hkdse chemistry bridging programe 1a

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First published July, 2009

Chapter 1 Fundamentals of Chemistry 1

1.1 What is chemistry about? 1

1.2 Chemistry in our lives today 1

1.3 Classification of matter 2

1.4 Properties of substances 8

1.5 Physical and chemical changes 10

1.6 Working in chemistry laboratory 11

Key terms 17

Summary 17

Part I Planet Earth

Chapter 2 The atmosphere 19

2.1 Getting to know our planet Earth 19

2.2 The atmosphere 20

2.3 Separation of oxygen and nitrogen from air 22

2.4 Properties of oxygen 23

Key terms 24

Summary 25

Chapter 4 Rocks and minerals 37

4.1 Rocks 37

4.2 Extraction of metals from their ores 38

4.3 Limestone, chalk and marble 40

4.4 Weathering and erosion of rocks 40

4.5 Chemical changes involving calcium carbonate 42

4.6 Tests for calcium carbonate in a sample of limestone/chalk/marble 45

/ /

Key terms 47

Summary 48

Chapter 3 Oceans 26

3.1 Introducing oceans and seas 26

3.2 Composition of sea water 26

3.3 Extraction of common salt from sea water 27

3.4 Tests for sodium and chloride ions in common salt 31

3.5 Tests for the presence of water in a sample 33

3.6 Electrolysis of sea water and uses of products 33

Key terms 35

Summary 36

1

Chapter 1 Fundamentals of chemistry

Chemistry is the study of substances, about their

compositions, structures, properties and the changes among

them.

N1

N2

Note 1Refer to Chapter 8 and Chapter 9 for structures of substances.

Chemistry is a branch of science. ‘Science’ means the

knowledge gathered systematically from observations and

experiments.

Clothing, food, housing, transport and medical care are the

basic necessities of life. Chemistry plays a major role in each of

these parts of our modern lives.

Class practice 1.1 1.1

(a) Clothing

polyester, nylon, dyes

(d) Transport

metals, alloys, fuels, glass, plastics

(e) Medicines

drugs, antibiotics, artificial hormones

(f) Amusement park facilities

metals, alloys, cement, glass, plastics, semi-conductors

(b) Food

fertilizers, insecticides, food additives

(c) Housing

metals, alloys, cement, glass, plastics

A1.1

1.1 What is chemistry about?

1.2 Chemistry in our lives today 1.2

1.1

The photos below are some commodities or facilities in our daily

lives which are affected by or related to chemistry. Write down the

names of chemicals in them. The first one has been done for you as

an example.

Note 2Refer to ‘Supplementary information: The scientific method’ in theTeacher’s Guide.

2


Elements

Oxygen, hydrogen and carbon are elements.

Until January 2008, scientists have discovered or reported

118 elements. (You can find the names of the elements in the

Periodic Table on the front inside cover of the book.)

Percentage by mass of elements in nature

The percentage by mass of elements in nature is shown in

Figure 1.1.

An element is a pure substance that cannot be broken down

into anything simpler by chemical methods.

2008 1 118

(

)

1 . 1

aluminium 8.1%iron

5.0%

calcium 3.6%

magnesium

2.1% sodium 2.8%

potassium 2.6%

all other elements

1.5%

oxygen 46.6%

silicon ( )27.7%

Figure 1.1 Percentage by mass of elements in nature.

1.3 Classification of matter 1.3N3

Note 3Refer to ‘Supplementary information: Kinetictheory of matter’ in the Teacher’s Guide.

3


Oxygen is the most abundant element in nature — it alone

takes up almost 50% by mass of all elements. See Figure 1.2. 50%

1.2

Figure 1.2 Oxygen is present as a freeelement in air, and in combined formsin water and sand.

air

water

sand

( )

(a) (b)

(c) (d)

(e) (f)

1.2

By referring to the Periodic Table (on the front inside cover), state

which of the following substances are elements:

(a) Phosphorus (b) Sodium chloride

(c) Ammonia (d) Glucose

(e) Sulphuric acid (f) Mercury

Class practice 1.2

Compounds

A1.2Phosphorus and mercury are elements. The others are not. (Note: A substance with a name consisting of two words (e.g. sodium chloride)is not an element. A substance with a name of only one word (e.g. ammonia)may or may not be an element. The only sure way is to check the nameagainst the Periodic Table.)

Another example of a compound is copper(II) chloride. It is

made up of the elements copper and chlorine chemically

combined. In this case, the word equation is:

copper + chlorine copper(II) chloride

reactants products

(reacting substances) (substance produced)

Many common substances are compounds, such as water,

common salt and sugar.

A compound is a pure substance made up of two or more

elements chemically combined together.

(II)

+ (II)

( ) ( )

4


Example 1.1Statements about elements and compounds

This question consists of two separate statements. Decidewhether each of the two statements is true or false; if bothare true, then decide whether or not the second statement isa correct explanation of the first statement.

‘When two or more elements are mixed and heated, acompound is always formed.’

‘A compound is a pure substance made up of two or moreelements that are chemically combined.’

Solution

The first statement is false. A compound may or may notform, depending on which elements are mixed and heatedtogether.

The second statement is true.

1.1

Decomposition of compounds

We may also decompose (break down) a compound into its

constituent elements (or simpler substances) using electricity

(electrolysis) or heat. However, we can never decompose an

element chemically.

( )

(

)

Properties of compounds compared with those ofconstituent elements

Once formed, a compound has its own physical and chemical

properties. The properties are entirely different from those of

the constituent elements.

Mixture

A mixture consists of two or more pure substances

(elements or compounds) which have not chemically

combined together.( )

5


A pure substance is either an element or a compound. An

impure substance is always a mixture. There are three kinds of

mixtures:

• element/element mixture

• element/compound mixture

• compound/compound mixture

• /

• /

• /

Example 1.2Distinguishing between elements, compounds andmixtures

(a) Give an example of (i) a pure substance which is an element.(ii) a pure substance which is a compound.(iii) an impure substance.

(b) Explain why the example given in (a) (iii) is a mixture.

Solution

(a) (i) Copper wire (used as electrical wire) is purecopper, an element.

(ii) Distilled water is pure water, a compound.(iii) Sea water is an impure substance (impure water).

(b) Sea water consists of water (a compound), sodiumchloride (a compound) and other substances(compounds and elements), which have not chemicallycombined together.

➲ Try Chapter Exercise Q33

1.2

(a)(i)(ii)(iii)

(b) (a) (iii)

(a) (i) ( )

(ii) ()

(iii) ()

(b) ( )( ) (

)

➲ 33

Differences between mixtures and compounds

If we just mix iron filings and sulphur powder, there is no heat

change. We get a mixture of the two elements. Iron and sulphur

still retain their original properties in the mixture.

6


black solid

Property or test Iron Sulphur Iron/sulphur mixture Iron(II) sulphide

Appearance yellow solid yellowish grey

solid

black solid

attracted by

magnet

Action of

magnet

not attracted by

magnet

only iron attracted

by magnet

not attracted by

magnet

Properties of iron, sulphur, iron/sulphur mixture and

iron(II) sulphide (the compound formed from iron and

sulphur) are compared in Table 1.1.

1.1

(II) ( )

sinksAction of water most sulphur

sinks, while a

little floats

all iron and most

sulphur sink,

while a little

sulphur floats

sinks

liberates

hydrogen gas

Action of dilute

hydrochloric

acid

no reaction only iron reacts to

liberate hydrogen

gas

gives toxic

hydrogen

sulphide gas (with

smell of bad eggs)

(

)

Structure

Table 1.1 Comparison of properties of iron, sulphur, iron/sulphur mixture and iron(II) sulphide.(II)

N4

Note 4Another example of element, compound and mixture that can be used to further elaborate the point (students can be involved in giving the differences between element and compound):

Appearance

When in a fire

Colourless gas

Hydrogen

Burns with apop sound

Colourless gas

Oxygen

Supportsburning

Colourless gas

Hydrogen/oxygen mixture

Burns explosively orburns smoothly with a

very hot flame

Colourless liquid

water

Puts out fire

Property or testPlaced into a

balloon Rises in air Sinks in air

Rises or sinksdepending on the

composition of the mixture

Falls freely inair

7


Table 1.2 summarizes the main differences between

mixtures and compounds.1.2

Mixture Compound

1. Composition by mass variable (the substances in the mixture

can be mixed together in any

proportion)

(

)

fixed

(e.g. in water, the ratio by mass of

hydrogen to oxygen is always 1 : 8)

(

1 8)

2. Changes in formation no chemical reaction takes place;

usually no heat change in making a

mixture

a chemical reaction takes place;

heat is usually given out or absorbed

when a compound is made

Table 1.2 Main differences between mixtures and compounds.

constituent elements can only be

separated by chemical methods, not

by physical methods

3. Melting point (m.p.)

and boiling point (b.p.)

melts or boils over a wide range of

temperatures

(i.e. does not have a sharp m.p. or b.p.)

( )

melts or boils at a definite

temperature

(i.e. has a sharp m.p. and b.p.)

( )

4. General properties each constituent substance retains its

own properties

properties are entirely different from

those of its constituent elements

5. Separation of

constituents

constituents can be separated by

physical methods, based on differences

in physical properties

List (a) five elements (b) five compounds and (c) five mixtures.

Class practice 1.3

(a) (b) (c)

1.3

A1.3(a) Hydrogen, oxygen, nitrogen, iron, sulphur(b) Water, carbon dioxide, carbon monoxide, sodium chloride, iron(II) sulphide(c) Air, sea water, town gas, sodium chloride solution, wine

(Other answers may be given.)

8


1.3

separation by physical methods

direct mixing

chemical decomposition

chemical combination

Matter

MixturesPure

substances

Compounds Elements

Figure 1.3 Classification of matter.

Classification of matter

Based on what we have discussed in this chapter, we can

classify matter as shown in Figure 1.3.

The properties of any substance can be classified into its

physical properties and chemical properties, as explained

below.

Physical properties

Physical properties of a substance are those properties that

can be determined without the substance changing into

another substance.

1.4 Properties of substances 1.4

9


Typical physical properties include appearance (colour and

physical state), odour (smell), taste, hardness, density,

solubility (in various solvents), melting point, boilng point,

malleability (ability to be rolled into sheets), ductility (ability

to be drawn into wires), electrical conductivity and thermal

conductivity.

N5

N6

Chemical properties

Chemical properties of a substance are the chemical

reactions of the substance, and the respective conditions

under which each reaction takes place.

(

)

( )

(

) (

)

Note 6Gold is the most malleable element — 1 cm

3of gold can be rolled into a

thin foil enough to cover up a football field! Gold is also the most ductileelement — 1 g of gold can be drawn to 2400 m (or 1 ounce to 43 miles)!

For example, an effervescent tablet reacts with water quickly

to release carbon dioxide. This is a chemical property of

effervescent tablet.

(

)

1.3

(a)

(b)

(c)(d)

(a)

()

(b) 100°C(c) (i)

(ii)= 0°C

(iii)= 1 g cm–3

(d)

➲ 31

Example 1.3Distinguishing between physical and chemicalproperties of substances

(a) Explain why ‘boiling point’ is regarded as a physicalproperty. Illustrate your answer by using water as anexample.

(b) What is the boiling point of water at 1 atmosphericpressure?

(c) State three other physical properties of water.(d) State one chemical property of water.

Solution

(a) We can determine the boiling point of water by heatingwater until it boils, and then measuring thetemperature of the boiling water. During themeasurement, liquid water changes to steam, but nonew substance is formed.Note: Steam is water in gaseous state — it is still water.

(b) 100°C(c) (i) Water is a colourless liquid at room conditions.

(ii) Melting point of water = 0°C at 1 atmosphericpressure

(iii) Density of water = 1 g cm–3 at room conditions(d) Water reacts with iron and air to form rust at room

conditions.


Note 5The solubility of a solute (X) in a solvent (Y), at a given temperature, isthe maximum mass (in g) of X that can dissolve in 100 g of Y at thattemperature. The relationship between solubility behaviour and solubility(at 20°C) is roughly as follows:Very soluble: > 10 g Soluble: 1 – 10 gSlightly soluble: 0.01 – 1 g Insoluble: < 0.01 g

10


A physical change is a change in which no new substances

are formed.

Changes can be classified as either a physical change or a

chemical change.

Physical change

1.5 Physical and chemical changes 1.5

Change of state is a common example of physical change.

See Figure 1.4. 1.4

sublimation

deposition

boilin

g

melting freezing

cond

ensa

tion

gassolid

liquid

heat absorbed heat given out

Figure 1.4 Change of state is a commonexample of physical change.

A chemical change is a change in which one or more new

substances are formed.

Chemical change

Thus, the main difference between chemical and physical

changes is whether new substances are formed.

N7

Note 7In some cases, classification into physical change or chemicalchange may not be easy (e.g. in dissolution process).

11


State whether each of the following is a physical change or a

chemical change. Give your reasons.

(a) A magnesium ribbon burns in air.

(b) Sugar dissolves in water.

(c) Water changes to ice in a freezer.

(d) Iron rusts.

Class practice 1.4

(a)

(b)

(c)

(d)

1.4

A1.4(a) Chemical change(b) Physical change(c) Physical change(d) Chemical change(b) and (c) are physical changes becauseno new substances are formed. (a) and (d)are chemical changes because newsubstances are formed.

Observation in chemistry

Observation in chemistry includes four activities:

• Seeing with eyes

• Feeling with hands

• Smelling with nose

• Hearing with ears

•

•

•

•

1.6 Working in chemistry laboratory 1.6

Tasting with the tongue is also one way of observation, but

it is not allowed in the laboratory. ( )

Note 8Some students have the misconception that they need notmention the colour of a substance if it is colourless or white.Another misconception is that when no observable change isnoticed, just write down ‘no observation’. Actually ‘noobservable change’ is an important observation!

Example 1.4Making observations in a reaction

Add a small piece ofmagnesium ribbon to a testtube containing dilutesulphuric acid (Figure 1.5).What changes can youobserve?

Solution

(1) There is effervescence —colourless gas bubblesare evolved from themagnesium surface.

(2) A steamy fume is givenout.

(3) The magnesium ribbon gradually becomes smaller insize; it eventually dissolves completely to form acolourless solution.

(4) The test tube becomes hot.

(5) A hissing sound is heard.

(Note: All the above are observable changes, but only (1), (2)and (3) are visible changes.)

1.4

( 1.5)

(1)

(2)

(3)

(4)

(5)

((1) (2) (3)

)

Figure 1.5 Magnesium reacting withdilute sulphuric acid.

dilutesulphuricacid

gas bubbles

magnesiumribbon

N9

N10

N8

Note 9This question asks for observations. Thus it is wrongto put down something like: ‘Hydrogen is evolved fromthe magnesium surface.’ as it is impossible to tellwhether the bubbles are hydrogen or not.

Note 10Another observation may be: A choking smellis detected. In fact hydrogen has no smell. Thechoking smell detected is due to other gases

formed by the reaction of dilute sulphuric acidand the impurities present in magnesium.

12


Interpretation and prediction in chemistry

For all the experiments you will do in this course, try your best

to:

(1) Observe carefully and fully when doing experiments.

(2) Report experimental results clearly and accurately.

(3) Analyse the results and try to interpret them. Then

draw conclusions and make predictions.

(1)

(2)

(3)

Scientists make predictions. Which of the following predictions,

do you think, are highly reliable?

(a) Movement of the planets

(b) Occurrence of a sun eclipse

(c) Weather forecast

(d) Occurrence of an earthquake

(e) Tidal movement

(f) Eruption of a volcano

Class practice 1.5

(a)

(b)

(c)

(d)

(e)

(f)

1.5

A1.5(a), (b) and (e).

Laboratory safety

To avoid accidents, always remember: ‘Laboratory safety is of

first importance in any experimental work.’

13


Basic laboratory safety rules

(1) Do not work in the laboratory unless your teacher is

present.

(2) Follow strictly the instructions given by your teacher.

(3) Never run around or play in the laboratory. Do not

leave your bench unless it is necessary.

(4) Dispose of solid waste (e.g. broken glass, filter paper,

copper turnings, etc.) in the waste bin, never in the

sink.

(5) Clean up all the spillage (on the floor or bench) at

once. Clean up the bench after experiment.

(6) Report all accidents and breakages to your teacher at

once.

(7) In case any chemical gets into eyes, flush the eyes with

running cold water immediately for at least three

minutes.

(8) For chemical burns on skin, place the affected area

under slowly running cold water until the pain fades.

(9) Take all necessary safety precautions.

N11

N12

N13

(1)

(2)

(3)

(4) (

)

(5)

(6)

(7)

3

(8)

(9)

Note 12In case students are working in groups, it is a goodpractice to appoint a group leader, by rotation eachtime. He is the only person allowed to get andreturn apparatus and chemicals. In so doing,‘movement’ in the laboratory can be minimized.

Note 13Remind students that the only reliable immediatetreatment for all chemical burns is washing with alot of water.

Note 11For example, never heat strongly when only gentle heating is required; never use concentrated acid/alkaliwhen only a dilute one is needed; never use excessive amounts of chemicals; never do any experiment notallowed/instructed by the teacher.

For more details on the safety precautions in a chemistry

laboratory, refer to the ‘Laboratory Handbook’.

14


Class practice 1.6

�

A1.6

Hazardous chemicals

Hazardous chemicals are substances which may cause injury to

people or damage to property. Chemicals can be classified

according to their hazardous nature. A bottle containing a

hazardous chemical should display the appropriate hazard

warning label(s) to warn the users. Figure 1.6 shows examples

of some common hazard warning labels.

N14

Note 14Some household chemicals have potential risks.Take the example of bleaching solution. Itliberates toxic chlorine gas when mixed with anacidic substance.

1.6

Figure 1.6 Some common hazard warning labels.

1.6

Study the following picture and point out all the improper actions

that are against the rules of laboratory safety.

15


Common laboratory apparatus

Many different pieces of apparatus are required when we do

experiments in the laboratory. Figure 1.7 shows some of the

common laboratory apparatus.1.7

Flat-bottomedflask

Round-bottomedflask

Clamp and stand Conical flask Wire gauze Evaporating dish(basin)

Tripod

Crucible Pipeclay triangle Bunsen burner Spatula Heat-resistantmat

Pestle Mortar

Desiccator Test tube holder Test tube Boiling tube Reagent bottle

Gas syringe Measuringcylinder

Beaker Funnel Plasticwashbottle

Safety spectacles

Test tube rack

Figure 1.7 Common laboratory apparatus.

N15

Note 15Brown reagent bottles are for holding chemicals that decompose in thepresence of light. Colourless reagent bottles are for most other chemicalsolutions.

16


Class practice 1.7

(a)

(b)

(e)

(c)

(d)

(f)

(h)

(g)

(l)

(i)

(j)

(k)

(o)

(p)

(q)

(r)

(t)

(s)

(w)(u) (v)

(x)

(z)(y)

(bb)(aa)

(cc)(dd) (ee) (ff) (gg)

(m)(n)

(a) _________________________

(b) _________________________

(c) _________________________

(d) _________________________

(e) _________________________

(f) _________________________

(g) _________________________

(h) _________________________

(i) _________________________

(j) _________________________

(k) _________________________

(l) _________________________

(m) _________________________

(n) _________________________

(o) _________________________

(p) _________________________

(q) _________________________

(r) _________________________

(s) _________________________

(t) _________________________

(u) _________________________

(v) _________________________

(w) _________________________

(x) _________________________

(y) _________________________

(z) _________________________

(aa) _________________________

(bb) _________________________

(cc) _________________________

(dd) _________________________

(ee) _________________________

(ff) _________________________

(gg) _________________________

Flat-bottomed flask

Round-bottomed flask

Clamp

Retort stand

Conical flask

Wire gauze

Evaporating basin

Tripod

Crucible

Pipeclay triangle

Bunsen burner

Crucible tongs

Spatula

Heat-resistant mat

Pestle

Mortar

Desiccator

Test tube holder

Test tube rack

Test tube

Boiling tube

Dropping bottle

Reagent bottle

Gas syringe

Measuring cylinder

Beaker

Funnel

Plastic washbottle

Teat pipette

Thermometer

Watch glass

Separating funnel

Glass rod

1.7

Name the apparatus in the following figures.

KK ee yy tt ee rr mm ss

1. chemical change 10

Page

3. chemistry 1

4. compound 3

7. laboratory safety 12

8. mixture 4

9. observation 11

10. physical change 10

11. physical property 8

12. sublimation

13. word equation

10

3

5. element 2

2. chemical property 9

6. hazard warning label 14

17


SS uu mm mm aa rr yy1.1 What is chemistry about?

1. is a branch of science. It is the study of various substances, about theircompositions, structures, properties and the changes among them.

1.2 Chemistry in our daily lives today

2. plays a major role in clothing, food, housing, transport and medical care.

1.3 Classification of matter

3. An is a pure substance that cannot be broken down into anything simpler bychemical methods.

4. A is a pure substance made up of two or more elements chemically combined.

5. A consists of two or more pure substances (elements or compounds) whichhave not chemically combined.

Chemistry

Chemistry

element

compound

mixture

1.4 Properties of substances

6. of a substance are those properties that can be determinedwithout the substance changing into another substance.

Examples: appearance, colour, odour, taste, hardness, density, solubility, melting point, boilingpoint, malleability, ductility, electrical conductivity, thermal conductivity.

7. of a substance are the chemical reactions of the substance andthe respective conditions under which each reaction occurs.

For example, a chemical property of sodium is: sodium reacts with water to form sodiumhydroxide.

1.5 Physical and chemical changes

8. A is a change in which no new substances are formed.

Examples: change of state, passing electricity through a light bulb.

9. A is a change in which one or more new substances areformed.

Examples: burning of a candle, rusting of iron.

1.6 Working in a chemistry laboratory

10. in chemistry includes four activities:

• Seeing with eyes

• Feeling with hands

• Smelling with nose

• Hearing with ears

11. is of first importance in any experimental work. Refer to p.13for basic laboratory safety rules.

12. are displayed on bottles containinghazardous chemicals, which must be handled with great care.

13. Some common laboratory apparatus are shown in Figure 1.7 on p.15.

18


Physical properties

Chemical properties

physical change

chemical change

Observation

Laboratory safety

Hazard warning labels

19

Chapter 2 The atmosphere

2.1 Getting to know our planet Earth 2.1

An introduction to the Earth

Here are some data about the Earth:

Age: 4.5 billion years

Shape and size: roughly spherical, about 6400 km in radius

Mass: 6 � 1024

kg

Surface: 70% covered by water, 30% covered by land,

surrounded by a gaseous layer (about 80 km thick)

called the atmosphere

N1

Note 199.99% of the total mass of the atmosphere iswithin 80 km of the surface of the Earth.

6400

6 � 1024

70% 30%

80

(a)

(b)

2.1

(a) Why did most people in ancient times believe that the

Earth’s shape was flat?

(b) Give one piece of evidence to support that the Earth is

spherical in shape.

Class practice 2.1

A2.1(a) People in ancient times had little scientific knowledge. In fact, any visible portion of the Earth

appeared more or less flat to the eyes.(b) Satellite photos clearly show that the Earth is roughly spherical.

(Other answers may be given.)

Structure of the Earth

The Earth has a layered structure

The Earth consists of four layers, namely crust (5–70 km thick),

mantle (about 2900 km thick), outer core and inner core. ( 5 – 70 ) (

2900 )

N2

Note 2Like the core (outer core: 2890–5150 km from surface, and inner core: 5150–6360 km from surface), themantle can also be divided into the upper mantle (35–660 km from surface) and the lower mantle(660–2890 km from surface).

20

Part I Planet Earth

(a)

(b)

(

)

2.2

(a) The diagram below is the structure of the Earth. Label the

different layers of the Earth.

(b) Add approximately to scale, the atmosphere to the diagram

and label it.

(Hints: atmosphere, crust, mantle, inner core, outer core)

Class practice 2.2

A2.2

outer coreinner core

crust

atmosphere

mantle

The Earth’s crust

The Earth’s crust is made up of rocks (see Chapter 4) and soils.

Planet Earth as a source of chemicals

The Earth’s crust, the oceans and the atmosphere are the major

sources of chemicals.

( )

2.2 The atmosphere 2.2

The atmosphere and air

The atmosphere is a gaseous layer (about 80 km thick)

surrounding the Earth.

Air is a gaseous mixture making up the atmosphere.

(

80 )

N4

Note 4Notice that ‘atmosphere’ and ‘air’ are two similarbut not identical terms. For example, we can say,‘We breathe in air’, but we cannot say ‘Webreathe in atmosphere’.

N3

Note 3While the Earth is a very abundant source of chemicals for humans, some resources (like petroleum orsome metals) are running out. Scientists are now looking into space for chemicals, including nearbyplanets (e.g. Mars) and satellites (e.g. the Moon). It is also hopeful that new chemicals that are notavailable on Earth may be found from other extra-terrestrial bodies.

N5

Note 5The atmosphere has a layered structure similar to that of the Earth. From lower layer to outer layer:troposphere, stratosphere, mesosphere, thermosphere and magnetosphere.

21


Example 2.1Understanding the importance of the atmosphere onthe Earth

Explain why the atmosphere is important to life on theEarth.

Solution

The atmosphere is important because it

(1) contains a lot of free oxygen which supports life.

(2) provides a moderate climate for living things to live.

(3) protects living things from the high-energy radiationsfrom outer space.

2.1

(1)

(2)

(3)

1. 8

(a)

(b)

2.

2.3

1. There are 8 planets in the solar system. Is the Earth the only

planet

(a) that has an atmosphere?

(b) that has an atmosphere which can support life?

2. Imagine you are on the Moon. Suggest why you could not

find any living things there.

Class practice 2.3

A2.31. (a) No. (7 planets have an atmosphere.)

(b) Yes.2. There is no air on the Moon.

Composition of air

Air is mainly a mixture of two gases — nitrogen and oxygen

(Figure 2.1).(

2.1)

1% other gases (including argon 0.93%, carbon dioxide 0.03% and smallamounts of water vapour)

1% ( 0.93% 0.03% )

oxygen 21%

nitrogen 78%

Figure 2.1 Percentage compositionby volume of clean air.

N6

Note 6The atmosphere is like a greenhouse, making weather on Earth suitable for living things to live. The atmosphere keeps in a lot of the heat energy from the Sunto give the Earth a small temperature difference between day and night (unlike the Moon, where day and night temperature changes are very extreme).However, too much greenhouse gases, like CO2, in the atmosphere can cause the temperature of the atmosphere to rise, causing ice caps at the pole to meltand subsequent catastrophic consequences. CO2, if present in a suitable concentration in the atmosphere, is NOT a pollutant at all as it is needed for green

plants, which are the starting point of nearly all food chains. SO2 and NOx are presentin the atmosphere long before there are humans on Earth. SO2 is produced in volcaniceruptions and hot springs, and NOx is produced during rainstorm with lightning.However as volcanoes on Earth became less active, the SO2 level had dropped to alevel suitable of living things to live. Human industrial activities raised these gas

concentrations to such levels as to endanger humans and otherspecies on Earth, which we now call air pollution.

N7

Note 7Refer to ‘Supplementary information: Asimple experiment to determine thepercentage of oxygen in air’ in theTeacher’s Guide.

22

Part I Planet Earth

We should note that air also contains small amounts of

water vapour and other gases. One of them is argon. It is very

unreactive and is called a noble gas.

2.4

Air contains mainly nitrogen and oxygen. It also contains other

gases such as water vapour, carbon dioxide, helium, neon,

argon, krypton and xenon.

Classify the constituents of air into elements and compounds.

Class practice 2.4

A2.4Elements Compondsnitrogen, oxygen carbon dioxidehelium, neon water vapourargon, kryptonxenon

2.3 Separation of oxygen and nitrogenfrom air

2.3

The components of air can be separated according to their

differences in boiling points. The air is first liquefied by

repeated cooling and compression (Figure 2.2). The liquid air is

then warmed up bit by bit very slowly. Different gases in air

boil at different temperatures, so we can collect them one by

one. Nitrogen (boiling point –196°C) boils off as gas first.

Argon follows (boiling point –186°C) and then oxygen (boiling

point –183°C). This process is called fractional distillation of

liquid air.

( 2.2)

(

–196°C)

( –186°C) (

–183°C)

air in

filter

liquefaction unitnitrogen gas(b.p. –196°C)

( –196°C)

argon gas (b.p. –186°C)

( –186°C)

oxygen gas(b.p. –183°C)

( –183°C)

air allowed to expand —it gets very cold (–200°C)and some turns to liquid

(–200°C)

liquid air at–200°C

–200°C

air compressedand then cooled

water vapour andcarbon dioxide removedas solids at –80°C

– 8 0 ° C

Figure 2.2 Separation of oxygen and nitrogen fromair by fractional distillation.

23


(a)

(b) – 2 0 0 ° C

2.5

The table on the right

shows the boiling points

of some of the gases

found in air.

(a) Rearrange the gases

into the order in

which they would

boil off during

fractional distillation

of liquid air.

(b) List the gases which

would still be

gaseous at –200°C.

Class practice 2.5

–153

–196

Boiling point (°C)(°C)

–186

Gas

Argon

–269Helium

Krypton

Nitrogen

Neon –246

Oxygen –183

Xenon –109

Carbon dioxide–78

Boiling points of some gases.

2.4 Properties of oxygen 2.4

Physical properties of oxygen

• Oxygen is a colourless, odourless gas.

• It is slightly denser than air (1.1 times as dense as air).

• It is slightly soluble in water.

Chemical properties of oxygen

Oxygen is very reactive. It reacts with many substances to form

oxides. In many reactions, so much heat is given out that the

substances burn in oxygen (or air) with a flame.

Test for oxygen

Oxygen is a good supporter of burning (combustion), that is, it

allows substances to burn in it.

Put the glowing splint into a test tube containing the gas to

be tested. If the gas is oxygen, the splint immediately relights

— that is, bursts into flame.

•

• ( 1.1

)

•

( )

( )

N8

Note 8Sometimes a small ‘pop’ sound is heard when a glowing splint is put into a tube of oxygen. This isbecause when the splint relights, it gets hot quickly and the wooden splint is decomposed to give offcombustible gases. The combustible gases then burn in oxygen to give a small ‘pop’ sound. This doesn’tmean the tube contains hydrogen.

A2.5(a)

(b) Neon and helium.

Helium –269Neon –246Nitrogen –196Argon –186Oxygen –183Krypton –153Xenon –109Carbon dioxide –78

24

Part I Planet Earth

✘ Oxygen gives a ‘pop’ sound with a burning splint.

✔ Oxygen relights a glowing splint, but does not give a ‘pop’sound with a burning splint. Hydrogen gives a ‘pop’ sound witha burning splint, but does not relight a glowing splint.

Check your concept

✘( )

✔

( )(

)

Oxygen relights a glowing splint. This can be used as a test

for oxygen.


1. argon 21

Page

3. fractional distillation 22

4. glowing splint 23

5. noble gas 21

2. atmosphere 20

25


SS uu mm mm aa rr yy2.1 Getting to know our planet Earth

1. The Earth is composed of the , , andsurrounded by the .

2. The atmosphere is a layer surrounding the Earth.

3. The Earth’s crust, the oceans and the atmosphere are major sources of useful .

2.2 The atmosphere

4. The atmosphere is important because it

• contains a lot of free which supports life

• provides a moderate for living things to live

• protects from the high-energy radiations from outer space

5. Air contains 78% by volume of nitrogen, 21% of oxygen, 0.93% of argon, 0.03% of carbon dioxide,trace amounts of other and water vapour.

2.3 Separation of oxygen and nitrogen from air

6. Nitrogen and oxygen can be obtained by of liquid air.

2.4 Properties of oxygen

7. Oxygen is a good supporter of (burning). It relights a splint.

8. We can test for oxygen with a splint.

crust mantle core

atmosphere

gaseous

chemicals

oxygen

climate

living things

noble gases

fractional distillation

combustion glowing

glowing

26

Part I Planet Earth

The Earth is sometimes called a ‘water planet’. This is because

70% of it is covered by water. 97% of this water occurs in oceans

and seas.

70%

97%

3.1 Introducing oceans and seas 3.1

Sea water is a solution containing about 3.5% by weight of

dissolved substances. (In other words, there is 35 g of dissolved

substances in 1000 g of sea water.)3.5% ( 1000 35

)

3.2 Composition of sea water 3.2

A solution is a homogeneous (uniform) mixture of two or

more substances.

Most of the dissolved substances are salts. The main one is

‘common salt’ — sodium chloride. The composition of sea

water is almost constant, although it may vary slightly

according to location and depth (Figure 3.1).

Figure 3.1 The salt composition by weight of a typical sea water sample.

magnesium chloride

14.6%

sodium sulphate

11.4%

calcium chloride

3.1%other salts

2.9%

common salt (sodium chloride)

( )68%

( )

(

68%)

( 3.1)

27

Chapter 3 Oceans

✘ All the dissolved substances in sea water are salts.

✔ There are dissolved substances in sea water other than salts.For example, gases like oxygen, carbon dioxide, organicsubstances like urea can also be found in sea water.

Check your concept

✘

✔

Common salt (sodium chloride) is the most abundant resource

in sea water. It is an important substance, useful at home and in

industry.

( )

3.3 Extraction of common salt from seawater

3.3

Evaporation of sea water

Common salt can be separated from sea water by evaporation.

Sea water (solution) is led into a special enclosure exposed to

direct sunlight. Water (solvent) evaporates and the sea water is

becoming more and more concentrated. At some point of the

process, the sea water becomes so concentrated that some salts

(solute) can no longer dissolve in it. Crystals of salts appear.

The sea water at this stage is said to be a saturated solution.

A saturated solution is a solution in which the solvent has

dissolved the maximum amount of the solute it can at a

particular temperature.

( )

( )

( )

Natural evaporation is a slow process. In the school

laboratory, we can obtain common salt from sea water quickly

by heating it to dryness. We may use either set-up, as shown in

Figure 3.2.3.2

evaporatingbasin

(a) Direct heating (b) Heating with a steam-bath

water

evaporating basin

heat heat

wire gauze

tripod

sea water

steam sea water

Figure 3.2 Getting common salt fromsea water in the laboratory.

28

Part I Planet Earth

To obtain pure sodium chloride

If sea water is heated to dryness as above, what is left would be

a powder, not crystals. Moreover, other salts would be present

besides sodium chloride.

To obtain pure sodium chloride, we can use filtration

followed by crystallization.

Filtration

Firstly, any insoluble substances such as sand should be

removed from sea water by filtration.

To filter, sea water is poured onto a piece of folded filter

paper in a filter funnel (Figure 3.3). A glass rod is used to guide

the flow (Figure 3.4). A piece of filter paper acts as a sieve in

filtration. There are many tiny holes in it. These holes allow

very small particles of solvent and dissolved solutes to pass

through as filtrate. Larger insoluble particles remain on the

filter paper as residue.

( )

( 3.3)

( 3.4)

foldfold

filter paper

one layer three layers

filter funnelFigure 3.3 A piece of filter paper is folded into a conical shape and placed in a funnel.

Figure 3.4 Filtration of sea water.

glass rod

sea water

folded filter paper residue

filter funnel

filtrate

stand

N1

Note 1Refer to ‘Supplementary information: Folding filter paperin fluted form’ in the Teacher’s Guide.

N2

Note 2Some teachers may like to teach students to fold thefilter paper in the fluted form. The fluted form of filterpaper is more difficult to fold but more efficient infiltration than the conical form as shown in figure 3.7.

29

Chapter 3 Oceans

Figure 3.5 Crystallization from sea water by slow evaporation.

Crystallization

After the removal of insoluble impurities, pure crystals of

common salt can be separated out by crystallization.

Sea water is allowed to evaporate slowly at room

temperature (Figure 3.5). The solution becomes more and more

concentrated. Eventually, the solution becomes saturated (with

respect to sodium chloride). Further evaporation of the

solution will cause pure sodium chloride crystals to separate

out. As evaporation continues, the solid crystals slowly grow in

size.

The sodium chloride crystals can be filtered from solution

and then dried by filter paper.

seawater

sodium chloridecrystals

water slowly evaporates at room temperature

moreconcentratedsolution

solutionsaturated withrespect to sodium chloride

N3

Note 3Other dissolved salts may be present in smaller amounts than sodium chloride.The solution is still not saturated as far as these salts are concerned.

(

3 .5)

Isolation of pure water from sea water

Figure 3.6 shows a simple set-up for distilling sea water to get

pure water. If we boil the sea water, water turns into vapour.

The hot water vapour condenses back to a liquid in the cold

receiver test tube.

3.6

The solution to be distilled should contain only non-volatile solutes,those which will not easily vaporize on heating.

Learning tip

()

30

Part I Planet Earth

Figure 3.6 Distillation of sea waterusing simple apparatus.

delivery tube

receiver test tube( )

end of delivery tube shouldbe above the distillate

boiling tubeclamp

anti-bumping granule(to prevent ‘bumping’of solution)

heat

pure water (distillate)( )

water (coolingagent)

( )

sea water

In distillation, the pure liquid that distils over is called the

distillate. The solid left behind is called the residue.

We can also carry out the above distillation using ‘Quickfit’

apparatus (Figure 3.7).

Distillation involves boiling of a solution followed by

condensation of the vapour formed.

thermometer

( 3.7)

Figure 3.7 (a) A set of ‘Quickfit’ apparatus. (b) Distillation using ‘Quickfit’ apparatus.

(a) (b)

thermometer

screw-cap adaptor

rubber tubing

receiveradaptor

test tube (as receiver)

cold water

distillate

Liebigcondenseranti-bumping

granule

pear-shapedflask solution

thermometer bulb

heat

water out (to sink)

cold water in(from tap)

(a)

(b)

31

Chapter 3 Oceans

Chemical analysis

Chemical analysis is an important part of chemistry. It is a

process to find the chemical identity or composition of a given

sample.

Test for sodium ions

Some metals and metal compounds, when burnt or heated

strongly, produce a characteristic coloured light. A simple test

— the flame test, is based on this principle.

3.4 Tests for sodium and chloride ions incommon salt

3.4

The procedure of the flame test is shown below:

1. Moisten a clean platinum wire with concentrated

hydrochloric acid (Figure 3.8a).

2. Dip the wire into a crushed sample (or solution) of the salt

to be tested (Figure 3.8b).

3. Heat the end of the wire strongly in a non-luminous flame

(Figure 3.8c).

N4

Note 4Often a nichrome wire is used in place ofthe platinum wire, which is very expensive.

1.

( 3.8a)

2. (

) (

3.8b)

3. ( 3.8c)

Figure 3.8 Performing a flame test to identify metal ions in a sample.

(a)

concentratedhydrochloricacid

platinum wire

sample of thesalt to be tested

flame colour dueto metal ions

(b) (c)

non-luminousflame

32

Part I Planet Earth

By observing the colour of the flame at the wire, we can

identify some types of metal ions.

The results of flame test of some metal compounds are as

follows:

Compound containing Flame colour

Potassium ion Lilac

Sodium ion Brilliant golden yellow

Calcium ion Brick red

Copper(II) ion Bluish green (II)

The flame colour due to potassium ion is lilac, but it is crimsonwhen viewed through cobalt glass.

Learning tip

The use of excess dilute nitric acid prevents formation of otherprecipitates (e.g. silver carbonate, silver sulphite) which will besoluble in dilute nitric acid.

Learning tip

()

Test for chloride ions

To show that chloride ions are present in sea water, we can use

the following test:

Silver nitrate solution is added to a sample of sea water,

followed by excess dilute nitric acid. The appearance of a white

precipitate (insoluble in acid) indicates the presence of chloride

ions.( )

The word equation for this reaction is:

sodium chloride + silver nitrate silver chloride + sodiumnitrate

+ + ( )(white precipitate)

33

Chapter 3 Oceans

Test by anhydrous copper(II) sulphate(II)

3.5 Tests for the presence of water in asample

3.5

Water turns white anhydrous copper(II) sulphate blue.

Test by dry cobalt chloride paper

(II)

Cobalt chloride test paper is also called cobalt(II) chloride testpaper.

Learning tip

(II)

✘ All liquids contain water.

✔ Many liquids do not contain water. Some liquids, like oil anddry-cleaning liquid, do not mix with water.

Check your concept

✘

✔

Water turns blue dry cobalt chloride paper pink.

Sea water is an important source of common salt (sodium

chloride) which has many uses. Moreover, by the electrolysis of

sea water, many useful products may be obtained.

( )

3.6 Electrolysis of sea water and uses ofproducts

3.6

34

Part I Planet Earth

Electrolysis means ‘decomposition by electricity’. It is

usually carried out by passing a direct electric current through

an aqueous salt solution. When sea water is electrolysed, the

products are hydrogen, chlorine and sodium hydroxide. See Figure

3.9.electrolysis

Sea water hydrogen gas + chlorine gas + sodium hydroxidesolution

3.9

+ +

Figure 3.9 Electrolysis of sea water.

hydrogen gas chlorine gas

sea water

graphiteelectrode (+)

graphiteelectrode (–)

(–)

direction ofelectron flow

Some uses of products from the electrolysis of brine.

Uses: as rocket fuel, make

margarine, ammonia and

fertilizers, make hydrochloric

acid, etc.

Uses: water sanitation, make

bleach, plastics (e.g. PVC),

solvents, pesticides, etc.

(

)

Uses: aluminium extraction,

soap, paper industry, treatment

of acidic/heavy metal effluents

from factories.

Brine

hydrogen chlorine sodium hydroxide

electrolysis

N5

Note 5During electrolysis of sea water or brine, theanodic gaseous product contains trace amount ofoxygen, though the major product is chlorine.

35

Chapter 3 Oceans


1. chemical analysis 31

Page

3. distillate 30

4. distillation 30

7. filtration 28

8. flame test 31

9. residue 28

10. saturated solution 27

11. solute

12. solvent

27

27

5. electrolysis 33

2. crystallization 28

6. filtrate 28

36

Part I Planet Earth

SS uu mm mm aa rr yy3.1 Introducing oceans and seas

1. 70% of the Earth is covered by . 97% of this occurs in oceansand seas.

3.2 Composition of sea water

2. Sea water contains about 3.5% by weight of dissolved substances. The main insea water is common salt (sodium chloride).

3. A is a homogeneous (uniform) mixture of two or more substances.4. A is a solution in which the has dissolved

the maximum amount of the solute it can at a particular temperature.

3.3 Extraction of common salt from sea water

5. Pure common salt can be extracted from sea water by and then.

6. Pure water can be isolated from sea water by .7. Distillation involves boiling of a solution followed by of the vapour formed.8. During distillation, the pure liquid that distils over is called the ; the solid left

behind is called the .

3.4 Tests for sodium and chloride ions in common salt

9. is a process to find the chemical identity or composition of agiven sample.

10. Some metal ions can be identified by the . Some characteristicflame colours are:

11. To test for chloride ions in sea water, we add solution,followed by excess dilute , to a sample. A white

precipitate forms if chloride ions are present.

3.5 Tests for the presence of water in a sample

12. and can beused to detect the presence of water in a given sample.

3.6 Electrolysis of sea water and uses of products

13. Electrolysis of sea water produces useful chemicals: , and. Refer to p.34 for their uses.

Compound containing

Potassium ion

Sodium ion

Calcium ion

Copper(II) ion

Flame colour

water water

solute

solution

saturated solution solvent

filtration

crystallization

distillation

condensation

distillate

residue

Chemical analysis

flame test

silver nitrate

nitric acid

silver chloride

hydrogen chlorine

sodium hydroxide

Anhydrous copper(II) sulphate dry cobalt chloride test paper

Lilac

Brilliant golden yellow

Brick red

Bluish green

37

Chapter 4 Rocks and minerals

What are rocks and minerals?

4.1 Rocks 4.1

In science, the word ‘rock’ has a more specific meaning:

A mineral can be a solid element (e.g. graphite), or in most

cases a solid compound (e.g. aluminium oxide).

A rock is a solid mass of a mineral or a mixture of minerals.

A mineral is a naturally occurring solid with a definite

crystalline structure and chemical composition.

( )

( )

Example 4.1Extracting minerals from rocks

Common salt (sodium chloride)is usually obtained from seawater. However, there areunderground deposits ofsodium chloride (called rocksalt) in some countries, e.g. U.K.

(a) Is rock salt a mineral?

(b) Suggest two methods toextract rock salt from underground.

Solution

(a) Yes.

(b) (i) By mining.

Use explosives to break up the rock salt. Then usetrucks to carry away the rock salt.

(ii) Pump water into the underground deposit. Rocksalt dissolves in the water, leaving most otherminerals behind. Then pump the salt solution upto the ground.

4.1

( ) (

)

(a)

(b)

(a)

(b) (i)

(ii)

Rock salt

38

Part I Planet Earth

Uses of minerals

There are more than 2200 minerals in the Earth’s crust. Most

are useful for many purposes.

Some uses of minerals:

• Graphite is used to make ‘pencil lead’.

• Rock salt is used in cooking.

• Jade is a gemstone used for decoration and in jewellery.

• Marble is used as floors in commercial buildings.

• Gold is used in jewellery.

2200

•

•

•

•

•

Minerals and ores

Most minerals require treatment before they become useful.

For example, many minerals are metal-containing compounds.

Before we can use the metals, we have to extract them from

their ores first.

Some common ores are shown below.

Bauxite — the main ore of aluminium. It is mostly aluminium

oxide.

Copper pyrite — the main ore of copper. It is mostly copper

iron sulphide.

Haematite — the main ore of iron. It is mostly iron(III) oxide.

An ore is a mineral from which a constituent (usually a

metal) can be profitably extracted.

(II)

(III)

(

)

Extracting metals from ores

An ore of a metal is typically a compound of the metal.

4.2 Extraction of metals from their ores 4.2

39


To obtain a pure metal from its ore, the following processes

are usually involved:

1. Mining of the ore (that is, digging the ore from the ground)

2. Concentrating the ore

3. Extraction of the metal from the concentrated ore

4. Purification of the impure metal

Extraction of iron from haematite

Haematite contains mainly iron(III) oxide. We can obtain iron

from the ore by heating it with coke (carbon) to a high

temperature in a blast furnace. The overall reaction can be

represented by a word equation:

heat

iron(III) oxide + carbon iron + carbon dioxide

Extraction of aluminium from bauxite

Bauxite is first treated to give pure aluminium oxide. The

aluminium oxide is then electrolysed in molten state to

produce aluminium metal.

electrolysis

aluminium oxide aluminium + oxygen

Extraction of silver from its ores

Silver is an unreactive metal. It can be extracted from its ores

(e.g. silver glance) by heating alone. Silver oxide decomposes

on heating to produce the metal and oxygen. A glowing splint

can be used to test for any oxygen evolved.

heat

silver oxide silver + oxygen

Limited reserves of natural resources

Natural resources such as ores are limited in amount and non-

renewable. It is obvious that we must use natural resources

wisely, so that they can last longer.

1. ( )

2.

3.

4.

(III)

( )

(III) + +

+

( )

+

( )

N1

Note 1The overall equation is a simplified one.The reactions involved are:carbon + oxygen carbon dioxidecarbon dioxide + carbon

carbon monoxideiron(III) oxide + carbon monoxide

iron + carbon dioxide

N2

Note 2Ma On Shan used to be an iron ore mineuntil after World War II. This means that theiron ore mine in Ma On Shan was operativeup to the 1950’s. As the amount of iron oresbecame limited, the mine closed down. Thisexample may help students to appreciatethat natural resources are non-renewable.

40

Part I Planet Earth

Rocks containing calcite

Limestone, chalk and marble are common rocks. They have

one thing in common — they all contain the same mineral

calcite (a crystalline form of calcium carbonate). These three

forms of naturally occurring calcium carbonate have different

appearances. Their hardness also differs.

Limestone is the most common form of calcium carbonate.

It is hard and strong, yet inexpensive. It is therefore widely

used in the building industry. Chalk is slightly softer and is

also used in buildings. Marble is a crystalline solid and is very

hard. It can be smoothly polished to give a beautiful

appearance. It is often used for building statues, monuments,

and as floors and walls in some buildings.

Uses of limestone

Limestone is used as a building material. Blocks of limestone

can be used to construct buildings and roads. Limestone has

many other uses as well.

• Limestone is used to make footpaths.

• Limestone is a raw material for making cement.

• Limestone is used in neutralizing water and soil affected

by acid rain.

( )

( )

•

•

•

4.3 Limestone, chalk and marble 4.3

Weathering and erosion

In fact, all rocks exposed on the Earth’s surface are slowly worn

away by weathering and erosion.

4.4 Weathering and erosion of rocks 4.4

Weathering of rocks is the slow process (usually over

thousands of years) in which exposed rocks are broken

down into smaller pieces.

(

)

41


Weathering occurs through the actions of water, wind, air

and changes in temperature.

Erosion can also have a broader meaning. It may also refer

to the process which involves both weathering of rocks and

transportation of weathered rock pieces to another place.

Types of weathering

Rocks can be weathered in two ways:

• Physical weathering

• Chemical weathering

Physical weathering

Weathering by temperature changes

Changes in temperature can break rocks. This happens when

rocks get hot in the daytime but cool down quickly at night.

The effect is even much more common in deserts.

Weathering by frost action

Rainwater can fill cracks in rocks. When the temperature drops

below 0°C, water freezes, and expands to form ice. This forces

rocks to break apart. We call this frost action. See Figure 4.1.

Erosion of rocks is the slow process in which weathered

rock pieces are transported away by gravity, wind and

water.

•

•

0°C

4.1

Figure 4.1 Expanding ice breaks rocks.

rainwatergathers in crack

water freezes and expands

eventually a pieceof rock breaks off

the crack gets bigger

temperature falls below 0°C0°C

ice

rock

42

Part I Planet Earth

Chemical weathering

Attack by acid

Rainwater attacks rocks, especially those containing calcium

carbonate. It is because carbon dioxide in air dissolves slightly

in rainwater, forming an acidic solution.

carbon dioxide + water carbonic acid(acidic)

The carbonic acid formed reacts with calcium carbonate:

calcium carbonate + carbonic acid calcium

hydrogencarbonate

Calcium hydrogencarbonate is soluble in water and thus

the limestone is slowly worn away. The results of this natural

chemical weathering process include:

• Formation of sinkholes in limestone areas

• Damage to limestone statues

Attack by oxygen

Oxygen in air can attack some rocks, especially those

containing iron. This causes the rock to wear away slowly.

+ ( )

+

•

•

4.5 Chemical changes involving calciumcarbonate

4.5

Thermal decomposition of calcium carbonate

When limestone (mainly calcium carbonate) is gently warmed

with a small Bunsen flame, there seems to be no visible change.

However, when it is heated strongly with a roaring non-

luminous flame (at about 900°C), it decomposes to give

calcium oxide and carbon dioxide gas (Figure 4.2).

(

)

( 900°C)

(

4.2)

N3

Note 3Rainwater containing dissolved carbon dioxide corrodes limestone areas and underground caves are resulted. Over a long time as more calciumhydrogencarbonate solution sips through cracks in cave ceilings, dissolved calcium carbonate slowly deposits and forms stalactites and stalagmites.

stalactite

stalagmite

43


Figure 4.2 Heating calciumcarbonate strongly to makequicklime (calcium oxide).

()

roaring non-luminous flame

Bunsenburner

calciumcarbonate

test tubeholder

roaring non-luminousflame

Bunsenburner

calciumcarbonate

test tubeholder

strong heat

calcium carbonate calcium oxide + carbon dioxide

Calcium oxide is commonly known as quicklime. When

treated with water, it turns into calcium hydroxide (slaked

lime), producing a lot of heat at the same time.

calcium oxide + water calcium hydroxide(slaked lime)

On stirring calcium hydroxide with water, a white

suspension is formed. (Calcium hydroxide is only slightly

soluble in water.) If the suspension is filtered, a clear solution

called limewater is produced.

Limewater test for carbon dioxide

Limewater is a saturated solution of calcium hydroxide in water.

It is a clear colourless solution, which turns milky when carbon

dioxide is passed through it for a few seconds (Figure 4.3). This

is because the white insoluble solid particles of calcium

carbonate formed are suspended throughout the mixture.

calcium hydroxide + carbon dioxide calcium carbonate + water(colourless solution) (white solid)

Although limewater is a saturated solution of calcium hydroxide inwater, it is only a dilute solution. This is because of the low solubilityof calcium hydroxide in water.

Learning tip

+

( )

+ ( )

( 4.3)

+ + ( ) ( )

44

Part I Planet Earth

limewaterturned milky

(a) (b)

limewater(colourlesssolution)

()

Figure 4.3 Carbon dioxide turns limewater milky.

Carbon dioxide is a colourless gas. It turns limewater milky.

The above changes can be summarized as shown in Figure

4.4:4.4

Figure 4.4 How some chemical changes involving calcium carbonate are related.

calcium carbonate

calcium hydroxide

slaked lime

calcium hydroxide solution calcium oxide

limestone

step 1strong heat

1

carbon dioxide given off

step 2add a little water

2

step 3add more water, stir well and then filter

3

step 4pass in carbon dioxide (limewater test)

4( )

quicklimelimewater

4 . 4 1 4

4.1

Write word equations for steps 1–4 in Figure 4.4.

Class practice 4.1

A4.1heat

Step 1: calcium carbonate calcium oxide + carbon dioxideStep 2: calcium oxide + water calcium hydroxideStep 3: calcium hydroxide + water

calcium hydroxide solution (limewater)Step 4: calcium hydroxide solution (limewater) + carbon dioxide

calcium carbonate + water

45


4.6 Tests for calcium carbonate in a sampleof limestone/chalk/marble

4.6 / /

Test for calcium ions

Calcium compounds give a brick red flame in a flame test.

Test for carbonate ions

Dilute hydrochloric acid is added to each sample (Figure 4.5). If

the sample is a carbonate, carbon dioxide is produced, which

turns limewater milky.

calcium carbonate + hydrochloric acid

calcium chloride + carbon dioxide + water

Limestone, chalk and marble all give a positive limewater

test.

(

4 .5)

+ +

+

dilute hydrochloric acid

solid sampleunder test

deliverytube

limewater

Figure 4.5 Test for a carbonate by limewater test.

✘ All rocks can react with dilute hydrochloric acid to give carbondioxide.

✔ Most rocks are made up of silicates. Only those made up ofcarbonates can react with dilute acids to give carbon dioxide.

Check your concept

✘

✔

4.2

Write a word equation for the reaction between chalk and dilute

nitric acid.

Class practice 4.2

A4.2calcium carbonate + nitric acid calcium nitrate + carbon dioxide + water

46

Part I Planet Earth

Example 4.2Action of dilute acid on some rocks

You are provided with distilled water, dilute hydrochloricacid (an acid stronger than carbonic acid) and small piecesof the following rocks:

limestone, chalk, marble, granite

(a) Which rock sampleshave a visible changein distilled water?

(b) Add dilutehydrochloric acid tothe rock samples, asshown in Figure 4.6.

(i) Which rocksamples wouldgive a visiblechange?

Write a word equation for any reaction that occurs.

(ii) What do you observe in (i)?

(iii) Based on the results in (i), which rocks would beattacked by carbonic acid in rainwater?

Solution

(a) None of them.

(b) (i) Limestone, chalk and marble.

calcium carbonate + hydrochloric acidcalcium chloride + carbon dioxide + water

(ii) There is effervescence. (Colourless gas bubbles aregiven out.)

(iii) Limestone, chalk and marble.


4.2

()

(a)

(b) 4.6

(i)

(ii) (i)

(iii) (i)

(a)

(b) (i)

(ii) ()

(iii)

➲ 35

Figure 4.6 Adding dilute hydrochloricacid to a rock sample.

rock sample

watch glass

dilute hydrochloricacid

dropper

47



1. bauxite 38

Page

3. calcium carbonate 40

4. calcium hydroxide 43

6. erosion 41

7. haematite 38

8. limestone 40

9. limewater 43

10. marble 40

11. mineral 37

12. ore 38

13. quicklime 43

14. slaked lime 43

15. weathering 40

5. chalk 40

2. calcite 40

48

Part I Planet Earth

SS uu mm mm aa rr yy4.1 Rocks

1. A is a solid mass of a mineral or a mixture of minerals.

2. A is a naturally occurring solid with a definite crystalline structure andchemical composition.

3. An is a mineral from which a constituent (usually a metal) can be profitablyextracted. Some common ores include bauxite, copper pyrite and haematite.

4.2 Extraction of metals from their ores

4. Extraction of a metal from its ore usually involves four steps:

• of the ore (that is, digging the ore from the ground)

• the ore

• of the metal from the concentrated ore

• of the impure metal

5. Two examples of extracting metals from ores:

(a) Iron can be extracted from iron ores by heating with carbon:

heatiron(III) oxide + carbon iron + carbon dioxide

(b) Aluminium can be extracted from by electrolysis:

electrolysisaluminium oxide aluminium + oxygen

4.3 Limestone, chalk and marble

6. , and are different natural forms of thesame compound, calcium carbonate.

7. Many minerals are very useful. An example is limestone. See p.40 for some of its uses.

4.4 Weathering and erosion of rocks

8. of rocks refers to the slow process (usually over thousands of years) in whichexposed rocks are broken down into smaller pieces.

9. of rocks refers to the slow process in which weathered rock pieces aretransported away by gravity, wind and water.

rock

mineral

ore

Mining

Concentrating

Extraction

Purification

haematite

bauxite

Limestone chalk marble

Weathering

Erosion

49


10. Rocks are weathered in two ways:

• weathering (e.g. by temperature changes, frost action)

• weathering (e.g. attack by acid, attack by oxygen in air)

4.5 Chemical changes involving calcium carbonate

11. attacks rocks, especially those containing calcium carbonate:

calcium carbonate + carbonic acid calcium hydrogencarbonate

12. On strong heating, limestone (mainly calcium carbonate) releases and is changed into (quicklime).

heatcalcium carbonate calcium oxide + carbon dioxide

13. When calcium oxide (quicklime) is treated with water, it turns into (slaked lime).

14. is a saturated solution of calcium hydroxide in water. It is a colourless solution,which turns milky when carbon dioxide is passed through it for a few seconds. This limewatertest can test for carbon dioxide gas.

calcium hydroxide + carbon dioxide calcium carbonate + water(colourless solution) ( solid)

4.6 Tests for calcium carbonate in a sample of limestone/chalk/marble

15. Calcium compounds give a colour in the flame test.

16. On treatment with dilute hydrochloric acid, calcium carbonate dissolves and releases.

calcium carbonate + hydrochloric acid calcium chloride + carbon dioxide + water

Physical

Chemical

Rainwater

carbon dioxide

calcium oxide

calcium hydroxide

Limewater

brick red

carbon dioxide

white

hkdse chemistry bridging programe 1a

Documents

front inside

key terms

roaring nonluminous

naturally

dilute hydrochloric

dilute sulphuric

dilute nitric

involving