02 Properties of matter and their uses - edupub.gov.lk 2.pdf · For free distribution 34 Properties of matter and their uses By the end of this chapter, you will be competent to

For free distribution34

Properties of matter and their uses

By the end of this chapter, you will be competent to...

• investigate about the structure of matter.

• investigate the properties and uses of elements.

• use the expansion of solids, liquids and gases in a usefull maner in day

today life.

• apply differences in density of matter in daily life.

• investigate the uses of compounds according to their properties

• investigate domestic uses of chemicals.

• use series and parallel connections of electrical appliances to suit human

needs.

• use of magnets in day today life.

02

35For free distribution

2.1 Structure of matter

In Grade 6 and 7 you have classified substances around you acccording to various

criteria. Given below are some such criteria and examples for each

Living : dog, coconut tree, ant.

Nonliving : table, pen, copper wire.

Natural : water, air, plants, animals.

Artificial : polythene covers, plastic goods, glass utensils

Solids : rock, chairs, tables,

Liquids : water, kerosine oil, coconut oil.

Gases : oxygen, carbon dioxide, water vapour.

Material : box of matches, water, air.

Non material : heat, light, sound.

What did you understand by the term material in the last classification ?

You will be able to get a better understanding about material by doing the activites

given below.

Activity 2.1 will show you that the granite, water filled glass, empty glass, air filled

volley ball , empty volley ball had a mass.

Also since the water filled glass weighed more, water too has a weight. The air

filled baloon has a bigger weight than when it is empty, hence air too has a weight. As

a result you can conclude that air, water and solid materials have a mass.

Material has a mass

Activity 2.1

• Find an air filled volley ball, piece of granite, glass filled with water, and a

triple beam balance.

• Find the weights of each of the above using the triple beam balance(with the

teachers help).

• Empty the glass and weigh the empty glass.

• Remove air from the volley ball and weigh


It will be observed that the water level goes down. The water level goes down

because it was replaced by air. Therefore, we can conclude that air occupies space.

You have already learnt in earlier grades that solid materials and liquids occupy

space. Now you know that air too occupies space.

Material occupies space

Now you know the materials around us have mass and occupy space

Such substances are called matter.

Substances which have a mass and occupy space are called matter

Sound, light, heat were classified as non materials that do not occupy space. They

have no mass. They are called energy

Our environment, including ourselves is built up of matter and energy

2.1.1 Particulate nature of matter

There were various ideas about how matter is made up of , that is about the

structure of matter.

Greek philosopher Democritus (460 - 370 B.C) and his followers stated that

matter is made up of tiny particles which cannot be divided further. Aristotle (384 -

270 B.C), a Greek philosopher put forward a different idea, is that matter is not made

up of particles. A public debate was held between Aristotle and the followers of

Democritus in the town of Athens in Greece. Democritus’ followers won the debate

and a majority of the public accepted his views. These particles are very tiny and

invisible to naked eye. These are also different to observe using very advance micro-

scopes.

Later, many scientific experiments confirmed the particulate nature of matter.

2.1.2 States of matter

Matter can be in 3 states, soild liquid and gas according to its physical nature.

Specific characters of solid, liquid and gas, are given below.

Activity 2.2

• Fill a test tube or glass with water. Invert it

carefully into a basin full of water.

• Blow into it using a saline tube as shown in

the diagram.

saline tube

glass

water basin


• Have a definite volume

• No definite shape

Takes the shape of the

container

• Flows easily

• Cannot be easily

compressed

• No definite volume Spreads

out filling the entire container

• No fixed shape

• Flows easily

• Can be compressed easily

The reason for the different properties of the solid, liquid and gaseous state is due

to the different arrangement of particles in the three states of matter.

• Particles are arranged in an

orderly pattern

• Particles are closely packed

and strongly attracted

• Particles do not move freely,

but they can vibrate

• Space between particles is

very little.

• Particles are arranged in

an orderly manner

• Particles are close but

not strongly attracted

• Particles can move over

each other

• Space between particles

comparatively little

• Particles are not arrang-

ed in an orderly manner

• Attractions among

particles are weak.

• Particles can move

freely

• Space between particles

is large

Fig. 2.1 (c) Arrangement

of particles in gass

Now you are aware of the arrangement of particles in solids, liquids and gases.

You will realise that the specific properties shown by them is due to the different

arrangement of particles in each of the states of matter.

Solids have got a fixed shape and hardness because their particles are arranged in

an orderly manner and closely packed together. Liquids have the ability to flow because

their particles can move freely over each other.

• Have a definite volume

• Have a definite shape

• Cannot flow

• Cannot be compressed

Solids Liquids Gases

Solid Liquid Gas

Fig. 2.1 (a) Arrangement

of particles in solid

Fig. 2.1 (b) Arrangement

of particles in liquid

Liquid

gas


Gases do not have a fixed volume because the particles of gas can spread out into

the entire available space.

Volume of a gas can be decreased (compressed) because of the space among

particles is large.

The following activities will explain that the above models of the different states of

matter can be accepted. They can also be accepted as evidence for the particulate

nature of matter.

1. Fill a gas jar with the brown-coloured gas nitrogen dioxide, and cover it with

another gas jar. In a short time you will observe that the gas spreads through both

jars. This is caused by nitrogen dioxide particles mixing into the normal air par-

ticles. This demonstrates the particulate nature of gases and presence of spaces in

between particles.

Some examples to demonstrate the nature of a gas in real life are the spreading of

fragrance from a bottle of perfume, smell of a flower throughout the surrounding

environment.

2. Add a crystal of potassium permanganate (condys) to a beaker of water. Its colour

spreads gradually throughout the water. This is caused by potassium permanganate

mixing into the water particles. This demonstrates the particulate nature of liquids.

Fig. 2.4 - Spreading of the colour throught the water

Water particle

potassium permanganate particle

Assignment -1

Make models to show particulate nature of solids, liquids and gases using suitable

materials.

Fig. 2.3 - particles of perfume

spreading through air

perfume particle

air particle

Fig. 2.2 - Spreading of colour of nitrogen dioxide

Normal

air

Nitrogen

dioxide

(brown

coloured gas)

the

colour

Spreadsthrough

two jars

Gas jars


Study the table 2.2 to find out the various uses of solids, liquids and gases and the

reasons for their use.

2.1.3 Applications of the different physical states of matter

Solids, liquids and gases are used for various purposes due to their differences in

properties and behaviours.

This can be easily understood by studying the different parts of a motor car. (Fig

2.5 and table 2.1)

Part of the motor car

Body of vehicle

In tyres

Fuel tank

Brake system

Substance used

Solids such as

Glass, Steel, Plastic

Air

Liquid fuel

Brake oil

Reason

Strength, having a fixed

shape.

Can change shape when tyre

moves along

Can flow from tank to

engine

Takes the shape of the pipe

and not very compressible

Table 2.1

Fig. 2.5

Glass

Steel

Plastic

Air

(In tyres)

Petrol

(In tank)

Table 2.2

Uses

1. The main reservoir in the urban water

distributing system is placed in a high

position and distributed by pipe lines

2. Making glass objects of different shapes

by melting glass and pouring into moulds.

3. Storing a large quantity of gas in the small

volume of gas jars

Properties of the substance used

The flowing nature of liquids

Flowing nature of liquids and ability

to take the shape of the container.

Gases can be compressed easily.


2.1.4 Building units of matter

You already know that matter is composed of particles. What are these particles?

Do all types of matter have the same type of particles?

Let us investigate these problems.

You have learnt that the particles making up matter cannot be seen by the naked

eye, not even by a microscope. Then these particles should be very small. These tiny

particles are called ‘atoms’.

The English word ‘atom’ was derived from the Greek word ‘atomos’ which meant

‘Cannot be divided any further’.

The diversity in the atom produces matter with different properties.

Different matter is made up of different types of atoms. For example iron is made

up of similar iron atoms while aluminium is made up of similar aluminium atoms. Since

iron and aluminium atoms are different to each other aluminium and iron metal show

different properties.

Two or more similar atoms, or two or more dissimilar atoms combine together to

form molecules

Chlorine gas is made up of chlorine molecules which are formed by the combination

of two chlorine atoms joined together

Water is made up of two hydrogen atoms joined with one atom of oxygen to form

a water molecule

Atoms combine together to form molecules

As per the facts revealed so far, it can be concluded that the basic building unit

of matter is the atom. (Fig 2.6 - 2.11)

Assignment - 2

List out as many examples as possible of different uses of matter and the prop-

erties of the matter that are used.

Do you know

The first scientist to find experimental evidence for the atom was Dalton (1766

- 1844 A.D). Accordingly all matter is made up of atoms.


The above piece of iron

is made up of billions of

iron atoms.

The above gas jar contains

chlorine gas which is

made up of billions of

chlorine molecules made up

of two chlorine atoms

joined together.

The above beaker contains

billions of water molecule each

of which is made up of two

hydrogen atoms joined with

one atom of oxygen to form

the water molecule

Fig. 2.6 Fig. 2.7 Fig. 2.8

Fig. 2.9

Arrangement of the iron

atoms in the small part

marked on the piece of iron

(Fig. 2.6 )

Fig. 2.10

Arrangement of the water

molecules in the small part

marked on water (Fig. 2.7)

Fig. 2.11

Arrangement of the chlorine

molecules in the small part

marked on gas jar (Fig. 2.8 )

The building units of matter is the atom

2.2 Properties and uses of elements

Iron is made up of iron atoms. Chlorine is made up of molecules of chlorine each

made up of two chlorine atoms joined together. Substances which are made up of

same type of atoms or molecules formed from the joining up of same type of atoms are

called elements.

According to that, iron and chlorine are elements.

Scientists have discovered about 118 naturally occurring elements, up todate.

Each of them is made up of atoms unique to them.

Elements are composed of only the same type of atoms, that cannot be divided

any further. They are pure substances.

Elements are formed of atoms or molecules composed of atoms of the same

type. They cannot be divided chemically into simpler substances any more.


2.2.1 Elements commonly used in daily life and their uses.

Iron, aluminium, sulphur, carbon, oxygen, nitrogen, mercury, copper, gold, silver,

lead, hydrogen, chlorine are some common elements used in our day to day life.

Symbols of elements

You have already learnt that various symbols are used to facilitate communication.

Symbols are used for identifying elements too. These symbols are universally accepted

for elements.

The basis of these symbols is the English or Latin name of the element. The first

letter of the name is used and should be in capital letters.

e.g. Carbon - Symbol C

Oxygen - Symbol O

If two or more elements start with the same letter, the 2nd letter or another letter

can be used. It should be in simple.

e.g. Calcium - Symbol Ca

Chlorine - Symbol Cl

Let us consider symbols of some elements. (table 2.3)

Carbon

Sulphur

ChlorineAluminiumCopper

Iron

Fig 2.12 - Some elements commonly used

Table 2.3 - Symbols of some elements

Element

Hydrogen

Carbon

Nitrogen

Oxygen

Aluminium

Silicon

Symbol

H

C

N

O

Al

Si

Element

Phospours

Sulphur

Chlorine

Argon

Calcium

Iodine

Symbol

P

S

Cl

Ar

Ca

I


Elements where the Latin name is used to derive the symbol. (table 2.4)

You have already learnt that elements can be classified as metals and non-metals.

This classification is based on their physical properties. Metals and non metals are

used for various purposes according to their properties.

Just as elements can be classified as metals and non-metals, they can also classi-

fied according to their state, solid, liquid or gas. There are over 100 elements already.

You can study further about them from the table 2.5.

Conduct heat

wellCan be hammered

into sheets (malleable)Conduct electricity.

(electrolytes)

Can be stretched

into wires (ductile)Gives a ringing sound

when dropped (sonorous)

Has a characteristic

shine (lustre)

Metals

Poor heat conductors

Brittle

(breaks easily)

No lustre

Poor electrical

conductors (except C)

Element

Silver

Gold

Iron

Lead

Mercury

Copper

Sodium

Latin name

Argentum

Aurum

Ferrum

Plumbum

Hydrargyrum

Cupprem

Natrium

Symbol

Ag

Au

Fe

Pb

Hg

Cu

Na

Table 2.4

Non-

metals


Fig 2.13 - Uses of element aluminium

2.2.2 Uses of elements

Properties of metallic and non metallic elements were discussed in the previous

section. Their different properties made them helpful for different purposes Let us

consider some examples of the uses.

Aluminium

Silvery white, strong light metal. Aluminium reacts with oxygen to

form a thin film, thereby protecting the metal from rusting. This is a

good conductor of heat and electricity. Aeroplane parts, compact discs

(CD), door and window frames, step ladders, saucepans, electric cables

are made up of aluminium.

Since they can be made into foils they are used for food packaging

Metal / Non-metal

Non-metal

Non-metal

Non-metal

Non-metal

Metal

Metal

Non-metal

Non-metal

Non-metal

Non-metal

Non-metal

Metal

Metal

Non-metal

Non-metal

Metal

State

(Solid, liquid, gas)

Gas

Solid

Gas

Gas

Solid

Solid

Solid

Solid

Solid

Gas

Gas

Solid

Solid

Liquid

Solid

Liquid

Element

Hydrogen

Carbon

Nitrogen

Oxygen

Sodium

Aluminium

Silicon

Phosporus

Sulphur

Chlorine

Argon

Calcium

Iron

Bromine

Iodine

Mercury

Symbol

H

C

N

O

Na

Al

Si

P

S

Cl

Ar

Ca

Fe

Br

I

Hg

Table 2.5


Iron

Various substances are mixed with iron and converted to alloys. We use these

alloys in our day to day lives for example: steel, is used for many purposes due to its

strength, high melting point and resistance to wear and tear

Iron is used in making bridges, buildings, machinery, arms, blades, cutlery, spoons

as well as iron nails, iron mesh, and barbed wire. Since iron can be magnetised easily

it is also used to make magnets

Sulphur

It is a yellow brittle non metallic element.

Occurs as crystals or powder which is used for

vulcanising rubber, making match sticks, fire

works, gun powder, pharmaceuticals and sulphuric

acid It is also used as a fungicide.

Carbon

It is a solid non-metal. It occurs in many forms, such as charcoal, soot, coal,

diamond, graphite. The Graphite form of carbon is a good conductor of electricity.

Coal is used as a fuel, Graphite as pencil rods, in dry cells for electrodes, and as

lubricants. Charcoal is used to remove poisons and pollutants in water purification.

Diamond is used for jewellery, gem cutting and glass cutting.

Fig 2.14 - Uses of element iron

Fig 2.15 - Uses of element sulphur

Fig 2.16 - Uses of element carbon


Oxygen

Oxygen is a colourless, odourless gas. It is essential for survival of organisms. It is

essential for burning, hence called a supporter of combustion used in the oxy-acety-

lene flame in welding. People with special oxygen needs such as mountaineers, patients

with breathing difficulities, divers and astronauts are provided with extra oxygen.

Nitrogen

Nitrogen is a colourless, odourless gas. It is an inert gas as it does not react easily.

Since it is inert it is used to fill electric bulbs. Nitrogen is used to prepare nitrogen

fertilisers such as urea, packing material for food and making explosive materials.

Copper

Copper has a characteristic colour. It does

not rust and is a good conductor of electricity. It

is used for making alloys such as brass, and for

making conducting wires.

Fig 2.17 - Uses of element oxygen

Fig 2.18 - Uses of element nitrogen

Fig 2.19 - Uses of element copper

Do you know ?

In Nanotechnology, carbon nano fibers are used

for their high strength. These fibers are used as part

of space craft, space suits and missiles.

Oxy-acetylene flame


2.3 Differences in densities among different substances

2.3.1 Density of substances

You have studied about ‘density’ as a characteristic property of substances in

Grade seven. Density is the mass of a unit volume of a substance. It can be shown by

the following equation.

Density =

Units of density are grams per cubic centimetre (g cm-3) or kilograms per cubic

metre(kg m-3).

The Standard International unit of density is kilograms per cubic metre (kg m-3).

You have already learnt about how to find the density of solid substances in

Grade 7.

If a piece of lead of volume 2 cubic centimetres has a mass of 22.6 grams, can you

find the density of lead?

Density of lead = 22.6 g

2 cm3

= 11.3 g/cm3

2.3.2 Differences in densities among substances.

Mass

Volume

Substance Density (g / cm3) Density (kg / m3 )

Iron 7.7 7700

Aluminium 2.7 2700

Copper 8.9 8900

Gold 19.3 19300

Brass 8.4 8400

Lead 11.3 11300

Cork (stoppers) 0.18 180

Ebony 1.2 1200

Wax 0.9 900

Table 2.6 Densities of some substances


How do we find the density of a liquid?

Finding the density of liquids

The density of liquids too can be found out if we can

measure the volume of the liquid and its mass.

A special instrument has been introduced to measure

the density of liquids. It is called a ‘hydrometer’. Fig. 2.20

shows a hydrometer found in the laboratory.

It is an air filled glass tube. The lead shots at the bottom

is for weight. It helps the hydrometer to keep erect when

immersed in a liquid. The tube is calibrated, and the density

can be read off straight away.

Let us find the density of some common liquids using

the hydrometer (Activity 2.3).

Activity 2.3

• Find • few tall bottles (measuring cylinders or plastic bottles with the upper

part removed) • water • kerosene oil • coconut oil

• Pour the liquids into the tall bottles. Immerse the hydrometer in each of the

liquids and read the value of the density.

• Compare your results with the values in density table (2.7).

Table 2.7 - Densities of some liquids

Liquid Density in kg / m3 Density in g / cm3

Mercury 13600 13.6

Glycerine 1262 1.26

Milk 1030 1.03

Sea water 1025 1.02

Water 1000 1.0

Olive oil 920 0.92

Coconut oil 900 0.90

Turpentine 870 0.87

Petrol 800 0.80

Alcohol 791 0.79

Kerosene oil 790 0.79

glass

tube

air

lead shots

Fig 2.20

Hydrometer


This hydrometer can be calibrated by immersing it in liquids whose densities are

known. If this cannot be done, we can use the above hydrometer for purposes of

comparing densities of different substances. Here the density can be taken as the

depth to which the hydrometer sinks in the liquid.

You would have observed in Activity 3 that

- the hydrometer sinks more in liquids of low density.

- the hydrometer sinks less in liquids of high density.

Experiment with the hydrometer you constructed and compare how much it sinks

in different liquids.Compare your observations with Fig 2.22.

You can also construct a simple hydrometer by doing Assignment 3.

Fig. 2.21 shows some hydrometers constructed by some groups of students.

Assignment - 3

• Observe the hydrometer in the laboratory carefully.

• Construct a hydrometer using materials such as a pencil, straw, drawing

pin, clay and sand.

Pencil

Straw

ClayDrawing

pin

Boiling tube

Sand

Fig. 2.21

Density of

kerosene oil

790 kg / m3

Density of

coconut oil

900 kg / m3

Density of

water

1000 kg / m3

Fig. 2.22


Sama wanted to prepare an orange drink with

fresh oranges. She extracted the juice of the orange

and added some water to it. She put two teaspoons

of sugar and started stirring. She noticed that the

seeds which were at the bottom started floating

gradually.

Yet some seeds remained at the bottom while

some floated on top. How can you explain this

phenomenon?

We can assume that some property of the water

changed as the sugar was dissolved in it. Could it

be the density of the water?

Let us do Activity 2.4 to find out whether the density of a substance changes

when some substance is dissolved in it.

Fig 2.23 -

Orange drink

Activity 2.4

• Find a tall vessel, constructed hydrometer, sugar, urea, salt, water and a

table spoon.

• Fill water up to about ¾ of the vessel, and note the height to which the

hydrometer will sink.

• Now gradually add 1, 2, 3 and 4 tablespoons of sugar and note the height

to which the hydrometer will sink at each instance.

• Repeat the activity with urea and next with salt.

• Present your conclusions about the change in density of water when

substances are dissolved in it.

Density of water increases when a substance is dissolved in it. The fact that the

hydrometer floats more as the density of the liquid increases shows that floating of an

object depends on the density of the liquid.

The density of the water in the orange juice increases gradually as more and

more sugar dissolves in it. Then the seeds begin to float. As all the sugar dis-

solve the density increased more and more seeds began to float. What is the reason

for some seeds to float and for some to remain at the bottom?

Do you know?

Density of sea water is higher than the density of river water. The reason is that

a large number of salts are dissolved in sea water.

seeds float

and sink

seeds float

seeds remain

at the

bottom


You would have observed that,

• Objects made of iron, aluminium which are denser than water will sink.

• Objects made of wax, styrofoam, cork, rubber which are less dense than

water will float.

• Objects whose density is similar to that of water will float and sink.

Similar observations can be made regarding any liquid other than water.

Accordingly we can come to the following conclusions regarding the float-

ing or sinking of objects.

Objects which are denser than a liquid, will sink in that liquid. Objects which

are less dense than a liquid will float in the liquid while those that have the same density

as the liquid will sink and float in the liquid.

From the above observations it is obvious that the density of the liquid is not the

only factor responsible for floating or sinking of an object. It may be that the density of

the object too is responsible. To find out, let us do activity 2.5

Activity 2.5

• Find some objects whose densities are known

e.g. iron nails, piece of wax, aluminium coin, piece

of styrofoam, cork stopper, polythene bag filled

with water, rubber stopper.

• Fill a basin with water, and add each of the objects you found one by one

• Prepare a table of objects which sink, objects which float and objects which

float and sink.

Do you know?

Just as objects which are less dense than the liquid float, liquids which are less

dense too float on that liquid. This is why kerosene oil floats on water.


2.3.3 Instances where differences in densities of substances

are used.

The differences in density are useful in our day-to-day activities. Let us find out

some of them.

Instances where differences in density are used;

1. Separating components from a mixture of solid substances

a) Separating stones from rice -

The density of stones and grit mixed with rice is more than the density of water or

rice. Therefore when water is added to rice and stirred, the stones and grit sink to the

bottom and rice comes to the surface.

b) Winnowing of rice to remove the husks from rice -

The rice which are more dense remain while husks which are less dense get blown

away.

c) Gem mining -

During the mining of gems, the gems mixed with soil and stones

are placed in baskets and turned around. The denser gems and

stones sink to the bottom, and the less dense sand and soil float on

the water. So gems can be separated off easily. (Fig 2.24)

2. Separating components from a mixture of liquids

Fig 2.2.5 shows the preparation of coconut oil at

domestic level. Coconut milk squeezed out of grated

coconut kernel is left on the fire to boil. As it boils the

water evaporates off. Coconut oil is less dense and

floats on water. The oil is removed with a spoon.

In the preparation of cinnamon oil, the cinnamon

leaves are boiled in water. Thereafter the mixture of

steam and cinnamon oil is cooled. Then it is condensed.

Cinnamon oil is denser than water. Therefore, water

is floating on the layer of cinnamon oil. Cinnamon oil

gets collected at the bottom of the vessel and can be

tapped off.

Fig 2.24

Fig 2.25

coconut oilwater +

sediment


2.4 Expansion of solids, liquids and gases

You learnt in Grade 7 and already know that solids, liquids and gases expand on

heating and contract on cooling. Expansion is increase in size. At some instances this

property may be disadvantageous, while at some instances it is useful. This property

has been used productively in the construction of machinery. We will be looking further

into these, in this chapter.

In the expansion of solids its length, breadth and height increase. Since the expansion

is so small, it cannot be easily observed with the naked eye. An iron rod of 1 m, when

increased the temparature by 10 C, the increase in length is as small as 1/100 mm. But

the expansion in liquids and gases is more, hence can be observed more easily.

Not only metals, substances such as glass, concrete and plastics also expand.

These expand at different amounts. Because of the problems caused by expansion,

the expansion of various substances should be taken into consideration in the con-

struction of machinery and other constructions. If not, machinery will not work prop-

erly and will get subjected to various distortions. An example, if allowance for expansion

is not considered when laying railway lines, they will press against each other and get

distorted. Concrete of concrete bridges will get cracked.

Phenomena associated with expansion that occur in nature are useful to us. Wind

and formation of soil are two such phenomena. The expansion of rock due to the sun’s

heat, followed by sudden cooling due to rain or differential expansion of different

types of rock breaks up the rock into fragments. This is one way of forming soil.

Warmed up air expands and decreases in density, becoming lighter. The light air goes

up and cool air flows in to fill its place, causing wind.

Let us investigate further about expansion which is so closely associated with our

day-to-day life activities.

2.4.1 Expansion of solids

Let us conduct Activity 2.6 to find out the expansion in length of a metal rod on

heating. Since expansion is difficult to be observed with the naked eye, a special

device has been constructed to observe this.


As the rod is heated, the wire used as the indicator will move to the right along the

scale. Reason is the increase in length of the wire. You can repeat this activity with rods

of different materials of equal length and find out their expansion too.

The different rates of expansion of different metals is used productively in the

construction of the Bimetallic strip. A bimetallic strip is constructed by riveting together

two strips of equal lengths of different metals together. The bimetallic strips in the

laboratory is made up of two strips of aluminium and brass. It is fixed with a wooden

handle. (Fig 2.26)

Let us do Activity 2.7 to find out the uses of a bimetallic strip.

Wooden handle

Aluminium strip

Brass strip

Fig 2.26

Activity 2.6

• Find a metal rod about 1 m. in length (e.g. Brass welding rod) a cardboard half

circle, pencil, a wire about 5 cm and 2 pieces of candle wax.

• Set up the equipment as shown in the figure.

• Observe what happens when the rod is heated.

Metal rodTable

Clamp

Cardboard half circle

marked with a scale

Piece of wire

Pencil,


It will be observed that the bimetallic strip curves inwards (Fig 2.27) and the bulb

goes off.

The reason for this is that aluminium expands more than brass. Now it is obvious

to you that a bimetallic strip can be used as a switch in an electric circuit.

Brass

Before heating After heating

Aluminium Aluminium

Brass

Fig 2.27

Expansion of solids can be productively used in our day-to-day activities. Let us

investigate some of them.

Activity 2.7

stand

stand

iron nail

bimetallic strip

bulb

• Set the above set up and lower the iron nail towards the bimetallic strip till the

bulb lights up.

• Observe what happens to the bimetallic strip when heated with the lighted

candle.

• Observe what happens when the candle is taken away and the bimetallic strip

is allowed to cool.

Assignment 4

Expansion of brass is greater than

that of iron. Select the correct way in

which a brass-iron bimetallic strip will

curve.2

Brass

iron1

Brass

Fig 2.28


Using expansion of solids productively

In Grade 7, you have studied about the use of expansion in fixing a frame to a

wooden wheel or removing a metal stopper from a glass bottle.

Let us find uses of bimetallic strips.

1. Used as a heat controller in an electric iron or electric oven.

In order to maintain the temperature of an electric iron or electric oven a bimetallic

strip is used as an automatic switch to control the current passing through the heating

coil. When the circuit breaks the bimetallic strip cools. Then it comes back to position

and the circuit is completed. The heating coil gets heated up again. (Fig 2.29)

If the adjustable nail near the handle of the iron is turned so that it presses on the

bimetallic strip a higher temperature can be obtained.

2. Used as an automatic switch in fire signals

A fire signal is an instrument which will give an alarm in case of a fire. When a fire

occurs the bimetallic strip will get heated and get bent. This completes the circuit and

sets off the signal. You can understand it’s working by studying the Fig 2.30.

Electric bell

Tuning nail

Brass Iron

Electric bell

Brass

IronNormal Stage During Fire

Fig 2.30

Bimetallic

strip

Brass

Iron

Heating coil

Adjustable nail with

plastic end.

Attached metal strip

Fig 2.29


2.4.2 Expansion of liquids

We know that all liquids, including water expand on heating. Do same volumes of

different liquids expand by the same amount?

According to the observations you will find that equal volumes

of different liquids expand by different amounts.

Let us consider some instances where expansion of liquids is

used in daily life.

Using liquid expansion productively

A thermometer is one example where expansion of liquids is

used productively. A thermometer can be constructed by turning the

end of a capillary tube into a bulb, filling it with mercury or coloured

alcohol, and closing up the other end. When the bulb gets heated,

the liquid expands and goes up the capillary tube. The temperature

can be read on the scale.

bulb

Fig 2.31

Mercury

or

coloured

alcohol

Capillary

tube

Do you know?

Automatic bulbs come on and off due to

a bimetallic strip connected to it. Study the

figure to understand the working of it.

Bimetallic strip

Glass cover

Filament

Contact with

bimetallic strip

Activity 2.8

• Fill 3 small bottles of same size (injection phials) with coloured water, kerosine

oil and coconut oil.

• Close the bottles with 3 rubber corks through which capillary tubes (Ball point

tubes) have sent.

• Mark the liquid level with a piece of thread.

• Place all 3 bottles in a vessel of warm water as shown in figure, and note the

level in about 1 minute.

• Present your conclusion about expansion of different liquids.

Warm water

Kerosine oil Coloured water Coconut oil


Expansion of gases

Now let us investigate expansion of gases and their uses. Expansion of gases can

be easily observed from the activity 2.9.

A science exhibit seen at an exhibition was named ‘Dancing coin’. One side of a

coin placed at the mouth of an empty aerated water bottle, got raised on one side

when the bottle is held with both hands (Fig 2.32) (The mouth of the bottle was

moistened with water before placing the coin). Expansion of air can be observed by

the following activity.

This phenomenon occurred because the air inside

the bottle expanded due to the warmth of the hands. The

expanded gas escaped between the mouth and the coin,

and the coin got raised. When the gas escaped the coin

falls back. This happens in a cycle. The mouth of the

bottle had to be moistened to prevent air seeping out.

Let us consider some instances where expansion of

gases is used.

When a volume of trapped air expands suddenly, a huge explosion takes place.

The energy generated is enormous. Explosives are used to break up huge rocks,

because when the explosive is burnt, the heat energy produced causes the gases to

expand suddenly. Now you can explain why a cracker makes a big noise when lit.

Fig 2.32

Air

Coin

Activity 2.9

Fix a baloon to the mount of a glass bottle and immerse the bottle in hot

water. The heat from the hot water raised the temperature of air inside the

baloon. Then the baloon blows up.Air filled baloon

Glass bottle

BasinWarm

waterAir

Baloon

Glass

bottle

Air


2.5 Properties and uses of compounds

2.5.1 Difference between elements and compounds

The basic building unit of matter is the atom. Substances

that are made up of the same type of atoms are called ele-

ments.

Two or more elements combine chemically to form a

compound. Although there are nearly hundred elements, there

are many ways in which they can combine, resulting in millions

of compounds. Compounds are also pure substances.

The different types of matter from which the earth is

made comprises of elements

Different types of elements chemically combine to form

compounds.

Let us investigate how elements combine chemically by

the following example

• Iron powder is a greish-black solid (Fig 2.34)

• Sulphur powder is a yellow solid (Fig 2. 35)

• When these two are mixed and heated till it melts, a black new compound is

formed (Fig 2.36)

It can be observed that the properties of the product formed is very different

from the substances at the beginning.

This shows that element iron has combined with element sulphur to form a new

compound called iron sulphide.

Elements mainly oxygen,

carbon, nitrogen,

calcium and iron

combine to make the

hundreds of compounds

in the human body

Fig 2.33

Fig 2.34 - Iron Fig 2.35 - Sulphur Fig 2.36 - Iron sulphide

Iron + Sulphur On heating Iron sulphide.

(element) (element) (compound)


According to the above information, the most important difference between an

element and a compound is that elements are formed of atoms of the same element

while compounds are formed of atoms of different elements.

2.5.2 Homo - atomic molecules and hetero atomic molecules

It was indicated in the above section that two or more atoms combine chemically

to give molecules. There are two basic ways in which two or more atoms can combine,

that is two or more atoms of the same kind can combine or two or more atoms of

different kinds of atoms can combine in the formation of the molecule.

Eg. 1

Hydrogen Hydrogen Hydrogen

atom atom molecule

A molecule of hydrogen is formed by two atoms of hydrogen joined together. There

fore it is a homo atomic molecule

Eg. 2

Hydrogen Chlorine Hydrogen chloride

atom atom molecule.

A molecule of hydrogen chloride is formed by one hydrogen atom combining with

one chlorine atom.

Therefore, it is a hetero atomic molecule.

Another two examples are given below

Hydrogen + Oxygen Water

(element) (element (compound)

Hydrogen + Chlorine Hydrogen chloride


+

+


Fig 2. 38

Chlorine, Oxygen and Nitrogen exist in nature as homo-atomic molecules.

Some gases are made up of one atom. For example, helium, argon, neon. They

are called mono atomic gases.

Carbon dioxide is a gas that is formed by the combi-

nation of two oxygen atoms with one carbon atom. It is a

hetero atomic molecule.

Water, carbon dioxide, ammonia, methane are hetero atomic molecules with

different types of atoms combined together. Therefore, they are known as compounds.

A few hetero atomic molecules are given in the table below:

Fig 2. 37 - Some homo atomic molecules

Compounds too can be classified as solids, liquid and gases according to the state

in which they are found.

Study the given table for further understanding.

Hetero atomic

molecule

Water

Ammonia

Methane

Atoms in the

molecule

Hydrogen, Oxygen

Hydrogen, Nitrogen

Hydrogen, Carbon

Structure of the molecule

2. 8 Table - Some hetero atomic molecules

HHO

H

H HN

C H

H

H

H

Activity 2.10

Create some homo atomic molecules and heteroatomic molecules and exhibit

in the classroom.


2.5.3 Properties and uses of some compounds

Different compounds are used for various purposes based on the differences in

their properties. We use many compounds in our daily life for various purposes. Indi-

cated below are properties of some substances and instances of their use.

Sodium chloride

Sodium + Chlorine Sodium chloride


Sodium is a solid metallic element.

Chlorine is a gaseous non-metal. These two

elements combine together to form sodium

chloride. It is a white solid. So sodium

chloride shows very different properties

from its components sodium and chlorine.

It dissolves well in water. Sodium chloride

is commercially produced by evaporating

sea-water. Such a place is called a saltern.

Sodium chloride is used as a flavouring in food preparation, as a preservative in

food preservation and as a raw material in various industries.

Sodium chloride is also used as a germicide and also in saline solution.

Fig 2. 39 - A saltern

Compound

Water

Sodium chloride

Carbon dioxide

Copper sulphate

Ethyl alcohol

Nitrogen dioxide

Physical state

Liquid

Solid

Gas

Solid

Liquid

Gas

Component elements

Hydrogen, Oxygen

Sodium, Chlorine

Carbon, Oxygen

Copper, Sulphur, Oxygen

Carbon, Hydrogen, Oxygen

Nitrogen, Oxygen

2. 9 Table


Water exists in the liquid state at

normal temperatures. When cooled

it goes into the solid state (ice) and

when heated it forms a gas (water

vapour). Since many substances can

be dissolved in water, its a good

solvent. Pure water is colourless. It

has no odour.

Water is used as a drink, as a

solvent and as a raw material in many

industries. Also it is used for cleaning

purposes, agricultural practices and to

produce hydroelectric power. It is also

used as a coolant. eg. In radiators. Due to the flowing nature of water, it is used as a

medium of transport. Study the Fig 2.40 and identify the various uses of water.

Carbon dioxide

Carbon + Oxygen Carbon dioxide


Carbon dioxide is

formed by the combination

of carbon and oxygen. It is

a colourless gas. It is

slightly soluble in water. It

is not a supporter of com-

bustion. Therefore it is used

as a fire extinguisher. Aer-

ated water has carbon di-

oxide dissolved under pressure. This provides it a freshness and a special taste. When

Carbon dioxide is cooled, it solidifies and is called ‘dry ice’.

Dry ice evaporates off from the solid state to the gaseous state straight away as

white fumes without becoming a liquid. Dry ice is used as a refrigeration agent and for

producing artificial rain.

Fig 2. 40 -Different uses of water

Water

Hydrogen + Oxygen Water


Fig 2. 41 - Different uses of carbon-dioxide.

Fire extinguisher Soft drinks


Among the chemical substances used in the house, we will consider detergents,

paints and pigments, flavouring, cosmetics, germicides and pharmaceuticals in this

chapter.

2.6.1 Soap and Detergents

We use various types of detergents for cleaning the body,

clothes, house hold equipment and floors. Things may not get

sufficiently cleaned by using only water. In such cases using

detergents is more successful, e.g. removing dirt cannot be

done only with water, but soap will have to be used. In the past,

people have used substances such as wood ash or certain plant

products. Soap was a popular substance even in the past. Soap has the function of

converting insoluble dirt into a soluble form.

In preparing soap a base such as sodium hydroxide and oil or fat are used. Here

plant oils, animal oils or fats are used. In Sri Lanka the commonly used oils are coconut

oil or other plant oils.

2.6 Domestic uses of chemical substances

Recall the activities you carried out from the time you woke up in the morning till

you went to sleep on any particular day. Some of the activities would be cleaning

yourself, drinking a cup of tea, taking your meals, getting dressed up for the day etc.

Prepare a list of some such activities and indicate the various substances you used

at each instance.

Check whether your list is similar to the one below

Activity Substances used

• Cleaning yourself − Soap, tooth paste

• Drinking tea − Sugar, tea, water

• Taking breakfast − Salt, vinegar, baking powder, flavouring

• Dressing up for the day − Powder, perfumes, oil

• Treating a simple wound − Germicide

• Treating a headache − Pain killer

Did you realise that all substances in the second column are chemical substances?

Assignment - 5

• Prepare a list of chemical substances used in various domestic activities.

• Classify the chemical substances in your list using suitable criteria.


When certain salts are dissolved in the water, the

action of soap is decreased. Presently there is a bigger

tendency to use detergents in place of soaps. Most

detergents are madeup of artificial substances. The

natural substances in soap get decomposed easily and

get removed from the environment. But, some

detergents are not easily removed, hence is a pollutant.

Due to the increased use of soaps and detergents they

collect in water ways and produce foam, causing

problems to aquatic organisms and create environmental problems.

2.6.2 Paints and pigments

Another group of chemicals commonly used

is the paints and the pigments that are used to

preserve wood floor, walls and metal surfaces.

These are often not pure substances but mixtures

of chemicals. Paints have three important

components. They are substances which form the

paint film, substances which give the colour (pigment) and substances which are used

to dissolve the chemicals. The pigment is often salts or powders of various metals.

You would have observed that the paint remains as a thin film on the surface of the

painted surface. After painting the volatile substance used in its’ preparation evaporates

off. The remaining component forms the film over the surface.

Not only the volatile substances used in the preparation of paints, but also the

substance such as thinner, turpentine used for making the paint less viscous are also

not environmental friendly. If they are inhaled in excess it may be harmful. It has been

mentioned that some paint pigments are also harmful.

2.6.3 Food additives

Chemical substances which are added to increase the taste, smell and nutritional

value as well as to preserve them in various different forms are called food additives.

According to the function, that they perform food additives may be classified into

groups.

Nutrients : A number of additives are added to food to increase nutrient values.

eg. Vitamins, minerals, Iodised salts are salts with added iodine which

are an important nutrient for the body.

Flavourings : Substances which are added to improve the flavour of food are

flavourings. Salt, condiments, mono-sodium glutamate (MSG) are

some of them.

Fig 2. 43 - Some varities of paints

Fig 2. 42 - A Water way

contaminated with foams


Preservatives : These are the additives that are used for preserving

food and preventing spoilage of food. Salt has

been used for this purpose for a long period of

time. Sugar, honey, vinegar are also used as

preservatives. In addition many other

preservatives are also used.

Colourings : Colouring is commonly used to improve the colour

of sweets, cordials etc.Saffron is a natural

colouring. Many artificial colourings are available.

Raising agents : These are used in the making of bread, biscuits etc. to give the

lightness and softness to the dough. Baking powder is an example.

Sweeteners : These improve the sweetness of food. Examples are sugar and

saccharin.

You should be particularly careful about substances that are added to food. Most

of them are artificial and may be harmful to the body. Most of the food stuff on road-

sides such as coloured sweet meats or preserved meals contain many harmful colourings,

preservatives etc. Hence such food taken in excess or too often should be avoided.

2.6.4 Cosmetics

Cosmetics are used to appear more attractive andto maintain cleanliness. Some examples areperfumes, powders, hair dyes, gel, deodorants, nailpolishes, lipsticks etc.

In the past, natural substances such as plant matter(fruit juice, sap of vegetable) and clay were used as

cosmetics. Sandal wood, Kokum, Aloe and Margosa were commonly used in this con-text.

Fig 2. 44 - Pre

served food types

Do you know ?

A specific number is given for each accepted presevative, colouring and sweetener.

It is named as E- numbers. Accepted ingredients in food can be identified by checking

these E- numbers.

Eg. - E223 -Recommended preservative

- E102 - Recommended food colouring

Assignment - 6• Examine various food packings.• Make a list of the substances added to the food as indicated on the packing.• Classify them according to the purpose for which they have been used.


Perfumes and deodorants only cover the bad odour of sweat. Some deodorants

control the micro-organisms that cause the smell. Certain powders absorb the sweat

making the skin dry, cover up the spots and blemishes in the skin.

However, some people may be allergic to these

chemicals. Some of the volatile substances in cosmetics

are harmful to the environment.

The dyes included in lipsticks, nail polish or hair

dyes may cause allergic reactions in some people. It is

very important that the contents in the advertisements

of these cosmetics are questioned as to whether they

can be accepted scientifically, before using them.

2.6.5 Pharmaceuticals and Germicides

There are many simple medicines we use without

medical advice. Paracetamol tablets are commonly used

to get relief from pain and fever. Milk of magnesia is used

to reduce the acidity of stomach. Antiseptics are applied

on wound and cuts and germicides are used to clean floors,

toilets etc.

Leaves, bark, roots and nuts of the Margosa

tree, saffron water and salt solution can be regarded

as natural germicides.

Antiseptics : Surgical spirits, Boric acid

Germicides : Phenol, Chlorine water

It is dangerous to use medicines without medical

advice. It is also important to follow the dosage and

the time intervals stated in the instructions in the pack-

ing. There have been instances where not following

proper instructions some times to be fatal. If the ill-

ness continues even after using them, it is important

to consult a medical doctor.

Using germicides will control the increase of the micro-organisms or destroy them.

At such times even useful micro-organisms may get destroyed. Particularly excessive

usage of germicides in toilets may suppress the decay of sewage where bacterial

action plays a vital role. Therefore, using such substances should be done carefully

making note of the proper dosage and frequency.

Fig 2. 45 - A skin allergy

Fig 2. 46 - A store of

pharmaceuticals

Fig 2. 47 - Using a germicides


2.7 Series and parallel connections of electrical appliances.

Madhawan observed that the entire set of red bulbs in a

particular Wesak decoration does not glow. On careful ob-

servation he noticed that only one of the bulbs in the whole

set had blown off resulting in the whole set of red bulbs failing

to work. He wondered why in his house, even though one

bulb goes off, the others remain lit.

This was a problem to him. He directed this problem to

the science teacher who requested him to do the following

activity in order to find an answer to his problem.

• In circuit (A), when 1 bulb was removed and replaced with the burnt bulb, the

remaining bulb too went off.

• In circuit (B) when 1 bulb was removed and replaced with the burnt bulb, there

was no change in the remaining bulb. It continued to light.

Fig 2.48 - Set of

decorative bulbs

Activity 2. 11

• Find a dry cell, two 2.5 V torch batteries, 1 burn 2.5 V bulb, 2 bulb holders and

a few pieces of wire.

• Try out different circuits in which you can light both bulbs simultaneously.

• From each of the circuits you made remove 1 bulb, replace it with the burnt bulb

and note the difference.

Shown below are some circuits that were put up by different groups in the class.

(A)

Circuit constructed by Madhavan’s group

(B)

Circuit constructed by Nimali’s group


2.7.1 Different ways of connecting bulbs

Bulbs can be connected in a circuit in two ways. Madhavan’s group had con-

nected the bulbs in a single row. That is called a series connection. In such a connec-

tion there is only one path for the current to flow through. As a result the same current

flows through all the bulbs.

Nimali’s group had connected the bulbs in another way. That is called a parallel

connection. In such a connection the current has several paths to take. Therefore, only

a part of the current goes through each bulb.

In a series connection if one bulbs goes off, all the rest of the bulbs also go off. In

a bulb set used for decoration there are a number of bulb sets connected in series.

Therefore, if one bulb goes off, all the rest of bulbs go off. This is because the current

is stopped at the bulb which got blown.

Fig 2.49 shows the circuit diagrams for series connection and for parallel connec-

tions.

At times the current from one bulb may not be sufficient. In such instances,

you may observe that a number of cells will be connected to the circuit. An electric

torch is an example of this.

Cells too can be connected in many ways, to get electrical energy.

Series circuit Parallel circuit

Fig 2.49

Think!

In the house, although one bulb goes off, the others remain lit. Can you think

whether the circuit connections in your house are in series or in parallel?


2.7.2 Different ways in which cells can be connected

There are two ways in which cells can be connected in order to light a bulb

(Fig 2.50). You too can try out the different ways.

In circuit (A) the Positive terminal of one cell is connected to the negative terminal

of the other. This is called series connection. In circuit (B) the positive terminals of all

cells are connected together while negative terminals are connected together. Such a

connection is called a parallel connection. Fig 2.51 shows the circuit diagram of a

series connection and a parallel connection.

You would have noticed that the brightness of the bulbs changed according to the

manner in which they are connected. Can you guess the reason for such a change in

brightness? Activity 2.12 will help you to find the answer.

Series connection Parallel connection

Fig 2.51

(A) (B)

Series connection Parallel connection

Fig 2.50


Compare your conclusions with the findings of the above groups of students who

conducted the above activity.

(1) When the number of bulbs connected in series is increased the

current flowing through decreases. Brightness of the bulbs decreases.

(2) When the number of bulbs connected in parallel are increased,

current flowing through increases. There is no change in the

brightness of the bulbs.

(3) When bulbs are connected in parallel, switches can be used to

switch off each bulb independently.

You would have noted how the brightness of the bulbs changed according to the

way the cells are connected. It may be presumed that there is a change in the voltage

across the two ends of the bulbs or in the current depending in the way the cells are

connected. Such a presumption is known as a hypothesis in science. We can test

whether our hypothesis is correct or not by the following activity.

Activity 2. 12

• Find 2 dry cells, 3 torch bulbs, 3 bulb holders, pieces

of wire, 3 switches and an ammeter.

(i) Connect the circuits as shown. Keep increasing the

number of bulbs, one by one and note the current in

the ammeter. Observe the changes in the brightness

of the bulbs.

(ii) Connect the circuits in parallel and keep increasing

the number of bulbs. Note the ammeter reaching at

each instance. Observe the changes in the brightness

of the bulbs.

• Investigate ways in which bulbs can be connected so that bulbs turn on and

off independently of each other.

i) If you are provided with 3 cells, 3 similar torch bulbs and 3 switches, draw a

circuit diagram of how you would connect them so that each bulb can be switched

off independently.

(Symbols: V Bulb, Cell, Switch).

ii) When all the bulbs in the above circuit are lit up, and if the current through 1

bulb is 0.2 amps, what is the total current that is released from the cells to the

circuit?


From the above activity you will be able to get the following observations;

• When the number of cells in series are increased the brightness of the bulbs increase.

The current flowing through the bulb increases. The potential difference across the

two ends increases. This potential difference is equal to the sum of the voltages of

the cells

.

• When the number of cells in parallel are increased the brightness of the bulbs do

not increase. The current flowing through the bulbs or the potential difference

across the two ends remains the constant.

In our day today life, we come across many kinds of circuits which contain bulbs

and cells. Let us consider some of them

Activity 2.13

• Find 3 torch batteries, 1 torch bulb, holder, an

ammeter, a voltmeter and a few pieces of wire.

• Keep increasing the number of cells in a series

connection and observe the following;

Change in the

• brightness of the bulbs

• current through the bulb

• potential difference between the 2 ends of

the bulb

• Keep increasing the number of cells in a parallel connection and make the

above three observations.

Do you know?

A battery is a collection of 2 or more cells. Very

often batteries are formed by connecting a number of

cells in series. In a vehicle battery of 6 cells of 2 V are

connected in series. The maximum voltage of such a

battery is 12 V.

In a motor cycle battery, 3 cells of 2V are

connected in a series. Hence maximum voltage is 6V.


2.7.3 Circuits with cells and bulbs

1) Electric Torch

Torch has an electric circuit where two or three cells are connected in series to

light a bulb. In the modern torches, instead of the normal filament bulbs, LEDs are

used. Since LEDs an consume little electricity, the cells can be used for a longer time.

The inside of a torch is shown below (Fig 2.52)

Remove the parts of a torch and note how the circuit is connected at A and B.

Note how the circuit is connected at B, in a torch with a metel case.

2) Decorating bulb sets

You already know that the bulb sets used in

decorations consist of a number of rows of bulbs connected

in series.If you examine such a set of bulbs you can see

that these rows are twisted together (Fig 2.53A).

In each row of bulbs an automatically switching on

and off bulb is included, then all the bulbs in the row get

switched on and off accordingly. In a decorating bulb set a number of rows of bulbs

connected in series are twisted in parallel.

A set of 100, 12V, bulbs are shown in Fig 2.53 B.

Fig 2.53 A

Fig 2.53 B

live wire

neutral wire

Fig 2.52

Reflector

Button of the swicth

Copper strip

Plastic bar

BulbMetal spring

Plastic lid

BA


Similar bulbs or any other electric appliance if connected in series will have the

voltage from the source equally divided among each.

The bulbs are connected to the main electric supply of 240 V. If each bulb in the

set is of 12V, let us see how many such bulbs can be connected so that they light up

with maximum brightness.

Number of bulbs that can be connected in series = —— = 20

If we insert more than 20 bulbs, the brightness of the bulbs will decrease.

You can show that the voltage across the two ends of a bulb, when you have 20

bulbs is 12 V by connecting a voltmeter to the circuit. Accordingly

240V

12V

3. Household circuits

Your domestic electricity supply comes through two cables (or a double cable)

from the main supply. One of the cables will be the live wire and the other will be the

neutral wire. The circuit in the house is called the ‘house circuit’. Every electrical

appliance in the house is connected to these two wires. In a house circuit, the bulbs or

other appliances are connected in parallel, hence they opprate independently. Figure

2.54 shows how the bulbs, plug bases and switches are connected.

Neutral wire

plug base with a switch

Live wire

Earth wire

Fig 2.54

Do you know?

In a row of bulbs connected in series, when one bulb goes off the rest should go off.

But in modern bulb sets this does not happen. When a bulb goes off there is a mechanism

for the curcuit to get completed within. Therefore the rest of the bulbs light up.

Assignment - 6

• Calculate the number of torch bulbs of 2.5 V that can be connected to a

12V battery to get a set of brightly burning bulbs.

• Take such a bulb and connect it to a 12V battery.

(Do not use the main supply)


2.8 Using magnets in day-to-day life

Sulochana’s grandmother was threading a needle, and she accidentally dropped

the needle. She failed to find it. Sulachana offered to find it, ‘Let me find it, Grandma’,

she offered. She tied a piece of magnet to the end of a stick and waved it across the

floor. ‘Here is your needle, Grandma!’ said Sulochana happily she picked up

the needle from the magnet to which it was attached. Do all things get attracted to

magnets?

2.8.1 Magnetic substances

All things do not get attracted to magnets. Things that are attracted to magnets are

called ‘magnetic substances’. Let us do activity 2.14 to identify the magnetic materials

around us.

It will be observed only some substances such as iron nails, file clips, needle, and

ferrite rods will be attracted to the magnet.

Some metals such as iron, cobalt and nickel have magnetic properties. Steel (an

alloy of iron) and ferrite (a synthetic material) are also magnetic substances.

Substances are attracted to magnets, but it is

surprising that they are attracted only to the ends of

the magnet. (Fig 2.55)

The magnetic property of magnets is localised

mostly at the two ends of the magnet. These ends

are called the ‘poles’ of the magnets.Fig 2.55

Activity 2.14

1. Collect as many objects as possible from around you (e.g. iron nails,

various coins, plastic, needles, bits of gold, file clips, pieces of video tape,

brass nails, pieces of aluminium, ferrite rods, copper wire etc.).

2. Hold a magnet to each of them and note the results.

Do you know?

Magnets can be made only from magnetic materials. Magnets made of ferrite has a

strong magnetic property, but since they are not made of metal, they can break easily.

Hence they should not be allowed to drop on the floor and should be carefully used.

Why do some coins which appear to be made of aluminium get attracted to magnets?


2.8.2 Magnetic poles of different types of magnets.

A magnet normally has two magnetic poles, that is the North pole and the South

pole. How are these poles identified?

The magnet will always come to rest along the North-South direction of the earth.

The pole pointing to the north is the North pole, while the opposite end is the South

pole.

Many magnets in the laboratory have their poles marked. The north pole will be

noted as N and the south pole as S. Often the North pole will be coloured red which

should make it easy for you to identify the poles.

There are magnets of various shapes. The naming of these magnets and how their

poles are indicated are shown in Fig 2.56. These magnets are used for various purposes.

The magnetic property of the above magnets remain for a long time. Hence they

are called permanent magnets.

Bar magnet Horse-shoe magnetRing magnetMagnadoor magnet

Fig 2.56 - Different types of magnets

S

N

N

S

NS

N S

Activity 2.15

• Find a magnet where the poles are not

indicated, and a piece of thread.

• As shown in the figure, hang the magnet with

a piece of thread, and tie it to a wooden

support. Take care not to use iron supports.

• Rotate the magnet and let it rotate freely.

• Note the direction in which the magnet comes

to rest.

• Repeat several times.

magnet

string


Scientist Oersted in 1820 A.D., noted that if current is

passed along a conductor the needle of a compass placed

close by showed deflection. (Fig 2.57)

This is how, almost accidentally it was discovered that a

magnetic field is created around a conductor carrying current.

Later it was discovered that by using a coil instead of a

straight wire, and placing a magnetic material inside the coil

would increase the magnetic effect. Such a magnet made by

passing current through a coil of wire is called an

electro-magnet.

You too can make an electro-magnet. Try Activity 2.16

According to the above observations;

• increasing the number of batteries

• increasing the number of turns in a coil

Fig 2.57

Steel needle

Do you know?

Permanent magnets are made of steel. Their

magnetism remains for a long time.

As shown in the figure, a large steel needle

can be made into a permanent magnet by rubbing

with a magnet.

Activity 2.16

• Find • about 1 metre of insulated wire coil

• a large iron nail

• few torch batteries

• few file clips

• Wind the wire around the iron nail in a coil.

• Remove the shellac covering at the two ends of

the wire.

Coil

Iron nail

• Pass a current through the wire. Note how the clips get attracted to the

ends of the coil.

• Increase the number of batteries and note how the magnetism increases.

(When the magnetism is increases the number of clips that get attached will

be more)

• Note what happens when you stop the current flowing.


result in increasing the magnetic effect of the magnet.

As the current flow is stopped the magnetic effect vanishes. Therefore electro

magnets can be used at times when we need the magnetic effect only temporarily.

You already would have noticed that the magnet has to be at a certain distance for

it to attract objects (file clips). Why is that? Is the magnetic effect only limited to

certain areas only? Find out by doing activity 2.17

Students who conducted this

activity noted that the iron filings

around the magnet spread out in a

definite pattern while the iron filings

further away were spread out at

random. (Fig 2.58)

What is the conclusion you can

arrive at?

The magnetic force around a

magnet is limited to a certain area and that it is spread out along definite lines. These

lines which the iron filings are arranged are called magnetic field lines.

The area arround a magnet where the magnetic force is called the magnetic field,

while the lines along which the magnetic force act are called magnetic field lines.

Iron powder

arranged in a

systematic

manner

Iron powder

spread out

unsystematically

N S

Fig 2.58

Activity 2.17

• Bar magnet, iron filings, (or some mineral sand isolated from sand), piece

of white paper.

• Place the magnet on the white paper and sprinkle the iron filings around the

bar magnet.

• Now tap the paper lightly

• What can you conclude from the observations?

Do you know ?

• Magnetic field is spread out around the

magnet in all directions.

• A set up as shown in figure will

demonstrate a 3 - dimensional magnetic

field.

test tube

glycerine/

coconut oil

magnet

magnetc field

Iron powder


In order to identify the magnetic field of the different types of magnets do

Assignment 6.

The magnetic effect of a magnet can be

preserved by packing them in such a way, that

is by using iron pieces so that the field lines are

not disturbed. (Fig 2.59)

Magnets may become weak by improper

storage.

Some student noted that although many

objects get attracted to a magnet, it may also

repel another magnet. They questioned their

science teacher about this, and they were asked to conduct Activity 2.18.

Do activity 2.18 to find out more about this.

Fig 2.59

Soft iron

pieceof

wood

Soft iron

Fig 2.60

N

N

N

N

N

N

S

S

S

S

SS

Assignment - 6

• Get the different magnetic fields using different types of magnets as

indicated in Activity 2.17.

• Draw the magnetic lines of force around the field.

• Similar magnetic poles repel each

other

• Opposite magnetic poles attract

each other

Compare your observations with the following (Fig 2.60)

Activity 2.18

• Find 2 bar magnets

• Place one on a watch glass as shown in the figure

• Bring the poles of the other magnet near the poles of the first magnet in the

following manner,

i) N - N

ii) N - S

iii) S - S

• Note the reactions at the ends of the two magnets

• Record your conclusion from the activity.

watch

glasses


2.8.3 Use of magnets in daily life

We started this chapter with a simple example of how a magnet is useful in our

daily life. Let us investigate further uses.

1. Compass

This is made from a permanent magnet. A small

magnet which can freely rotate around a pivot is used

here. The North direction to which the needle will point is

made as an arrow. If not, it will be marked with red, blue

or white (Fig 2.61).

A dial indicating the 8 directions is fixed underneath

the needle.

The directions are indicated by letters

North - N North East - NE

South - S South East - SE

East - E South West - SW

West - W North West - NW

The North and South directions of the earth can be easily found out by using the

compass. In order to find the other directions the compass must be turned so that the

arrow of the needle is in line with the letter N of the dial.

The dial then indicates the other directions straight away. The compass helps navi-

gators, pilots, soldiers and scouts to find out the directions in which they should travel.

2. Electro Magnetic crane

Electro-magnetic crane is another instance where

strong magnets are used. Such cranes are used to lift

iron or steel objects (Fig 2.62).

When the metal is lifted and shifted to wherever it is

needed, the current is switched off. Then the iron or

steel objects leave the magnet.Fig 2.62

Fig 2.61- Compass

Assignment - 7

• When a compass is used there should be no magnets around.

Can you think why?

• Using a needle magnetised by the contact method, make a simple compass.


3. Other uses of magnets

• Refrigerator doors are kept tightly closed by fixing a magnetic tape to the door

frames.

• Letters, numbers to which magnetic tapes are attached are used to fix them on

magnetic boards.

• Drawers, cupboards are fixed with permanent magnets to ensure that they are

closed well.

• The door of your box of instruments is kept well closed by two pieces of permanent

magnets.

• Permanent electro-magnets are used in,

• electric bells

• electric motors and dynamos

• to remove a piece of iron/steel from the eyes in surgery.

A freely moving magnet always hangs in the North - South directions. Have you

ever thought why? The Earth’s magnetic field is the cause for this.

Earth’s Magnetism

There is a magnetic field around the earth. That

is, it behaves as if there is a huge bar magnet inside

the earth. This magnetism associated with the earth

is called earth’s magnetism. This is caused by the

rotation of the liquid metals in the outer core of the

earth round the earth’s axis.

Freely moving magnets will align themselves in

the directions of the magnetic field of the earth.

Therefore they turn the north - south direction of

earth. (Fig 2.63)

The poles of the magnet are not exactly aligned

along the geographical North and South poles but just a few degrees away. The North

is situated a little shifted towards the North West and the South is situated a little

shifted away from the geographical South East.

The charged particles which come to the earth constantly from the sun to the earth

may be harmful to organisms. The earth’s magnetic field will attract these particles,

thus protecting the organisms on earth.

Assignment - 8

Construct an equipment using a permanent or an electro-magnet. (eg. Motor/

electric bell) Collect the constructions of the whole class and exhibit them.

Magnet

North

geographical

North

geographical

South

Magnet

South

Fig 2.63

N

S


Exercises

(1) Given below are 4 models prepared by 4 student groups to show the arrangement

of particles in different states of matter.

A B C D

Select the statement which best describes each model and write the relevant letter in

the space provided.

1) An element in the solid state -------------------------

2) An element in the gaseous state -------------------------

3) A compound in the gaseous state -------------------------

4) A compound in the solid state -------------------------

(2) The table indicates the principles and uses of certain elements and compounds

Select the appropriate element / compound and fill in the blanks.

(Carbon, Oxygen, Water, Sulphur, Sodium Chloride)

Properties and uses

1) Colourless compound Exists as a

liquid at room temperature. Good

solvent.

2) Non - metal. Black in colour.Good

conductor of electricity.

3) A gas composed of di - atomic

molecules. Essential for survival

of organisms. Colourless

4) Solid compound. Used for

flavouring and preservation of

food.

5) Yellow in colour element in the

solid state. Used as a fungicide.

Element / Compound


3. A, B and C are three immiscible liquids. They are differently coloured and placed

in 3 beakers. What is the method you can adopt to arrange them in order of their

densities using only these solutions and equipment.

4. The picture shows a ‘density ladder’ in a science exhibition of school. Indicated

are the densities of the different substances. densities of X and Y are not given.

i) Why does the piece of Mahogany

float on water but not on kerosine oil?

ii) Which of the following could be

the density of X?

a) 0.7 g cm-3

b) 0.8 g cm-3

c) 0.9 g cm-3

iii) Which one of the following could

be the density of Y?

a) 1.2 g cm-3

b) 7.5g cm-3

c) 8.9g cm-3

5. a) A group of students passing a concrete bridge noticed gaps between the concrete

slab and the side walls.

kerosine oil

Mahogany

Perspex

Brass coin

Mercury

water

0.8 gcm-3

0.9 gcm-3

1.0 gcm-3

1.2 gcm-3

8.9 gcm-3

13.6 gcm-3


i) What is the necessity of such gaps?

ii) What effect would there be on the bridge if the gaps were not kept?

iii) What was the reason for filling the gaps, using ‘tar’?

iv) State another construction of cement and concrete where similar gaps are left?

b) The gap between two railway lines is to allow for expansion.Underground oil

pipes in deserts also expand on heating. Suggest a method that could be used to

allow the expansion in the second example.

6. ‘It is more effective washing with soap and water than with water alone’.

a) Is the above statement true or false? Give reasons for your answer.

b) Suggest a simple test to prove the correctness / incorrectness of the above

statement.

7. You have been supplied three torch batteries and three torch bulbs. Connect the

three cells in series.

a) i. Use standard symbols and draw a circuit of the 3 bulbs connected in series.

ii. Use standard symbols and draw a circuit of the three bulbs connected in

parallel. Put a switch for each bulb.

b) i. Why is it necessary to connect bulbs parallely to a domestic electric circuit.

ii. How is the battery of a vehicle connected to signal lights and horn? (Parallely

or as a series)

8. When a compass which gives correct reading was placed on the table it pointed,

not in the North direction but in the East - West direction. State two reasons for

such an observation.

a) ‘A’ and ‘B’ are two plastic toy boats. A piece of iron has been fixed on ‘ A’

while a bar magnet has been fixed on ‘B’.

i. Which boat can be pushed or pulled by using a magnet?

ii. Which boat can only be pulled by using a magnet?

iii. Give explanations for above (i) and (ii) answers.

You have been supplied with a magnet and a piece of iron which were

similar in shape, size and colour.

How can you identify the magnet only by using the two things provided.

A B

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