yourself in the glass. Next putthe glass slab on a black sheet of paper. Again look into the glass.In which case you see yourself better and why?

2. Make friends with some visually challenged students. Enquire fromthem how they read and write. Also find out how they are able torecognise objects, hurdles and currency notes.

3. Meet an eye specialist. Get your eye sight checked and discuss howto take care of your eyes.

4. Survey your neighbourhood. Find out how many children belowthe age of 12 years use spectacles. Find out from their parentswhat, in their view, could be the reason for the weak eyesight oftheir children.

You can read more on this topic on the following websites:� www. glenbrook.k12.il.us/gbssci/phys/mmedia/optics/

ifpm.html.� www.glenbrook.k12.il.us/gbssci/phys/class/refln/

u1311b.html.

Did You Know?

Eyes can be donated by any person as an invaluable gift to visuallychallenged persons suffering from corneal blindness, The person may be:

(a) male or female

(b) of any age

(c) of any social status

(d) using spectacles

(e) suffering from any normal disease but not AIDS, Hepatitis B or C,Rabies, Leukemia, Tetanus, Cholera, Encephalitis.

The eyes have to be donated within 4-6 hours after death at anyplace, home or hospital.

A person who wants to donate the eyes has to pledge eyes during his/herlifetime to any registered eye bank. He/she should also inform his/herrelatives about this pledge so that they can take necessary action afterhis/her death.

You can also donate a Braille kit. Contact :

Give India, National Association for the Blind. [The cost of a Braille kit isRs. 750/-]

STARS AND THE SOLAR SYSTEMSTARS AND THE SOLAR SYSTEM

Paheli and Boojho visited theirgrandparents’ village duringsummer break. After dinner, they

went on to the roof of the house. It was aclear cloudless night. They weresurprised to see a large number of brightstars in the sky. They had never seen sucha beautiful scene in their city (Fig. 17.1).

at a place where there are no bright lightsand the atmosphere is clear.

Look at the sky on a dark, clear night.You see the entire sky dotted withcountless stars, some bright and somenot so bright. Observe them carefully.Do all of them appear to twinkle? Doyou find any star-like object which does

Fig. 17.1: Night sky

Paheli wondered, why the village skywas so different from the night sky inbig cities? Her grandfather explained thatdue to bright light, smoke and dust, skyin big cities is rarely clear. Night skywatching can be a fascinating experience

not twinkle? The objects which do nottwinkle are planets.

The moon is the brightest object inthe night sky. The stars, the planets,the moon and many other objects in thesky are called celestial objects.

SCIENCE218

Are all celestial objects similar? Letus find out.

17.1 The Moon

Activity 17.1

Observe the moon continuously forseveral nights, preferably from onefull moon to the next. Make a sketchof the moon every night in your notebook and note from the day of thefull moon. Also note everyday thepart of the sky (east or west) inwhich the moon is seen.

Is there a change in the shape of themoon everyday? Are there days whenthe shape of the moon appears to beperfectly round? Are there days whenthe moon cannot be seen at all even ifthe sky is clear?

The day on which the whole disc ofthe moon is visible is known as the fullmoon day. Thereafter, every night the

size of the bright part of the moonappears to become thinner and thinner.On the fifteenth day the moon is notvisible. This day is known as the newmoon day. The next day, only a smallportion of the moon appears in the sky.This is known as the crescent moon.Then again the moon grows larger everyday. On the fifteenth day once again weget a full view of the moon.

The various shapes of the bright partof the moon as seen during a monthare called phases of the moon (Fig. 17.2).

Fig.17.2 : Phases of the moon

The time period between one full moonto the next full moon is slightly longerthan 29 days. In many calendars thisperiod is called a month.

Why does the moon changeits shape every day?


Fig. 17.3 : Moon is visible due to reflectedsunlight

Fig. 17.4: The moon appears different at different positions in its orbit

Let us try to understand why phasesof the moon occur. You have studied inChapter 16 that the moon does notproduce its own light, whereas the Sunand other stars do. We see the moonbecause the sunlight falling on it getsreflected towards us (Fig. 17.3). We,therefore, see only that part of the moon,from which the light of the Sun isreflected towards us.

Activity 17.2

Take a big ball or a pitcher. Painthalf of it white and half black incolour.

Go out into the playground withtwo of your friends. Draw a circle ofradius about 2 m on the ground.Divide the circle into eight equalparts as shown in Fig. 17.4.

Stand at the centre of the circle.Ask a friend to hold the ball atdifferent points of the circle. Ask herto keep the white portion of the ballalways towards the Sun. If you areperforming this activity in themorning then the white portion ofthe ball should be kept towards the

east. If the activity is beingperformed in the afternoon then thewhite portion of the ball should bekept towards the west. In each casethe line dividing the white andblack portions is kept vertical.

Standing at the centre of thecircle observe the visible whiteportion of the ball while your friendstands at the points on the circlemarked earlier. Draw the shape ofthe white portion as you see it.Compare your drawings with thedifferent phases of the moon asshown in Fig. 17.5.

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Fig. 17.5: Positions of the moon in its orbit and its corresponding phases

Fig. 17.6 : Earth accompanied by moonrevolving round the Sun

I have heard that wenever see the back side ofthe moon from the Earth.

Is it true?

Remember that the moon revolvesaround the Earth. The Earth alongwith the moon, revolve around theSun (Fig. 17.6).

these positions in your notebook. Inwhich part of the sky would you lookfor the full moon?

The size of the illuminated part ofthe moon visible from the Earthincreases each day after the new moonday. After the full moon day, the sunlitpart of the moon visible from the Earthdecreases in size every day.

Can you now guess the relativepositions of the Sun, moon and theEarth on the day of the full moon andon the day of the new moon? Sketch

Activity 17.3

Draw a circle of about 1m diameteron the ground. Ask one of yourfriends to stand at the centre of thiscircle. You revolve around your


friend in such a manner that yourface always remains towards him.Can your friend see your back? Howmany rotations did you complete inone revolution? The moon revolvesaround the Earth in a similarmanner.

The moon completes one rotation onits axis as it completes one revolutionaround the Earth.

The Moon’s Surface

The moon is a fascinating object forpoets and story-tellers. But whenastronauts landed on the moon, theyfound that the moon’s surface is dustyand barren. There are many craters ofdifferent sizes. It also has a large numberof steep and high mountains (Fig. 17.7).Some of these are as high as the highestmountains on the Earth.

Can we hear any soundon the moon?

We learnt in Chapter 13that sound cannot

travel when there is nomedium. Then, how canwe hear any sound on

the moon?

Fig. 17.7 : Surface of the moon

The moon has no atmosphere. It hasno water. Can any life exist on themoon?

Did You Know?

On July 21, 1969 (Indian time) theAmerican astronaut Neil Armstronglanded on the moon for the first timefollowed by Edwin Aldrin.

17.2 The Stars

What other objects do you see in thenight sky? There is a large number ofstars in the sky. Observe carefully on adark night and from a place away froma big city. Are all the stars equally

NASA

NASA

Fig. 17.8 : An astronaut on the moon

SCIENCE222

bright? Are they of the same colour? Infact, stars emit light of their own. TheSun is also a star. Why does it appearso large compared to the other stars?

Which appears bigger, a footballplaced near you, or a football placed ata distance of 100 m? The stars aremillions of times farther away than theSun. Therefore, the stars appear to uslike points.

The Sun is nearly 150,000,000kilometres (150 million km) away fromthe Earth.

The next nearest star is AlphaCentauri. It is at a distance of about40,000,000,000,000 km from theEarth. Can you read this distance inkilometres conveniently? Some starsare even further away.

Such large distances areexpressed in another unit known aslight year. It is the distance travelledby light in one year. Remember thatthe speed of light is about 300,000km per second. Thus, the distance ofthe Sun from the Earth may be saidto be about 8 light minutes. Thedistance of Alpha Centauri is about4.3 light years.

In fact, the stars are present in thesky during the day-time also. However,they are not visible then because of thebright sunlight.

Observe some prominent star or agroup of stars in the sky for about twohours or more. What do you find? Doyou find any change in the positions ofstars in the sky?

You will find that the stars appearto move from east to west. A star whichrises in the east in the evening, sets inthe west in the early morning.

Why do stars appear to move fromeast to west? Let us find out.

Activity 17.4

Stand in the centre of a big roomand start rotating. In whichdirection will the objects in the roomappear to move? Do you see themmoving in the direction opposite toyour motion?

Paheli recalls that when she is in amoving train the nearby trees andbuildings appear to move in thebackward direction.

If the stars appear to move from eastto west, could it mean that the Earth,rotates from west to east?If light from stars takes

years to reach us, Iwonder if we are lookinginto the past when we

look at stars.

I want to know why we donot see the stars duringthe day. Why are theyvisible only at night?

Now I understandwhy the Sun appearsto rise in the east andset in the west as the

Earth rotates fromwest to east on

its axis.


My grandfather told me thatthere is one star in the skywhich does not move at all.

How is it possible?

Fig 17.10: The Pole star lies close to the axis ofrotation of the Earth

There is actually a star, the pole star,which is situated in the direction of theearth’s axis. It does not appear to move(Fig. 17.10).

Constellations were devised byancient people to be able to recognisestars in the sky. The shapes ofconstellations resemble objectsfamiliar to those people.

17.3 Constellations

Look at the sky for some time. Can yousee some stars forming groups withshapes like those in Fig. 17.11.

The stars forming a group that has arecognisable shape is called aconstellation.

Fig. 17.9 : Pole star does not appear to move

You can easily identify someconstellations in the night sky. For this,you should know how a particularconstellation looks like and where tolook for it in the night sky.

One of the most famousconstellations which you can see during

Activity 17.5

Take an umbrella and open it. Makeabout 10-15 stars out of whitepaper. Paste one star at the positionof the central rod of the umbrellaand others at different places on thecloth near the end of each spoke(Fig. 17.9).

Now rotate the umbrella by holdingits central rod in your hand. Observethe stars on the umbrella. Is thereany star which does not appear tomove? Where is this star located?If there were a star located where theaxis of rotation of the Earth meetsthe sky, could this star also bestationary?

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I have heard that wecan locate the Pole

Star with the help ofUrsa Major.

Fig. 17.12: Dipper used for drinking water inancient times.

(a) Great Bear (b) Orion (c) Cassiopeia (d) Leo Major

Fig. 17.11 : Some constallations in the night sky

Activity 17.6

Observe this constellation for a fewhours. Do you find any change inits shape? Do you find any changein its position?

You will observe that the shapeof the constellation remains thesame.

You will also find that theconstellation appears to move in thesky from east to west.

summer time in the early part of thenight is Ursa Major [Fig.17.11 (a)].

It is also known as the Big Dipper,the Great Bear or the Saptarshi.

There are seven prominent stars inthis constellation. It appears like a bigladle or a question mark. There are threestars in the handle of the ladle and fourin its bowl (Fig. 17.12).

Activity 17.7

This activity should be performedon a clear moonless night duringsummer at about 9.00 pm. Looktowards the northern part of the skyand identify Ursa Major. You mayget help from elders in your family.


Fig. 17.14: Ursa Major moves around the PoleStar.

Look at the two stars at the end ofUrsa Major. Imagine a straight linepassing through these stars asshown in Fig. 17.13. Extend thisimaginary line towards the northdirection. (About five times thedistance between the two stars).This line will lead to a star which isnot too bright. This is the Pole Star.Observe the Pole star for some time.Note that it does not move at all asother stars drift from east to west.

Fig. 17.13: Locating the pole star

Pole star

Activity 17.8

During a summer night, observeUrsa Major 3-4 times at an intervalof 2 to 3 hours. Also locate the PoleStar each time. Does Ursa Majorappear to move from east to west?Does it appear to revolve around the

Pole Star? Compare yourobservations with those in Fig.17.14.

In fact, all the stars appear to revolvearound the Pole Star.

Note that the Pole Star is not visiblefrom the southern hemisphere. Someof the northern constellations likeUrsa Major may also not be visiblefrom some points in the southernhemisphere.

Orion is another well-knownconstellation that can be seen duringwinter in the late evenings. It is one ofthe most magnificent constellations inthe sky. It also has seven or eight brightstars [Fig. 17.11(b)] Orion is also calledthe Hunter. The three middle starsrepresent the belt of the hunter. Thefour bright stars appear to be arrangedin the form of a quadrilateral.

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17.4 The Solar System

The Sun and the celestial bodies whichrevolve around it form the solar system.It consists of large number of bodiessuch as planets, comets, asteroids andmeteors. The gravitational attractionbetween the Sun and these objectskeeps them revolving around it.

The Earth, as you know, alsorevolves around the Sun. It is a memberof the solar system. It is a planet. Thereare seven other planets that revolvearound the Sun. The eight planets intheir order of distance from the Sunare: Mercury, Venus, Earth, Mars ,Jupiter, Saturn, Uranus and Neptune.

The star Sirius, which is thebrightest star in the sky, is locatedclose to Orion. To locate Sirius, imaginea straight line passing through thethree middle stars of Orion. Look alongthis line towards the east. This line willlead you to a very bright star. It isSirius. (Fig. 17.15).

Fig. 17.15: Locating - Sirius

Cassiopeia is another prominentconstellation in the northern sky. It isvisible during winter in the early partof the night. It looks like a distortedletter W or M [Fig. 17.11(c)].

Did You Know?

A constellation does not haveonly 5-10 stars. It has a largenumber of stars (Fig. 17.16).However, we can see only the brightstars in a constellation with ournaked eye.

All the stars which make up aconstellation are not at the samedistance. They are just in the sameline of sight in the sky.

Fig. 17.16


of heat and light. The Sun is the sourceof almost all energy on the Earth. Infact, the Sun is the main source of heatand light for all the planets.

The Planets

The planets look like stars, but they donot have light of their own. They merelyreflect the sunlight that falls on them.Can you distinguish between planetsand stars?

The simplest method of identifyingplanets from stars is that stars twinkle,whereas planets do not. Also the planetskeep changing their positions withrespect to the stars.

A planet has a definite path in whichit revolves around the Sun. This path iscalled an orbit. The time taken by aplanet to complete one revolution iscalled its period of revolution. The period

I had read that there are nineplanets in the solar system

Did you know?

Till 2006 there were nine planets inthe solar system. Pluto was thefarthest planet from the Sun.

In 2006, the InternationalAstronomical Union (IAU) adopted anew definition of a planet. Pluto doesnot fit this definition. It is no longera planet of the solar system.

Fig. 17.17 : The solar system (not to scale)

Figure 17.17 shows a schematic viewof the solar system.

Let us know about some members ofthe solar family.

The Sun

The Sun is the nearest star from us. Itis continuously emitting huge amounts

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of revolution increases as the distanceof the planet increases from the sun.

Fig. 17.18 : Planets move in their own orbits

I wonder why the planets donot collide while revolving

around the Sun.

Activity 17.9

Go out into the playground with fouror five of your friends. Draw fourcircles of radii 1m, 1.8m, 2.5m and3.8m, all having a common centre(Fig. 17.18).

Ask one of your friends to standin the centre and represent the Sun.Your other four friends mayrepresent Mercury, Venus, Earthand Mars.

Ask your friends to move aroundthe Sun in anti-clockwise directionin their own orbits (Fig. 17.18). Dothey collide with one another?

Besides revolving around the Sun, aplanet also rotates on its own axis like atop (Fig. 17.19). The time taken by aplanet to complete one rotation is calledits period of rotation.

The Earth revolvesaround the Sun. Does itmake Earth a satellite

of the Sun?

Fig. 17.19 : A Planet rotates on its own axislike a top

Some planets are known to havemoons/satellites revolving round them.Any celestial body revolving aroundanother celestial body is called itssatellite.


The Earth can be said to be a satelliteof the Sun, though generally we call it aplanet of the Sun. We use the termsatellite for the bodies revolving aroundplanets. Moon is a satellite of the Earth.

There are many man-made satellitesrevolving round the Earth. These arecalled artificial satellites.

Activity 17.10

Find out from some newspaper orfrom an almanac the time whenVenus is visible in the sky. You caneasily recognise Venus by itsbrightness. Remember that Venuscannot be seen very high in the sky.You must try to observe Venuseither 1-3 hours before sunrise or1-3 hours after sunset.

Venus has no moon or satellite of itsown. Rotation of Venus on its axis issomewhat unusual. It rotates from eastto west while the Earth rotates from westto east.

Mercury (Budh)

The planet mercury is nearest to theSun. It is the smallest planet of our solarsystem. Because Mercury is very closeto the Sun, it is very difficult to observeit, as most of the time it is hidden in theglare of the Sun. However, it can beobserved just before sunrise or just aftersunset, near the horizon. So it is visibleonly at places where trees or buildingsdo not obstruct the view of the horizon.Mercury has no satellite of its own.

Venus (Shukra)

Venus is earth’s nearest planetaryneighbour. It is the brightest planet inthe night sky.

Sometimes Venus appears in theeastern sky before sunrise. Some timesit appears in the western sky just aftersunset. Therefore it is often called amorning or an evening star, although itis not a star. Try to locate Venus in thenight sky.

If you get a chance, try to observeVenus through a telescope. You willobserve that Venus shows phases justlike the moon (Fig. 17.20).

Fig. 17.20 : Phases of Venus

The Earth

The Earth is the only planet in the solarsystem on which life is known to exist.Some special environmental conditions

Does it mean that on Venus,the Sun would rise in thewest and set in the east?

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are responsible for the existence andcontinuation of life on the Earth. Theseinclude just the right distance from theSun, so that it has the right temperaturerange, the presence of water and suitableatmosphere and a blanket of ozone.

We must take special care toprotect our environment so thatlife on Earth is not disturbed.

From space, the Earth appears blue-green due to the reflection of light fromwater and landmass on its surface.

The axis of rotation of the Earth isnot perpendicular to the plane of itsorbit. The tilt is responsible for thechange of seasons on the Earth. TheEarth has only one moon.

Fig. 17.21 : Earth rotates on a tilted axis

You are familiar with the equator ofthe Earth. The plane of the equatoris called the equatorial plane (Fig.17.21). The plane in which the Earthrevolves round the Sun is called theorbital plane of the Earth (Fig. 17.21).These two planes are inclined to eachother at an angle of 23.5º. Thismeans that the axis of the Earth isinclined to its orbital plane at anangle of 66.5º.

Mars (Mangal)

The next planet, the first outside theorbit of the Earth is Mars. It appearsslightly reddish and, therefore, it is alsocalled the red planet. Mars has two smallnatural satellites.

Jupiter (Brihaspati)

planet. However, the mass of Jupiter isabout 318 times that of our Earth. Itrotates very rapidly on its axis.

If I am 13 years old, how manytimes have I gone round the sun?

Jupiter is the largest planet of the solarsystem. It is so large that about 1300earths can be placed inside this giant

I have an idea! If you take alarge ball which can

accommodate about 1300peas, then the ball will

represent Jupiter and onepea will represent the Earth.


Jupiter has a large number ofsatellites. It also has faint rings aroundit. You can easily recognise Jupiter as itappears quite bright in the sky. If youobserve it with the help of a telescope,you can also see four of its large moons(Fig. 17.22).

Beyond Jupiter is Saturn which appearsyellowish in colour. What makes itunique in the solar system is its beautiful

Fig. 17.22 : Jupiter and its four large satellites.

Saturn (Shani)

Boojho has anaughty idea! “If

we imagine Saturnin a large pool ofwater then it willfloat (Fig. 17.23)

Fig. 17.23 : Saturn is less dense than water

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rings. These rings are not visible withthe naked eye. You can observe them witha small telescope. Saturn also has a largenumber of satellites.

One interesting thing about Saturnis that it is the least dense among allthe planets. Its density is less than thatof water.

Uranus and Neptune

These are the outermost planets of thesolar system. They can be seen only withthe help of large telescopes. Like Venus,Uranus also rotates from east to west.The most remarkable feature of Uranusis that it has highly tilted rotational axis(Fig. 17.24). As a result, in its orbitalmotion it appears to roll on its side.

The first four planets, Mercury,Venus, Earth and Mars are much nearer

the Sun than the other four planets.They are called the inner planets. Theinner planets have very few moons.

The planets outside the orbit of Mars,namely Jupiter, Saturn, Uranus andNeptune are much farther off than theinner planets. They are called the outerplanets. They have a ring system aroundthem. The outer planets have largenumber of moons.

17.5Some Other Members ofthe Solar System

There are some other bodies whichrevolve around the Sun. They are alsomembers of the solar system. Let uslearn about some of them.

Asteroids

There is a large gap in between the orbitsof Mars and Jupiter (Fig. 17.25). Thisgap is occupied by a large number ofsmall objects that revolve around theSun. These are called asteroids.Asteroids can only be seen through largetelescopes.

Fig. 17.25 : Asteroid belt

Fig. 17.24 : Uranus in its orbital path

Asteroids


Comets

Comets are also members of our solarsystem. They revolve around the Sunin highly elliptical orbits. However, theirperiod of revolution round the Sun isusually very long. A Comet appearsgenerally as a bright head with a longtail. The length of the tail grows in sizeas it approaches the sun. The tail of acomet is always directed away from thesun (Fig. 17.26).

Fig. 17.26 : Different position of a Comet

Many comets are known to appearperiodically. One such comet is Halley’scomet, which appears after nearly every76 years. It was last seen in 1986. Canyou tell, when Halley’s comet will bevisible again?

Superstitions about the Comets

Some people think that comets aremessengers of disasters, such aswars, epidemics and floods. But theseare all myths and superstitions.Appearance of a comet is a naturalphenomenon. We have no reason tobe afraid of it.

Meteors and Meteorites

At night, when the sky is clear and themoon is not there, you may sometimes

see bright streaks of light in the sky (Fig.17.27). These are commonly known asshooting stars, although they are notstars. They are called meteors. A meteoris usually a small object thatoccasionally enters the earth’satmosphere. At that time it has a veryhigh speed. The friction due to theatmosphere heats it up. It glows andevaporates quickly. That is why thebright steak lasts for a very short time.

Fig. 17.27 : Streak of a Meteor

Some meteors are large so that theycan reach the Earth before theyevaporate completely. The body thatreaches the Earth is called a meteorite.Meteorites help scientists ininvestigating the nature of the materialfrom which the solar system was formed.

Meteor Showers

When the Earth crosses the tail of acomet, swarms of meteors are seen.These are known as meteor showers.Some meteor showers occur atregular intervals each year. You canfind the time of their appearance froma scientific magazine or from theinternet.

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Artificial Satellites

You must have heard that there are anumber of artificial satellites which areorbiting the Earth. You might wonderhow artificial satellites are different fromthe natural satellites. The artificialsatellites are man-made. They arelaunched from the Earth. They revolvearound the Earth much closer thanearth’s natural satellite, the moon.

India has built and launched severalartificial satellites. Aryabhatta was thefirst Indian satellite. Some other Indian

satellites are INSAT, IRS, Kalpana-1,EDUSAT, etc. (Fig. 17.28).

Artificial satellites have manypractical applications. They are used forforecasting weather, transmittingtelevision and radio signals. They arealso used for telecommunication andremote sensing.

Fig. 17.28 : Some Indian satellites

I want to tell you that by remotesensing we mean collectinginformation from a distance.

ISRO


KEYWORDS

ARTIFICIAL

SATELLITES

ASTEROIDS

CASSIOPEIA

CELESTIAL OBJECTS

COMETS

CONSTELLATIONS

LIGHT YEAR

METEORITES

METEORS

NATURAL SATELLITES

ORBIT

ORION

PHASES OF MOON

PLANETS

POLE STAR

REMOTE SENSING

SOLAR SYSTEM

STARS

URSA MAJOR


� The phases of the moon occur because we can

see only that part of the moon which reflects

the light of the Sun towards us.

� Stars are celestial bodies that emit light of

their own. Our sun is also a star.

� It is convenient to express distances of stars

in light years.

� Stars appear to move from east to west.

� The pole star appears to be stationary from

the Earth, because it is situated close to the

direction of the axis of rotation of the Earth.

� Constellations are groups of stars that appear

to form recognisable shapes.

� The solar system consists of eight planets and

host of asteroids, comets and meteors.

� A body revolving around another body is called

a satellite.

� Moon is the natural satellite of the Earth.

Some planets also have natural satellites.

� Venus is the brightest planet in the night sky.

� Jupiter is the largest planet of the solar

system.

� The artificial satellites revolve around the

Earth. They are much closer than the moon.

� Artificial satellites are used for weather

forecasting, long distance communication and

remote sensing.

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Exercises

Choose the correct answer in Questions 1-3 :

1. Which of the following is NOT a member of the solar system?

(a) An asteroid

(b) A satellite

(c) A constellation

(d) A comet

2. Which of the following is NOT a planet of the sun?

(a) Sirius

(b) Mercury

(c) Saturn

(d) Earth

3. Phases of the moon occur because

(a) we can see only that part of the moon which reflects light towardsus.

(b) our distance from the moon keeps changing.

(c) the shadow of the Earth covers only a part of moon’s surface.

(d) the thickness of the moon’s atmosphere is not constant.


(a) The planet which is farthest from the Sun is ____________ .

(b) The planet which appears reddish in colour is ____________ .

(c) A group of stars that appear to form a pattern in the sky is known asa ____________ .

(d) A celestial body that revolves around a planet is known as __________.

(e) Shooting stars are actually not ____________.

(f) Asteroids are found between the orbits of _________ and _________ .

5. Mark the following statements as true (T) or false (F):

(a) Pole Star is a member of the solar system. ( )

(b) Mercury is the smallest planet of the solar system. ( )

(c) Uranus is the farthest planet in the solar system. ( )

(d) INSAT is an artificial satellite. ( )

(e) There are nine planets in the solar system. ( )

(f) Constellation Orion can be seen only with a telescope. ( )


6. Match items in column A with one or more items in column B:

A B

(i) Inner planets (a) Saturn

(ii) Outer planets (b) Pole Star

(iii) Constellation (c) Great Bear

(iv) Satellite of the Earth (d) Moon

(e) Earth

(f) Orion

(g) Mars

7. In which part of the sky can you find Venus if it is visible as an evening star?

8. Name the largest planet of the solar system.

9. What is a constellation? Name any two constellations.

10. Draw sketches to show the relative positions of prominent stars in

(a) Ursa Major and (b) Orion

11. Name two objects other than planets which are members of the solar system.

12. Explain how you can locate the Pole Star with the help of Ursa Major.

13. Do all the stars in the sky move? Explain.

14. Why is the distance between stars expressed in light years? What do youunderstand by the statement that a star is eight light years away fromthe Earth?

15. The radius of Jupiter is 11 times the radius of the Earth. Calculate theratio of the volumes of Jupiter and the Earth. How many Earths can Jupiteraccomodate?

16. Boojho made the following sketch (Fig. 17.29) of the solar system. Is thesketch correct? If not, correct it.

E X

E R

C I

S E

S

Fig. 17.29

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Extended Learning — Activities and Projects1. If possible, visit a planetarium. There are planetariums in many

cities. In a planetarium you can see the motion of the stars,constellations and planets on a large dome.

2. On a moonless night observe the sky for a few hours. Look out for ameteor, which appears as a streak of light. September-November isa good time for observing meteors.

3. Learn to identify the planets visible to the naked eye and someprominent constellations such as Great Bear (Saptarshi) and Orion.Also try to locate the Pole Star and the star Sirius.

4. Select a convenient place on the roof of your house or in someplayground from where you can watch sunrise clearly. Rememberthat you will have to do this activity for a few months. So, choosethe place carefully. On a sheet of chart paper draw the outline ofthe eastern horizon indicating big trees, poles, etc. Mark the outlineof the sheet so that you can place it in the same position everytime. After every two weeks, note the position from where the Sunrises and mark this on your chart paper (Fig. 17.30). Also note thedate of your observation. Repeat this for a few months.

It is advisable that you start observations in the month of Novemberor May.

Fig. 17.30 : Position of sunrise on different dates

Does the Sun always rise from the same direction? Discuss yourobservations with you teacher, parents and elderly people in yourfamily or in the neighbourhood.

Did You Know ?Only on two days in a year, March 21 and September 23, the Sun risesexactly in the east. On all other days, the Sun rises either north of eastor south of east.

From summer solstice (around June 21) the point of sunrise graduallyshift towards the south. The Sun is then said to be in dakshinayan (movingsouth). It keeps moving towards south till winter solstice (aroundDecember 22). Thereafter, the point of sunrise changes direction andstarts moving towards north. The Sun is then said to be in uttarayan(moving north).


Table 17.1

Name Approximate Approximate Period Periodof Planet radius distance of of

(assuming from the Sun revolution rotationEarth as 1 unit) (Assuming

distance ofthe Earthas 1 unit)

Mercury 0.40 0.39 88 days 59 days

Venus 0.95 0.72 225 days 243 days

Earth 1.00 1.00 365.25 days 24 hours

Mars 0.55 1.50 687 days 24 hours 37 min

Jupiter 11.00 5.20 12 years 9 hours 55 min

Saturn 9.00 9.50 29.46 years 10.66 hours

Uranus 4.00 19.20 84 years 17.2 hours

Neptune 3.90 30.00 165 years 16.1 hours

5. Form a group of students. Prepare a model of the solar systemshowing the planets, and their relative sizes. For this take a largechart paper. Make spheres representing different planets accordingto their relative size (Use Table 17.1). You may use newspaper, clayor plasticine to make spheres. You can cover these spheres withpaper of different colours. Exhibit your models in the class.

6. Try to make a scale model of the solar system showing distances ofthe planets from the Sun (Use Table 17.1). Did you face anydifficulty? Explain it.

7. Solve the following riddle and try to make similar riddles yourself:

My first is in VAN but not in PAN

My second is in EARTH and also in HEAVEN

My third is in ONE and not in TWO

My fourth is in BUN and also in FUN

My last is in STAR but not in RADAR

I am a planet that moves round the Sun.

You can read more on the following websites :� http://www.nineplanets.org� http://www.kidsastronomy.com� http://www.solarsystem.nasa.gov/planets� http://aa.usno.navy.mil/fazz/does/moon-phases.html

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Did you know ?

In ancient times, it was believed that the Earth was at the centreof the universe and the moon, the planets, the Sun and starswere orbiting around it. About 500 years ago, a Polish priestand astronomer, named Nicolaus Copernicus (1473 – 1543),stated that the Sun was at the centre of the solar system andthe planets revolved around it. It was a revolutionary idea. EvenCopernicus hesitated to publish his work. His work waspublished in the year of his death in 1543.

In 1609, Galileo designed his own telescope. Through thistelescope Galileo observed moons of Jupiter, phases of Venusand rings of Saturn. He argued that all the planets must orbitthe Sun and not the Earth.

Thus you can see that ideas and concepts grow and change.How about your own ideas? Do you keep your mind open toaccept new ideas when there is good evidence to support it?

POLLUTION OF AIR AND WATERPOLLUTION OF AIR AND WATER

Paheli and Boojho were very excitedto hear the news that TajMahal in Agra is now one of the

seven wonders of the world. But theywere disappointed to hear that thebeauty of this monument in whitemarble is being threatened by airpollution in the area surrounding theTaj. They were eager to know ifsomething can be done to fight the airand water pollution.

We are all aware that ourenvironment is not what it used to be.Our elders talk about the blue sky, cleanwater and fresh air that was availablein their times. Now the media regularlyreports on the falling quality of theenvironment. We ourselves feel theimpact of the falling quality of air andwater in our lives. The number of peoplesuffering from diseases of the respiratorysystem, for example, is steadily rising.

We dread to imagine a time whenclean air and water may no longer beavailable! You have learnt about theimportance of air and water in yourprevious classes. In this chapter, we willstudy about the harmful changes takingplace in our surroundings and theireffects on our lives.

18.1 Air Pollution

We can survive for some time without food,but we cannot survive even for a fewminutes without air. This simple fact tellsus how important clean air is to us.

You already know that air consistsof a mixture of gases. By volume, about78% of this mixture is nitrogen andabout 21% is oxygen. Carbon dioxide,argon, methane, ozone and watervapour are also present in very smallquantities.

Activity 18.1

You may have covered your nosewhile passing a brick kiln emittingsmoke or started coughing whilewalking on a busy road (Fig. 18.1).On the basis of your experience,compare the quality of air at theplaces given below:

� A park and a busy road.� A residential area and an

industrial area.� A busy traffic intersection at

different times of the day e.g.early morning, afternoon andevening.

� A village and a town.

Fig. 18.1 : A congested road in a city

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Fig. 18.3 : Air pollution due to automobiles

One of your observations in theabove activity could be the differencesin the amount of smoke in theatmosphere. Do you know where thesmoke could have come from? Additionof such substances to the atmospheremodifies it. When air is contaminatedby unwanted substances which havea harmful effect on both the living andthe non-living, it is referred to as airpollution.

18.2 How does Air Get Polluted?

The substances which contaminate theair are called air pollutants. Sometimes,such substances may come from naturalsources like smoke and dust arising fromforest fires or volcanic eruptions.Pollutants are also added to theatmosphere by human activities. Thesources of air pollutants are factories(Fig. 18.2), power plants, automobileexhausts and burning of firewood anddung cakes.

Activity 18.2

You might have read in thenewspapers that respiratoryproblems amongst childrenare rising day by day. Conduct asurvey of households in yourneighbourhood and among friendsto find out how many children aresuffering from respiratory problems.

Many respiratory problems arecaused by air pollution. Let us now tryto find out the substances or pollutantswhich are present in the polluted air.

Have you noticed how rapidly thenumber of vehicles is increasing in ourcities?

Vehicles produce high levels ofpollutants like carbon monoxide, carbondioxide, nitrogen oxides and smoke (Fig.18.3). Carbon monoxide is producedfrom incomplete burning of fuels suchas petrol and diesel. It is a poisonousgas. It reduces the oxygen-carryingcapacity of the blood.

Fig. 18.2 : Smoke from a factory

Do you know?

If the vehicles registered in Delhi arelined up one after the other, the totallength would be nearly equal to thecombined lengths of the two longestrivers in the world, Nile and Amazon!


Boojho remembers seeing a thickfog-like layer in the atmosphere,especially during winters. This is smogwhich is made up of smoke and fog.Smoke may contain oxides of nitrogenwhich combine with other air pollutantsand fog to form smog. The smog causesbreathing difficulties such as asthma,cough and wheezing in children.

Many industries are also responsiblefor causing air pollution. Petroleumrefineries are a major source of gaseouspollutants like sulphur dioxide andnitrogen dioxide. Sulphur dioxide isproduced by combustion of fuels likecoal in power plants. It can causerespiratory problems, includingpermanent lung damage. You havealready studied about the burning offossil fuels in Chapter 5.

Other kinds of pollutants arechlorofluorocarbons (CFCs) which areused in refrigerators, air conditionersand aerosol sprays. CFCs damage theozone layer of the atmosphere. Recallthat the ozone layer protects us fromharmful ultraviolet rays of the sun. Haveyou heard of the ozone hole? Try to findout about it. Thankfully, less harmfulchemicals are now being used in placeof CFCs.

In addition to the above mentionedgases, automobiles which burn dieseland petrol, also produce tiny particleswhich remain suspended in air for longperiods (Fig. 18.3). They reducevisibility. When inhaled, they causediseases. Such particles are alsoproduced during industrial processeslike steel making and mining. Powerplants give out tiny ash particles whichalso pollute the atmosphere.

Activity 18.3

Prepare a table using the pollutantsmentioned above. You may even addmore data to the following Table.

Table 18.1

Air Pollutants Sources Effects

18.3 Case Study : The TajMahal

Over the past 2 decades, India’s mostfamous tourist attraction, Taj Mahallocated in Agra (Fig. 18.4), has becomea matter of concern. Experts havewarned that pollutants in air arediscolouring its white marble. So, it isnot only living organisms that getaffected by polluted air but non-livingthings like buildings, monuments andstatues also get affected.

The industries located in and aroundAgra like rubber processing, automobile,chemicals and especially the Mathura oilrefinery, have been responsible forproducing pollutants like sulphurdioxide and nitrogen dioxide. Thesegases react with the water vapour presentin the atmosphere to form sulphuric acidand nitric acid. The acids drop down withrain, making the rain acidic. This iscalled acid rain. Acid rain corrodes themarble of the monument. Thephenomenon is also called “Marblecancer”. Suspended particulate matter,such as the soot particles emitted byMathura oil refinery, has contributedtowards yellowing of the marble.

The Supreme Court has takenseveral steps to save the Taj. It has

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and a part is reflected back into space.A part of the reflected radiation istrapped by the atmosphere. The trappedradiations further warm the earth. If youhave seen a greenhouse in a nursery orelsewhere, recall that the sun’s heat isallowed to get in but is not allowed togo out. The trapped heat warms thegreen house. The trapping of radiationsby the earth’s atmosphere is similar.That is why it is called the greenhouseeffect. Without this process, life wouldnot have been possible on the earth. Butnow it threatens life. CO2

is one of thegases responsible for this effect.

You know that CO2 is one of thecomponents of air. You have also studied

I am reminded of thechapter on crops. I

wonder whether acid rainaffects the soil and

plants also.

But how does CO2 contentrise in the atmosphere and

become excessive?

ordered the industries to switch tocleaner fuels like CNG (CompressedNatural Gas) and LPG (LiquefiedPetroleum Gas). Moreover, theautomobiles should switch over tounleaded petrol in the Taj zone.

Discuss with your elders and seewhat they have to say about thecondition of the Taj, 20 or 30 years ago!Try to procure a picture of the Taj Mahalfor your scrap book.

18.4 Greenhouse Effect

You know that the sun’s rays warm theearth’s surface. A part of the radiationthat falls on the earth is absorbed by it

Fig. 18.4 : Taj Mahal


the role of carbon dioxide in plants. Butif there is excess of CO2 in the air, itacts as a pollutant.

Can you help Paheli find out theanswer to her question?

On the one hand, CO2 is continuouslybeing released because of humanactivities. On the other hand, area underforests is decreasing. Plants utilise CO2

from the atmosphere for photosynthesis,thereby decreasing the amount of CO2

in the air. Deforestation leads to anincrease in the amount of CO2 in the airbecause the number of trees whichconsume CO2 is reduced. Humanactivities, thus, contribute to theaccumulation of CO2 in the atmosphere.CO2 traps heat and does not allow it toescape into space. As a result, the averagetemperature of the earth’s atmosphereis gradually increasing. This is calledglobal warming.

Other gases like methane, nitrousoxide and water vapour also contribute

Global Warming

A Serious Threat!

Global warming can cause sea levelsto rise dramatically. In many places,coastal areas have already beenflooded. Global warming could resultin wide ranging effects on rainfallpatterns, agriculture, forests, plantsand animals. Majority of people livingin regions which are threatened byglobal warming are in Asia. A recentclimate change report gives us onlya limited time to keep the greenhousegases at the present level. Otherwise,the temperature may rise by morethan 2 degrees Celsius by the end ofthe century, a level considereddangerous.

towards this effect. Like CO2, they arealso called greenhouse gases.

Global warming has become a majorconcern for governments worldwide.Many countries have reached anagreement to reduce the emission ofgreenhouse gases. The Kyoto Protocolis one such agreement.

Boojho is surprised to hear that anincrease in the earth’s temperature byeven as little as 0.5 ºC can have such aserious effect! Paheli tells him that shehad read in the newspapers recently thatthe Gangotri glacier in the Himalayashas started melting because of globalwarming.

18.5 What can be Done?

What can we do to reduce airpollution?

There are many success stories in ourfight against air pollution. For example,a few years ago, Delhi was one of themost polluted cities in the world. It wasbeing choked by fumes released fromautomobiles running on diesel andpetrol. A decision was taken to switchto fuels like CNG (Fig. 18.5) and

Fig. 18.5 : A public transport bus powered by CNG

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unleaded petrol. These measures haveresulted in cleaner air for the city. Youmight know of some other examplesfrom your area regarding reduction ofair pollution. Share these with yourfriends.

fuels for our energy requirements. Thesecould be solar energy, hydropower andwind energy.

Activity 18.4You have various options ofcommuting to your school such aswalking, going by bicycle, travellingby bus or other public transport,using a car individually, travellingby car pool. Discuss in your classthe impact of each of these optionson the quality of air.

Small contributions on our part canmake a huge difference in the state ofthe environment. We can plant trees andnurture the ones already present in theneighbourhood. Do you know about VanMahotsav, when lakhs of trees areplanted in July every year (Fig. 18.6)?

Fig. 18.6 : Children planting saplings

Do you know about the “Say no tocrackers” campaign which wasorganised by children from manyschools? This made a big differenceto the air pollution levels aroundDiwali.

The quality of air at various locationsis monitored regularly by governmentand other agencies. We can use this datato generate awareness about airpollution among friends andneighbours.

There is a need to switch over toalternative fuels instead of the fossil


Boojho and Paheli once happened togo to an area where some people wereburning dry leaves. They startedcoughing because the entire area was fullof smoke. Paheli thought it would be abetter option to put them in a compostpit rather than burning. What do youthink?

18.6 Water PollutionIn Class VII you learnt that water is aprecious resource. Think and list thevarious activities in which we needwater. We saw that water is becomingscarce due to increase in population,industries and agricultural activities.You have also studied how waterbecomes “dirty” after we use it forwashing clothes, bathing, etc. Thismeans that we are adding somematerials to the water, which spoil itsquality and change its smell and colour.

Whenever harmful substances suchas sewage, toxic chemicals, silt, etc., getmixed with water, the water becomespolluted. The substances that pollutewater are called water pollutants.

Activity 18.5

Try to collect samples of water froma tap, pond, river, well and lake.Pour each into separate glasscontainers. Compare these forsmell, acidity and colour. Completethe following Table.

Table 18.2

Smell Acidity Colour

Tap Water

Pond Water

River Water

Well Water

Lake Water

18.7 How does Water GetPolluted?

Case Study

Ganga is one of the most famous riversof India (Fig. 18.7). It sustains most ofthe northern, central and easternIndian population. Millions of peopledepend on it for their daily needs and

Fig. 18.7 : Course of the river Ganga

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livelihood. However, recently a study bythe World Wide Fund for Nature (WWF)found that Ganga is one of the ten mostendangered rivers in the world. Thepollution levels have been rising formany years. We have reached this stagebecause the towns and cities, throughwhich the river flows, throw largequantities of garbage, untreated sewage,dead bodies, and many other harmfulthings, directly into the river. In fact theriver is “dead” at many places wherethe pollution levels are so high thataquatic life cannot survive.

An ambitious plan to save the river,called the Ganga Action Plan waslaunched in 1985. It aimed to reducethe pollution levels in the river. However,the increasing population andindustrialisation have already damagedthis mighty river beyond repair.

Let us take a specific example tounderstand the situation. The Gangaat Kanpur in Uttar Pradesh (U.P.), hasone of the most polluted stretches ofthe river (Fig. 18.8). Kanpur is one ofthe most populated towns in U.P. Peoplecan be seen bathing, washing clothes

and defecating in the river. They alsothrow garbage, flowers, idols of godsand goddesses and non-biodegradablepolythene bags into the river.

At Kanpur the amount of water iscomparatively small and the flow of theriver is very slow. In addition, Kanpurhas more than 5000 industries. Theseinclude fertiliser, detergent, leather andpaint industries. These industrial unitsdischarge toxic chemical wastes into theriver.

Based on the above informationthink of the answers to the followingquestions :

� What are the factors responsible forpollution of the river ?

� What steps can be taken to restorethe river Ganga to its past glory ?

� How would the disposal of garbage,etc., affect the living organisms in theriver ?

Many industries discharge harmfulchemicals into rivers and streams,causing the pollution of water(Fig. 18.9). Examples are oil refineries,paper factories, textile and sugar mills

Fig. 18.9 : Industrial waste discharged into ariver

Fig. 18.8 : A polluted stretch of the riverGanga


and chemical factories. These industriescause chemical contamination of water.The chemicals released include arsenic,lead and fluorides which lead to toxicityin plants and animals. There areregulations to prevent this. Industriesare supposed to treat the wasteproduced before discharging it intowaters, but quite often the rules are notfollowed. The soil is also affected byimpure water, causing changes inacidity, growth of worms, etc.

We have learnt in Chapter 1 theimportance of pesticides and weedicidesfor the protection of crops. However, allthese chemicals dissolve in water andare washed into water bodies from thefields. They also seep into the groundto pollute ground water.

Have you seen ponds which lookgreen from a distance because they havea lot of algae growing in them? This iscaused by excessive quantities ofchemicals which get washed from thefields. These act as nutrients for algaeto flourish. Once these algae die, theyserve as food for decomposers likebacteria. A lot of oxygen in the waterbody gets used up. This results in adecrease in the oxygen level which maykill aquatic organisms.

Recall Activity 18.6

You had investigated the sewagedisposal system of your locality inClass VII.

Do you remember how thesewage was collected from yourhome and where it went thereafter.

Sometimes untreated sewage isthrown directly into rivers. It contains

food wastes, detergents, microor -ganisms, etc. Can ground water getpolluted by sewage? How? Watercontaminated with sewage may containbacteria, viruses, fungi and parasiteswhich cause diseases like cholera,typhoid and jaundice.

The bacteria present in the faecesof mammals are indicators of thequality of water. If water has thesebacteria, it means that it has beencontaminated by fecal matter. If suchwater is used by us, it can cause variousinfections.

Do you know?

Hot water can also be a pollutant !This is usually water from powerplants and industries. It is releasedinto the rivers. It raises thetemperature of the waterbody,adversely affecting the animals andplants living in it.

18.8 What is Potable Waterand How is WaterPurified ?

Activity 18.7

Let us construct a water filter withsimple, everyday materials.

Take a plastic bottle and cut itinto 2 halves at the centre. Use theupper half as a funnel by putting itupside down in the lower half. Makelayers in it with paper napkin or afine cloth followed by, cotton, sandand then gravel. Now pour dirtywater through the filter and observethe filtered water.

SCIENCE250

Discuss the following questionsamongst yourselves and with yourteacher:� Why do we need to filter water

before drinking ?� Where do you get your drinking

water from?� What will happen if we drink

polluted water?

Boojho is very upset. He tells Pahelithat he drank water which looked clearand was without any smell, but still hefell sick !

Paheli explains that water whichlooks clean may still have disease-carrying microorganisms and dissolvedimpurities. So, it is essential to purifywater before drinking, for example, byboiling.

Water which is suitable for drinkingis called potable water. You have seenhow various physical and chemicalprocesses in the sewage treatmentplants help to clean water beforedischarging it into water bodies.Similarly, municipal bodies treat thewater before supplying it to households.

water. Boiling kills the germs presentin the water.

� Chlorination is a commonly usedchemical method for purifying water.It is done by adding chlorine tabletsor bleaching powder to the water. Wemust be cautious. We should notuse more chlorine tablets thanspecified.

18.9 What can be Done?

Activity 18.8

Investigate the level of awarenessabout water pollution in your area.Collect data on the sources ofdrinking water and the methods ofsewage disposal.What are the common water-bornediseases in the community? You canconsult your local doctor/healthworker for this.Which are the governmental andnon-governmental organisationsworking in this field? What are themeasures being taken by them forgenerating awareness?

Laws for industrial units should bestrictly implemented so that pollutedwater is not disposed off directly intorivers and lakes. Water treatment plantsshould be installed in all industrialareas (Fig. 18.10). At our individuallevels we should consciously save waterand not waste it. Reduce, reuse andrecycle should be our mantra !

Think of your daily routine – Howcan you save water?

We can think of creative ideas likereusing water used for washing and forother household tasks. For example,

Do you know?

25% of the world’s population iswithout safe drinking water !

Let us see how water can be madesafe for drinking:� You have already seen how water is

filtered. This is a physical method ofremoving impurities. A popularhousehold filter is a candle typefilter.

� Many households use boiling as amethod for obtaining safe drinking


water used for washing vegetables maybe used to water plants in the garden.

Pollution is no longer a distantphenomenon. It is affecting the quality

of our daily lives. Unless we all realiseour responsibility and start usingenvironment-friendly processes, the verysurvival of our planet is in danger.

Fig. 18.10 : Water treatment plant

Do you know?

While brushing your teeth, leaving the tap running may waste several litres ofwater. A tap that drips once every second wastes a few thousand litres of waterevery year. Think about it !

SCIENCE252

KEYWORDS

ACID RAIN

AIR POLLUTION

CHEMICAL

CONTAMINATION

GLOBAL WARMING

GREENHOUSE

EFFECT

POLLUTANTS

POTABLE WATER

WATER POLLUTION


� Air pollution is the contamination of air by

impurities which may have a harmful impact

on the living organisms and the non-living

components.

� Pollutants are the substances which

contaminate air and water.

� Carbon monoxide, nitrogen oxides, carbon

dioxide, methane and sulphur dioxide are the

major pollutants of air.

� Increasing levels of greenhouse gases like CO2

are leading to global warming.

� Water pollution is the contamination of water

by substances harmful to life.

� Sewage, agricultural chemicals and industrial

waste are some of the major contaminants of

water.

� Water which is purified and fit for drinking is

known as potable water.

� Water is a precious natural resource. We must

learn to conserve it.

Exercises

1. What are the different ways in which water gets contaminated ?

2. At an individual level, how can you help reduce air pollution?

3. Clear, transparent water is always fit for drinking. Comment.

4. You are a member of the municipal body of your town.

Make a list of measures that would help your town to ensure the supplyof clean water to all its residents.

5. Explain the differences between pure air and polluted air.

6. Explain circumstances leading to acid rain. How does acid rain affect us?

7. Which of the following is not a greenhouse gas?

(a) Carbon dioxide

(b) Sulphur dioxide


(c) Methane

(d Nitrogen

8. Describe the ‘Green House Effect’ in your own words.

9. Prepare a brief speech on global warming that you have to make in yourclass.

10. Describe the threat to the beauty of the Taj Mahal.

11. Why does the increased level of nutrients in the water affect the survivalof aquatic organisms?

E X

E R

C I

S E

S


1. In some cities, a pollution check has been made compulsory forvehicles. Visit a petrol pump in order to learn about the process ofconducting a pollution check. You may systematically record yourfindings in the following areas:

� Average number of vehicles checked per month.

� Time taken to check each vehicle.

� Pollutants checked for.

� The process of testing.

� Permissible levels of emission of various gases.

� Measures taken if the emitted gases are above the permissiblelimits.

� How frequently is a pollution check required ?

2. Conduct a survey in your school to investigate various environmentrelated activities that have been undertaken. The class can divideitself into two groups, with each group looking at a different area.For example, one group can find out whether there is anenvironment club in the school. What are its objectives ? What isits calendar of events ? How can you become a member ?

If your school does not have such a club, you can even think ofstarting one along with a few of your friends.

3. Organise a field visit to a river in or around your town with the helpof your teachers.

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Observations followed by discussion could focus on :

� The history of the river.

� Cultural traditions.

� Role of the river in meeting the town’s water needs.

� Pollution concerns.

� Sources of pollution.

� Effects of pollution on the people living by the riverside as wellas those living far away.

4. Find out with the help of your teachers and the internet (if possible),whether there are any international agreements to control globalwarming. Which are the gases covered under these agreements?

� www.edugreen.teri.res.in/explore/air/air.htm

� www.edugreen.teri.res.in/explore/water/pollu.htm

� www.cpcb.nic.in/citizen’s%Charter/default_citizen’s.html

� coe.mse.ac.in/kidswater.asp

� coe.mse.ac.in/kidsair.asp

INDEXINDEX

AAcid Rain 73, 241

Acrylic 34

Adam’s Apple 116

Adolescence 120

Adrenalin 120

Agricultural Practices 1, 2

Air Pollution 240, 241

Algae 17-19

Amplitude 164, 165

Angle of Incidence 200

Angle of Reflection 200

Animal Husbandry 12

Antibiotics 20,21

Antibodies 21

Artificial Satellites 232

Artificial silk 33

Asexual Reproduction 100, 106, 107

Asteroids 230

Atmospheric pressure 140, 141

Atom 52

Audible 166

BBacteria 17-21

Balanced Diet 121

Ball Bearing 153

Binary Fission 107

Biodiversity 78, 79

Biosphere Reserve 79-82

Blind spot 206

Braille 209, 210

Budding 107

CCalorific Value 72

Carrier 23

Cassiopeia 224

Celestial Objects 215, 216

Cell 90-96, 173

Cell Membrane 93, 94

Cell Wall 94

Chemical Contamination 247

Chloroplast 96

Chromosome 95

Coal 44, 45, 57, 59-61

Coal Gas 58

Coal Tar 58

Coke 58

Combustion 64, 65

Comets 251

Communicable Diseases 23

Conductor 45, 175, 176

Cones 206

Constellations 221-224

Contact Force 134, 135

Cornea 206

Crop 1-3, 10

Crust 192

Cytoplasm 94

DDeforestation 75, 77, 78, 84

Desertification 78

Diffused/Irregular reflection 202

Discharge 188, 189

Dispersion 205

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Displacement Reaction 50, 51

Drag 153

Ductility 45

EEardrum 161, 162

Earthquake 190-195

Earth’s Plates 190, 193

Ecosystem 83

Eggs 90, 91, 102-106, 108, 116-118

Electrode 176-178

Electroplating 178, 179

Electrostatic Force 136, 138

Elements 52

Embryo 104

Endangered Species 83

Endemic Species 81

Endocrine Glands 117

Estrogen 117

Eukaryotes 95

Explosion 70

External Fertilisation 103

Extinct 83

FFauna 80, 81

Fermentation 20

Fertilisation 102

Fertiliser 5-7

Fire Extinguisher 69

Flame 64, 65, 70-72

Flora 80-82

Fluid Friction 153

Foetus 105

Force 127-138, 141

Fossil Fuel 57, 61

Friction 146-153

Fuel 64, 68, 69, 72, 73

Fuel Efficiency 72

Fungi 11, 17-19

GGene 95Global Warming 61, 73, 243

Good Conductor 45, 172, 173, 175, 176, 187Granaries 12

Gravitational Force 137Gravity 137Green House Effect 242, 243

HHardness 44

Harvesting 10, 11

Hertz (Hz) 164, 166

Hormones 117, 119, 120

IIdeal Fuel 72Ignition Temperature 67

Incident Rays 199-201Inflammable Substances 68

Insulin 120Interlocking 151Internal Fertilisation 102

Iris 206Irrigation 7-9

KKaleidoscope 204, 205

Kharif 2

LLactobacillus 19

Larynx 160

Lateral Inversion 202

Laws of Reflection 199-201

LED 174

Light Year 220

Lightning 184, 188, 189

Lightning Conductor 189, 190

Loudness 164, 165

Lubricants 151, 152

INDEX 255

MMagnetic Force 135

Malleability 45, 46

Manure 5

Metalloids 52

Metals 44-46, 48-52

Metamorphosis 106

Meteorites 231

Meteors 231

Microorganism 17, 19-26

Migratory Birds 84

Multicellular 91

Muscular Force 134, 135

NNational Park 82, 83

Natural Gas 56, 57, 59-61

Natural Satellites 228

Negative Charge 187, 188

Nitrogen Cycle 27

Nitrogen Fixation 26

Noise 166, 167

Non-Contact Force 136, 137

Non-Metals 44, 46-52

Nuclear Membrane 95

Nucleolus 95

Nucleus 94, 96

Nylon 33, 34

OOrbit 217, 225, 226

Organ 92, 93

Organelles 95

Orion 222, 224

Oscillation 164

Oviparous Animals 105

PPasteurisation 26

Pathogen 23

Petroleum 56-61Petroleum Refinery 60Phases of Moon 216-218Pitch 165, 166Pituitary Gland 117Planets 225-228, 230Plasma membrane 93, 94Plastic 36-39Plastid 96Plough 3Pole Star 221Pollutants 240, 241, 243, 245Polyester 34, 35Polymer 33, 36Polythene 34, 36Poor Conductor 172, 175Positive Charge 188Potable Water 248Preservation 25, 26Pressure 137-142Prokaryotes 95Protozoa 17Pseudopodia 92Puberty 117, 118Pull 127-132Pupil 204, 206Push 127-138

RRabi 2Rayon 33Red Data Book 83, 84Reflected Rays 199-201Reflection 199-202Reforestation 84Regular Reflection 202Remote Sensing 232Reproductive Health 120Retina 204, 206Rhizobium 26Richter Scale 193, 194Rods 208

Rolling Friction 152

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SSanctuary 78, 79, 82Secondary Sexual Characters 116

Seeds 4-6Seismograph 194

Sex Chromosomes 118Sexual Reproduction 100Shooting Stars 231

Shrillness 165Silo 12

Sliding Friction 152Solar System 215, 224, 228Sonorous 46

Sowing 4, 5Sperms 101, 102

Stars 219, 222Static Friction 149

Storage 11, 12Synthetic Fibres 32, 33, 35, 36

TTarget Site 117

Terylene 34Testosterone 117

Thermoplastics 37Thermosetting Plastics 37Threshing 11

Thunder 189Thunderstorm 189, 190

Thyroxine 119, 120

Time Period 164

Tissue 92, 93, 95

Transfer of Charge 187

Tremor 193, 194

Tsunami 192

UUnicellular 91

Ursa Major 221-223

VVaccine 21

Vacuole 96

Vibration 158

Virus 17, 19, 23, 24

Viviparous Animals 105

Voice Box 115, 158

WWater Pollution 245

Weedicide 10

Weeds 10

White Blood Cell (WBC) 92

Wind Pipe 160

Winnowing 11

YYeast 20

ZZygote 102

ncert science viii book

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