form 2 science chapter 5

Form 2 Chapter 5: WATER AND SOLUTION

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5.1 PHYSICAL CHARACTERISTICS OF WATER

Pure water is colourless, odourless and tasteless liquid

Water exists in 3 states: solid (ice), liquid (water) and gas (steam/water vapour)

Water can change state from one state to another:

The changes in the state of water occur at specific temperature

Point Explanation Example

Melting point The temperature at which ice (solid) becomes water (liquid)

Melting point of ice = 0oC

Freezing point The temperature at which water (liquid) becomes ice (solid)

Freezing point of water = 0oC

Boiling point The temperature at which water (liquid) becomes steam (gas)

Boiling point of water = 100oC

The temperature of a substance remains constant, that is it does not rise or drop, during the

change of state.

The density of water is 1 g per cm3 at 4oC. 1 cm3 of water has a mass of 1 gram. For example,

50 cm3 of water has a mass of 50 gram.

Water is a poor heat and electrical conductor. This means that water does not conduct heat

and electric current readily.

5.1.1 Determining the freezing point of water

Water changes from liquid to solid at its freezing point.

Freezing point of water is the temperature at which water freezes into ice.

Liquid solid

The freezing point of pure water is 0oC.


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The temperature remains constant at 0oC until water freezes completely.

Kinetic theory of freezing point of water.

The water is cooled.

When the temperature drops, the water particles lose kinetic energy and move slowly.

Therefore, the forces of attraction among the particles grow stronger.

At 0oC, the forces of attraction are so great that the water particles are held in fixed

positions.

Water freezes into ice.

Unlike other liquids, water expands upon freezing. Thus, the ice is less dense than water and

floats on water.

This characteristic allows aquatic life to survive during winter. When the lake water freezes, ice

floats on the surface. Since ice is also a good thermal insulator, the water underneath it does

not freeze. This allows life to continue under the freezing lake.


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5.1.2 Determining the boiling point of water

Water boils at its boiling point.

Boiling point of water is the temperature at which water boils and becomes steam.

Liquid gas

The boiling point of pure water is 100oC.

The temperature remains constant at 100oC until water boils completely.

Kinetic theory of boiling point of water

The water is heated.

When the temperature rises, the water particles gain energy, and the particles move faster

and further apart.

Therefore, the forces of attraction among the particles grow weaker.

At 100oC, water particles move so fast that they overcome the forces of attraction and

leave the liquids surface.

Water turns into gas (steam).


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5.1.3 The Effects of Impurities on the Physical Properties of Water

The presence of impurities can change the physical properties of water.

Examples of impurities are salt, sugar, acid and alkali.

The effect of impurities in water:

Change the colour, taste or smell of water

Increase the density of water.

Increase the boiling point of water to above 100oC

Lower the freezing point of water to below 0oC

Improve thermal and electrical conductivity of water

Sea water is denser than fresh water. This is why ships can float better in sea water than in

fresh water. Sea water also has lower freezing point and higher boiling point than pure water.

5.1.4 Water Test

The presence of water can be determined by the tests as shown in table below.

Water test Observation/Results

The melting/freezing point and the boiling point

The melting/freezing point of water is 0oC and the boiling point of water is 100oC (also used to determine the purity of water)

Anhydrous cobalt chloride paper Changes from blue to pink

Anhydrous copper (II) sulphate Changes from white to blue

5.2 COMPOSITION OF WATER

Water is a compound. It is made of two different elements.

Hydrogen (H2)

Oxygen (O2)

The chemical formula of water is H2O. Two hydrogen atoms and one oxygen atom combine

chemically to form one molecule of water. The ratio of hydrogen to oxygen is 2:1

Water can be broken down to its elements through electrolysis.

Electrolysis is a method to break down a compound by passing an electric current through its

liquid or solution

In the process of electrolysis, oxygen is discharged at the anode (positive electrode) and

hydrogen is discharged at the cathode (negative electrode).

The volume of hydrogen released is always twice the volume of oxygen released.

The acid helps water to conduct electricity.

O H H


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Confirmation Test of Water Composition

Oxygen is tested with a glowing wooden splint. The wooden splint lights up when oxygen is

present. This is due to oxygen supports combustion.

Hydrogen is tested with a lighted wooden splint. Pop sound is produced when hydrogen is

present.

5.3 EVAPORATION OF WATER

Water evaporation is a process where water molecules are released as vapour into air below

boiling point of water.


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Evaporation occurs at any temperature and at any time.

During evaporation, water molecules at the surface gain enough energy from surroundings to

escape from the surface into the air as water vapour.

The water left behind becomes cooler because the evaporating water molecules remove some

energy from the water.

The rate of water evaporation is influenced by a few factors and can be explained using the

Kinetic Theory.

5.3.1 Factors Affecting the Rate of Evaporation of Water

Temperature of surroundings

When the temperature of surrounding increased, water molecules gain more energy, move

faster and escape into the air faster.

The higher the temperature of surroundings, the higher the rate of evaporation.

Example, wet clothes dry faster under the hot sun.

Surface area of water

When the exposed surface area is larger, it allows more water molecules to escape from

the water surface.

The larger the surface area of water, the higher the rate of evaporation.

Example, wet towels dry faster when they were spread widely.

Air humidity

Humidity refers to the amount of water vapour in the air.

When humidity is high, it is more difficult for water to evaporate.

The higher the humidity, the lower the rate of evaporation.

Example, when raining, the air contains a lot of water vapour, wet clothes dry slower.

Movement of air

When evaporation takes place, water vapour gathers above the water surface.

When there is the movement of air, the water vapour above the water surface is removed

as soon as it evaporates. This allows more water molecules to evaporate into the air.

The rate of evaporation is greater in moving air.

5.3.2 Comparison between evaporation and boiling

Evaporation Boiling Similarities - Heat energy is absorbed and converted into kinetic energy.

- Liquids become gases Rate Slow Fast

Temperature At any temperature below boiling point At the boiling point

Site Occurs only on the surface of water Occurs throughout the water

Observation No visible effect Air bubbles are formed in water Factors - Humidity

- Surrounding temperature - Surface area of water - Air movement

- Air pressure - Presence of impurities - Rate of heating - Volume of water


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Water vapour has the same temperature as the air, while the temperature of steam is above

the boiling point of water

5.3.3 Applications of Evaporation

Drying clothes

Wet clothes are hung out to expose a large surface area to the sun and moving air.

Drying hair

We dry our hair by using a hair dryer to fasten evaporation of water from the hair.

Cooling the body

Evaporation of sweat from our body cools the body.

Preserving agriculture products

Evaporation of water is used to dry and preserve agriculture products such as groundnuts,

paddy, chillies and coffee.

Preserving food

Evaporation of water is used to dry and preserve shrimps, fish, squids and vegetables.

Processing of milk powder

Hot air is used to evaporate water from milk to get milk powder.

Obtaining common salt

Common salt is obtained by evaporating sea water in large pans.

5.4 SOLUTION AND SOLUBILITY

5.4.1 Solute, Solvent and Solution

Solute A substance that dissolves in other substance (solvent)

Solvent A liquid that dissolves other substances (solute)

Solution A mixture that is formed when one or more solutes dissolve in a solvent


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Figure above shows that when some sugar is added to a beaker of water and stirred, the sugar

is no more visible.

The sugar which is soluble in water is called solute.

The water is called solvent because it dissolves the sugar.

The mixture that is formed when a solute dissolves in a solvent is called a solution.

The particles of a solute and a solvent are evenly distributed in a solution. At any part of the

solution, the taste, appearance and composition are the same. Thus, a solution is

homogeneous.

When a solution is formed, the solute cannot be separated from the solvent by filtration.

However, solute can be obtained by evaporating the solution to dryness.

Water is the most common solvent because it dissolves many types of substances.

Petrochemical solvents are used to dissolve oil and grease.

5.4.2 Solution and Suspension

Mixtures of liquids can be divided into :

Solutions

Suspensions

Mixtures which contain dissolved substances are called solutions.

Mixtures which contain insoluble substances are called suspensions.

A suspended substance is a substance that does not dissolve in water.

A suspension can be separated by filtration.

The differences between a solution and a suspensions


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5.4.3 Dilute, Concentrated and Saturated Solution

Dilute solution

Contains very little solute

Still can dissolve much more solute

There is a lot of space between the particles of solvent

Concentrated solution

Contains a lot of solute

Can only dissolve a little more of the solute

There is a little space between the particles of solvent

Saturated solution

Has dissolved the maximum amount of solute at that particular temperature

Cannot dissolve any more solute

Any extra solute added into the solution will be deposited at the bottom of the solution

There is very little space between the particles of solvent

5.4.4 Solubility

Solubility is the maximum amount of solute in grams that will dissolve in 100 g of solvent at a given

temperature.

The unit of solubility is g per ml of solvent.

For example, the solubility of sodium chloride is 38 g per 100g of water at 20oC. This means that

we can dissolve not more than 38 g of sodium chloride in 100 g of water at 20oC.

The solubility of a solute is affected by :

the nature of a solvent

the nature of a solute

the temperature of a solvent


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The nature of solvents.

A substance has different solubility in different solvents.

The examples are :

Sugar (solute) dissolves faster in water (solvent) than in other solvents like alcohol and

oil.

Ink (solute) dissolves quickly in acetone and alcohol but dissolves very slowly in water.

The nature of solutes.

Different solutes have different solubility in a given solvent.

For example, salt is more soluble in water than in a suspension of wheat flour.

The temperature of solvents.

The solubility of a solid in a liquid solvent increases if the temperature of solvent increases.

The solubility of a gas in a liquid solvent decreases if the temperature of solvent increases.

Most solutes are more soluble at higher temperatures than at lower temperatures.

For example, common salt dissolves faster in a hot water than in cold water.

5.4.5 The Rate of Dissolving a Solute

The rate of dissolving a solute indicates how fast a substance can dissolve in a given amount of

solvent, at a certain temperature.

The time when a solute is added to a solvent until it has completely dissolved determines the

time for dissolving process.

Factors affecting the rate of dissolving are:

The size of solute particles.

A solute in powder form dissolves faster than in one large piece. This is because tiny

pieces of solute offer a larger surface area to come into contact with the solvent.

The rate of dissolving increases with smaller solute particles.

The rate of stirring the solvent.

A solute dissolves faster in the solvent if the solute is stirred. Stirring breaks up the

solute and causes its particles to move faster into the space between the particles of

the solvent.

The rate of dissolving increases at a higher rate of stirring.

The temperature of solvent.

The hotter the solvent, the faster the solute dissolves. A hot solvent supplies more

energy to the particles in the solute. As a result the particles break away from the

solute faster.

The rate of dissolving increases at higher temperatures.

The volume of solvent.

A large volume of solvent dissolves a solute faster. A large volume of solvent has more

empty spaces between its particles for the particles of solute to enter.

The rate of dissolving increases with larger volume of solvent.


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5.4.6 Water as a Solvent

Water is known as universal solvent because water can dissolve many types of substances.

A solution in which water is the solvent is called an aqueous solution.

The importance of water as a solvent

Water is used as solvent during preparation of foods and drinks like tea, coffee and fruit

juices.

Water is also needed to dissolve detergent for washing clothes, cars, cooking utensils and

lab apparatus.

Water which covers 70% of the Earth's surface is the solvent in rivers, seas, ponds and

lakes.

Water acts as a medium in many chemical reactions in our body.

Human body processes like digestion of food, respiratory, excretory and blood circulatory

need water as a solvent.

Water is used as a solvent in many types of industries that manufacture foods, drinks,

medicines, detergents and textiles.

Plants need water for photosynthesis and transpiration processes.

Aquatic organisms are able to live in water because oxygen and carbon dioxide dissolve in

water.

5.4.7 Organic Solvent

Some organic substances are not soluble in water, but they are soluble in organic solvents like

alcohol, petrol, kerosene, turpentine, acetone and ether.


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Organic solvents are volatile. Therefore, products such as paints, lacquers, varnishes and inks

become dry in a short period of time.

However, products containing organic solvents should be handled carefully.

They are flammable and should be stored away from heat.

They are toxic and carcinogenic (likely to cause cancer).

5.5 ACIDS AND ALKALIS

Acid and alkali will show their properties in the presence of water.

Two main groups of solutions are acids and alkalis.

5.5.1 Acid

An acid is a substance that has a hydrogen atom, which can be replaced by a metal or

ammonium.

Acids can be classified as :

Organic acids

Inorganic or mineral acids.


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Organic acids.

Organic acids which contain carbon are normally found in plants and animals.

Common organic acids are shown in the Table above.

Inorganic acids

Mineral salts are used to prepare inorganic acids. Therefore, it is also called mineral acids.

Common inorganics acids are :

Nitric acid

Sulphuric acid

Hydrochloric acid

Acids exist in three states which are :

Solid - tartaric acid

Liquid - ethanoic acid ( acetic acid )

Gas - hydrogen chloride

Acids have the following properties :

Taste sour

Corrosive if it comes into contact with skin, it will burn

Change blue litmus paper to red

Have pH values of less than 7

React with carbonates to release carbon dioxide and form salt and water


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Example :

React with reactive metals (magnesium / aluminium / zinc / iron) to release hydrogen and

form salts

Example :

Reacts with alkalis to form salt and water (neutralisation process)

5.5.2 Alkali

An alkali is hydroxide or metal oxide that dissolves in water.

Example: potassium hydroxide, sodium hydroxide, calcium hydroxide (lime water), ammonium

hydroxide (ammonia solutions).

Alkalis have the following properties :

Taste bitter

Feel slippery like soap when touched with the fingers

Corrosive

Change red litmus paper to blue

Have pH values of more than 7

Reacts with ammonium salts to release ammonia when heated

Example:

React with acids to form salt and water (neutralisation process)

5.5.3 pH values

The pH value of a substance shows if the substance is acidic, neutral or alkaline.

The pH scale has values from 1 to 14.

Acidic conditions (pH 1 - 6) : a substance with a value of pH 1 is far more acidic than a

substance with a pH value of 6

Neutral condition (pH 7) : neither acidic nor alkaline

Alkaline conditions (pH 8 - 14) : a substance with pH value of 14 is far more alkaline than a

substance with a pH value of 8


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The pH values of substances are determined by using pH paper or universal indicator solutions.

Indicator Colour of solution

Acidic Neutral Alkaline

Litmus solution Red Purple Blue

Methyl orange/red Red Orange Yellow Phenolphthalein Colourless Pink Red

Universal indicator Red/orange/yellow Green Purple

5.5.4 Acidic and alkaline substances in everyday life

Lemon juice and vinegar have a sour taste. Things with sour waste contain acids.

Many things in our daily lives contain alkalis.

The following procedure is used to determine if a substance is acidic or alkaline in our daily life :

Add two drops of universal indicator solution to a test tube containing the substance to be

tested. Mix the substance with water if needed.

Compare the colour of the solution formed with the universal indicator chart.

Determine the pH of the solution and the acidic or alkaline conditions of the substance.


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5.5.5 Neutralisation

Neutralisation is the reaction that occurs between an acid and an alkali to form salt and water.

The word equation below represents the neutralisation reaction between an acid and an alkali.

At the neutralisation point, all the acid completely reacts with the alkali; they cancel out each

others properties. The solution formed does not show acidic or alkaline properties and has a

pH value of 7.

The salt formed from the neutralisation process depends on the type of acid and alkali used, as

shown in the following table.

The method of mixing an acid with an alkali using a burette to achieve the point of

neutralisation is known as the titration method. In this method,

The correct amounts of an acid and an alkali are mixed using a burette.

An acid-alkali indicator is used to detect the end-point of neutralisation.

To neutralise an acid, an alkali is dripped into conical flask containing acid and stop once the

colour changes from red (acidic pH) to green (neutral pH), using universal indicator.


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The applications of the neutralisation process in everyday life are shown below:

Using hair shampoo that is alkaline which is neutralised by hair conditioners that are acidic

so that the hair looks clean and soft?

Applying alkaline medicine on insect bites that are acidic. For example, a bee sting, a

mosquito or an ant bite, which is acidic, can be neutralised by applying lotion that contains

zinc carbonate.

Reducing soil acidity by adding slaked line that is alkaline.

Gastric patients drink milk of magnesia or take pills such as magnesium hydroxide or

aluminium hydroxide that is alkaline to reduce stomach acidity

A weak acid such as vinegar can be used to neutralise a wasp sting that is alkaline

Bacteria in the mouth change sugary food into acid that attacks the teeth causing tooth

decay. Toothpaste contains weak alkalis to neutralise the acid in the mouth.

5.6 Water Purification

5.6.1 Natural sources of water

Natural water contains impurities and needs to be treated before it is safe for drinking.

Water source Contents of water Rain The cleanest source of water

Has very little dissolved substances and a little dust

Groundwater, wells, springs

Quite clean as it originates from rain water that has seeped through the ground

It contains dissolved minerals and microorganisms

Rivers, streams Dirtier

It contains dissolved and decaying substances, mud and microorganisms

Lakes, ponds Much dirtier as it is stagnant

It contains more decaying substances, mud and microorganisms

Seas, ocean The dirtiest source of water

It contains lots of dissolved and decaying substances and microorganisms

Usage of Water

Domestic use of water

- Drinking

- Washing

- Watering plants

Agriculture use of water

- Irrigation

- Aquaculture

Direct use of rivers and streams

- Generate hydroelectric power

- Recreation

- Transportation

Industrial use of water

- Cooling

- Washing

- Disposal of waste


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5.6.2 Methods of Water Purification

Natural water is not pure. It contains water pollutants such as

Dissolved salts

Microorganisms

Harmful chemicals

Rocks, clay, sand and plants

Human and animal wastes

Natural water needs to be purified before being used or consumed.

Purified water should be free from water pollutants, bad odours and unpleasant taste.

The methods of water purification includes:

Filtration

Boiling

Chlorination

Distillation

Ozonation

Ultraviolet irradiation

Water purification method Advantage Disadvantage Filtration

Using a filter (such as filter paper) to remove insoluble solids such as dust, sand and other particles from a solution

Removes solid particles

Environmentally friendly

Beneficial minerals are not removed from the water

Simple and cost effective

Does not kill microorganisms

Does not remove dissolved mineral salts

Distillation

Using heat to boil water. The steam condenses into water when cooled. The water produced is called distilled water or pure water.

Removes solid particles

Kills microorganisms

Removes dissolved mineral salts

Requires heat energy

Distilled water lacks essential minerals

Expensive to operate


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Water purification method Advantage Disadvantage

Boiling

Boiling water at its boiling point. Water is then cooled before drinking.



Does not remove solid particles


Requires heat energy

Chlorination

Dissolving chlorine in water to purify water in water treatment plants or swimming pools.


Decolourises water


Does not remove solid particles and dissolved mineral salts

Chlorine is harmful if used excessively

Changes the taste of water

Ozonation

Using low levels of ozone to sanitise water in the swimming pools and spas, and to disinfect laundry in hospitals and food factories.






Ultraviolet irradiation

Using ultraviolet light to sterilise the water in fish ponds, food factories and portable drinking water dispensers.


Chemicals are not added to the water






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5.6.3 Application of water purification methods

Filtration

River water or well water is filtered before being sent to consumers at home

Boiling

Piped water or well water is boiled before drinking

Surgical equipment is sterilised in boiling water to kill microorganisms

Chlorination

Piped water that has been filtered at a water treatment plant will have chlorine added to it

before it can be used

Chlorine is added to the water in a swimming pool to kill microorganisms that cause

diseases like skin infections.

Distillation

Distilled water is used in clinics and hospitals to prepare medicines.

Distilled water is prepared in school laboratories to prepare chemical solutions.

Distilled water is used in car batteries.

5.7 Water Supply System

Diagram below shows the stages in a water treatment plant.


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Properties of the drinking water that is supplied to consumers:

Does not contain colour or smell.

Does not contain suspended particles or harmful substances.

Does not contain harmful microorganisms.

Contains dissolved mineral salts.

5.7.1 Water purification and supply system

Storage reservoir

River water is pumped and collected in a storage reservoir.

The stagnant water allows mud, silt and other suspended solid particles to sink to the

bottom of the reservoir.

As the water is exposed to the sunlight, some of the dangerous bacteria in the water are

killed by the sunlight.

Some of the large solid particles are removed by a metal screen.

Coagulation tank

From the reservoir, water is forced through an aeration system to dissolve oxygen.

Aeration system helps to remove unpleasant smell from the water.

In the coagulation tank, slaked lime and alum (aluminium sulphate) are added to the water.

Alum sticks fine solid particles together to form larger lumps. While, slaked lime reduces

the acidity of water.

Sedimentation tank

Then, water flows into the sedimentation tank.

The larger lumps settle down to the bottom of the tank.

Filtration tank

As water flows into the filtration tank, it passes through layers of sand filters.

Suspended particles and sediment are trapped by the sand filters, and then removed.

Chlorination and fluoridation tank

Clean water flows into the chlorination tank.

A small amount of chlorine is mixed to the water to kill microorganisms, like bacteria and

fungi.

The water is enriched by adding a small quantity of fluoride salt to prevent tooth decay.

Storage

Then, treated water flows into the storage tank.

From the storage tank, water is pumped into the supply ponds which are situated on a

higher ground than consumers' houses.

Clean water is supplied to the consumers' houses through pipes.


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Summary of Water Purification

Water from the rivers is channel into the reservoir.

In the Aeration Tank bad smells will be aired and removed from the water.

Alum and Lime are added into the water in the coagulation tank. Alum will make particles join

together to form larger particles while lime will decrease the acidity of water.

In the mixing tank, alum and lime will be evenly distributed in the water the water.

The water travels into the sedimentation tank so that large particles will sediment to the

bottom.

Lighter particles which floats on the water will flow into the filtration tank, suspended particles

will be filtered.

Clean water will then be stored at the storage tank. In the storage tank, chlorine and fluorine

will be added in the water.

5.8 Preservation of Water Quality

Water pollutant Effects Ways to control

Domestic waste

Rubbish, garbage

Untreated sewage

Rubbish chokes up rivers causing flash floods.

Plastics wastes are non-biodegradable (cannot be broken down by microorganisms). Thus, they remain in water for a long time, killing many

Educate the public

Reuse and recycle waste materials

Encourage the use of degradable and non-phosphate


pg. 23

seabirds and aquatic animals that swallow them.

Harmful microorganisms in faeces can cause diseases such as cholera, dysentery and hepatitis A.

Detergents reduce dissolved oxygen in water, killing many aquatic organisms due to phosphate in detergents are nutrients to plants, causing algal bloom (rapid growth of algae and aquatic plants). When the plants die, large population of bacteria decompose them, using up oxygen in water.

based detergents

Relocate residents in squatter areas as these areas lack proper sewage systems

Agriculture waste

Pesticides and fertilisers

Animal waste

In high concentrations, pesticides poison fish, animal and humans that drink the polluted water.

Non-biodegradable pesticides remain in the water for a long time. They are passed along in food chains and become concentrated in the final consumers, causing many health problems.

Nitrates and phosphates in fertilisers are nutrients to algae and aquatic plants. The algae bloom that occurs will eventually reduce dissolved oxygen in water, killing many aquatic organisms.

Feeding on animal waste, the population of bacteria increases, and reduces the oxygen level in water.

Educate farmers on the proper use of pesticides and fertilisers

Encourage the use of biological control

Implement laws regarding disposal of waste from farms

Industrial waste

Toxic waste

Acidic or alkaline substances

Heavy metals

Radioactive residues

Toxic wastes poison aquatic life.

Acidic and alkaline substances change the acidity of water, causing death to sensitive aquatic organisms.

Heavy metals accumulate and are passes along food chains. In high concentration, they are poisonous to our body.

Radioactive residues are carcinogenic (can cause cancer).

Implement strict laws regarding disposal of industrial waste

Treating waste before they are disposed into rivers

Siltation

Mud and sand from deforestation or construction sites

Water becomes muddy, preventing sunlight from penetrating the water.

Submerged plants cannot photosynthesise and die.

Rivers become shallow, causing flash floods.

Control deforestation to prevent severe soil erosion

Oil spillage

From oil rigs, oil tankers and illegal cleaning of vessels

The oil floats on the surface of the sea, killing seabirds and marine life.

Constant surveillance of the waters


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In short, the main steps of water pollution control are prevention, enforcement and

monitoring.

Ways of conserving and preserving water and its quality

Water should be used carefully in order to avoid wastage and shortage of water.

To ensure continuous supply of clean water, we need to conserve and preserve water and its

quality.

Several steps need to be taken to save or conserve water, which includes:

Closing the water tap after using it.

Using water from cup and not running water from tap to rinse teeth after brushing.

Reusing the water used for washing fruits and vegetables to water plants.

Using buckets of water and not running tap water connected to a hose to wash car.

Collect rainwater to water plants and wash floor.

Use an energy efficient washing machine, and wash only full loads.

Install low-flush or dual-flush toilet cisterns.

Some examples of measures that can be taken for preserving water:

Cleaning up polluted rivers.

Promoting public awareness through education, seminars, media campaigns, exhibition

and talks.

Proper disposal of waste.

Strict enforcement of laws in pollution control.

form 2 science chapter 5

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