HPE-book-7-final-for-press

8. Differentiate between

a. Luminous and non luminous body

b. Ray and beam of light

c. Regular and irregular reflection of light.

d. Incident ray and reflected ray

e. Periscope and kaleidoscope

9. Study the figure and answer the question.

a. Which phenomena is shown in the figure?

b. In the given figure, identify

i. incident ray X N Y
ii. reflected ray O Q

iii. normal

iv. points of incidence

v. angle of incidence P
vi. angle of reflection

Project work

How can you prepare a kaleidoscope? Prepare a kaleidoscope with the
help of your friends.

Science and Environment Book - 7 97

8Lesson Sound

Specific After the completion of the lesson, students will be able to:
Objectives
• define sound.
• explain different factors that affects the transmission of sound.
• explain the variation of speed of sound with the medium.

Preliminary Syllabus
Concept • Sound
• Wave and its type
We are surrounded by different types of sound. Some • Transverse wave
are sweet and some are harsh. Thunders are loud • Longitudinal wave
and sound produced by a baby is soft. Have you ever • Speed of sound in
thought what produces these types of sound? How
does the sound reach us? Why does the chirping sound different medium
of a bird sweet and the sound of bursting crackers
irritates us?

Each and every sound has its typical characteristics.
Verbal communication with spoken words is one of
the most important modes of communication. When
you hear the mixer running in the kitchen, you know
your mother must be preparing a delicious food. But
the alarm clock reminds you to get ready for the day.
In this chapter, we will explore how sound is produced,
what are the characteristics of a sound and how it
propagates in different media.

98 Science and Environment Book - 7

Introduction

In our surrounding, we hear different types of sound such as barking of dog,
horn of vehicles, shouting of people, chirping of birds, sound of television,
musical instruments, etc. Among all these sounds, some gives pleasure to our
ears and some don't. All sounds produce sensation of hearing to our ears.
Hence, sound is a form of energy that produces the sensation of hearing to
our ear. Sound is produced from all the vibrating bodies. Sound travels in all
direction and it requires a medium to propagate. Sound cannot travel through
vacuum. It can travel through solid, liquid and gas.

Sources of sound

An object which produces sound is called source of sound. Television, Radio,
musical instrument, horn of automobiles, animals, human, machine, etc are
some sources of sound.

A rapid back and forth motion of a particle or a body about its mean position
is called vibration. A body can be vibrated by hitting, strucking, plucking,
blowing, etc.

When the particles of a body vibrate, sound is produced. The sound produced
from the object propagates in our surrounding in the form of waves. Those
waves reach our ears and vibrate our eardrum. The vibration in the eardrum
transmit sound impulses to the brain through auditory nerve. As the result, we
hear the sound.

Activity 1

Take a scale (a metal scale is preferred
better). Hold one end of the scale
firmly on the table with one of your
hands as shown in the figure. Flick
the free end of the scale with the
other hand. What do you observe?
The scale starts vibrating to and
fro and a sound is heard. The sound is produced until the scale keeps
vibrating. So, we can conclude that the sound is produced due to the
vibration of the object.

Science and Environment Book - 7 99

Activity 2

Take a steel utensil and hang it vertically as shown in
the figure. Take an iron rod and strike it against the
utensil. You can hear a sound. You can also feel the
vibration of the utensil, if you touch it gently with
your finger, don't you? But as you touch it harder,
the vibration stops and so does its sound. So, it is
another demonstration to show that the sound is
produced due to the vibration.

Wave motion

Take a small piece of stone and throw it into the middle of a calm water pond.
You can see the waves in the water. The ripples slowly move from the point of
impact towards the edge of pond in a circular manner.

Sound cannot travel in a
vacuum. It needs some material
medium to travel, it may be
air as in most of the time or it
may be any other solid object or
even the liquid medium. Sound
travels in a form of a wave. A wave can be described as a disturbance that
travels through a medium from one location to another location. It can travel
in all possible direction through a medium at a constant speed. During a wave
motion, the particles of the medium do not actually move from one place to
another but only pass its energy to its adjacent particle.

There are two main types of wave which are discussed below:

Transverse wave: The wave in which the Crest
particles move perpendicular to direction Wavelength

of propagation of wave is called transverse

wave. The ripples produced in the pond,

vibration of a stretched wave in musical

instruments, etc are example of a transverse Trough

wave. Fig. Transverse wave

A transverse wave is produced in form of crest or trough. Crest is the

100 Science and Environment Book - 7

point of maximum positive displacement and the trough is the point
of maximum negative displacement in a transverse wave. The distance
between two consecutive crests or two consecutive troughs are known as
the wave length.

Longitudinal wave:

Longitudinal waves are

the waves where the compression Fig. Longitudinal wave rarefaction
vibration move parallel

to the direction of propagation of wave. Sound waves are the longitudinal

waves. The longitudinal waves travels from one place to another making

compressions and rarefactions.

Compression is the part of the longitudinal waves in which the particles of
medium are closer to one another and the rarefaction is the part in which the
particles are far from each other. The compressions and rarefactions are formed
alternately. The compressions and rarefactions together form a complete
longitudinal wave which carry sound from one place to another.

Difference between longitudinal and transverse wave

Transverse wave Longitudinal wave

1. Displacement of the medium is 1. The displacement of the medium

perpendicular to the direction of is parallel to the direction of

propagation of the wave. propagation of the wave.

2. It is produced only in solids and the 2. It is produced in all solid, liquid

surface of the liquid. and gas.

3. It consists of crests and troughs. 3. It consists of compressions and
rarefactions.

Propagation of sound wave

The process of transmission of sound from one place to another is known as
the propagation of sound. Sound travels in the form of longitudinal wave.
Unlike light, the sound wave cannot travel in the vacuum. It needs medium
to travel. That's why, one cannot hear a sound on moon. The speed of sound
differs according to the nature of the medium it travels through.

Science and Environment Book - 7 101

1. Propagation of sound through solids

Sound can travel through solid objects. The speed of sound is maximum when
it is traveling through a solid medium.

Activity 3

To show sound can travel through solid.
Materials required: two tin cans, 50 m long string, nail, hammer.
Procedure:
1. Punch a hole in the bottom of each

can using the hammer and nail.
Work the nail around each hole to
flatten the sharp edges as much as
possible.
2. Run one end of the string through
the hole at the bottom of the can and
tie a knot so that the thread does not
come out of the hole. Do the same
thing with the other can.
3. Take one can and give the other to a friend. Stretch the string taut, but
not so tight that the string will break.
4. Have your friend to speak into the can while you listen. Then talk
while your friend listens.
Observation: The sound was carried between the cans by the vibrating
string.
When you or your friend speak into the can, the sound waves from the air
vibrate the can, which caused the taut string to vibrate. These vibrations
travelled along the length of the string and caused the can at the other end
to vibrate. In turn, the vibrated air, which produced the sound waves was
heard by you and your friend.

2. Propagation of sound through the liquid:

Sound waves can travel through the liquid medium as well. The speed of
sound is less when it travels through the liquid than in solid.

102 Science and Environment Book - 7

Activity 4

To show sound can travel through liquid
Take a bucket full of water. Hold two pieces
of stones, one in each hand. Now, take
them into the water and strike them against
each other. Can you hear the sound? You
can also try this experiment with some
other objects like iron pieces steel or bell.
When these objects are struck against each
other even inside the water, we can hear its
sound. So, we can conclude that the sound
can travel through the liquid medium as
well. The speed of sound in water is about 1500 m/s.

3. Propagation of sound through air

We all are familiar to the fact that the sound can travel through the air because
the sound we generally listen passes through the air medium. When we talk to
people, listen to music, hear the sounds of animals and birds and vehicles, the
sound travels through the air and reach into our ear, then we get the sensation
of hearing. But very unlikely to our thinking, the speed of sound in the air is
less than that in solid or liquid.

Activity 5

To show that sound needs a medium to travel.
Materials Required: a bell jar,
manual wind up alarm clock,
vacuum pump
Procedures
1. Get a bell jar with a valve at its

base.
2. Place the bell jar on a smooth

and flat surface and connect
the valve to a vacuum pump.

Science and Environment Book - 7 103

3. Now, place a manual wind up alarm clock under the bell jar.

4. Seal the bottom of the bell jar with petroleum jelly so that there are no
gaps between the bell jar and the surface. Can you hear the ringing of
the clock?

5. Start the vacuum pump and note how the sound gets fainter at the air
goes out of the jar.

Again stop the pump and let air enter the jar. Can you hear the ring again?
This shows that sound needs a material medium for its propagation.

Factors affecting the speed of sound in air

Density

Density is the amount of mass per unit volume of a body. Speed of sound is
more in the air with less density. Due to this reason, speed of sound is more in
hydrogen (1270 m/s) than that in carbondioxide (260 m/s),

Temperature: The speed of the sound increases with the increase in temperature.
It is because the warmer particles have more kinetic energy and move faster.
Hence, the speed of sound is faster in summer than in winter.

Humidity: The amount of water vapour present in the air is called humidity.
The speed of sound increases with the increase in humidity of the atmosphere.
Thus, the speed of sound is greater in a humid air than in dry air.

Direction of wind: Speed of sound is obviously greater in the direction of the
wind than in the opposite direction.

Speed of sound in different medium

Water 1498 m/s

Wet air 400 m/s

Dry air 340 m/s

Copper 3700 m/s

Steel 5200 m/s

Reflection of sound

Like the light, the sound can also bounce back when it strikes on a surface, it is
known as the reflection of sound. The reflection of sound also follows the laws
of the reflection of light.

104 Science and Environment Book - 7

Reflection of sound waves from the surface can lead to one of the two
phenomena - an echo or a reverberation. A repeated sound that is caused
due to the reflection of the sound wave from a surface is called echo. An echo
can be heard only when the distance between the source of the sound and
the reflecting body is at least 17m. The reflecting body may be a wall, hill,
mountain, cleft, cave, etc.

Reverberation is defined as the prolongation of sound occured due to its
reflection. It can occur in big halls, auditorium, etc. The distance between
the source of sound and the reflector should be lesser than 17 m for the
reverberation of sound.

Points to Remember

1. Sound is a form of energy that gives us sensation of hearing.
2. Sound are produced due to the vibration of a body.
3. The object that produces sound are known as the sources of sound.
4. Sound travel in a form of wave in the presence of a medium. It cannot

travel in a vacuum.
5. The periodic disturbance travelling through a medium from one

location to another is called wave.
6. The wave in which the displacement of the medium is perpendicular

to the wave direction is called transverse wave.
7. The wave in which the displacement of the medium is parallel to the

wave direction is called longitudinal wave.
8. A sound wave is a longitudinal wave.
9. The transmission of sound from one place to another is called

propagation of sound.
10. A repeated sound caused by the reflection of the sound is called echo.
11. The prolongation of sound occured due to reflection is called

reverberation.

Science and Environment Book - 7 105

Exercise

1. Choose the best answer from the given alternatives.

a. What is the SI unit of speed of sound?

i. metre/second ii. hertz iii. kilohertz iv. joule

b. Sound cannot travel through_______.

i. solid ii. liquid iii. gas iv. vacuum

c. The speed of sound is maximum in_______.

i. solid ii. liquid iii. gas iv. same in all

d. The point of maximum positive displacement in transverse wave is

called_______.

i. crest ii. trough iii. compression iv. rarefaction

e. What is the speed of sound in air?

i. 3,00000km/sec ii. 320 m/s iii. 332 m/s iv. 1500 m/sec

2. Fill in the blanks with suitable words.

a. Sound needs medium to __________ .

b. A wave of a sound is a __________ wave.

c. A __________ wave consists of crest and trough.
d. The __________ of sound increases with the increase in temperature.
e. Sound travels with maximum speed in __________.
f. The return of the waves of sound after striking on a surface is a ____ .
g. Reverberation is caused due to the __________ reflection of sound.

3. Put a tick (√) for the correct statement and a cross (×) for the incorrect one.

a. Sound gives us sensation of hearing.
b. A body can also produce sound without its vibration.

c. A sound wave is a transverse wave.

d. The substance through which sound travels is called medium.

e. Sound wave cannot travel through liquid medium.

f. The speed of sound decreases with the decrease in humidity.

106 Science and Environment Book - 7

g. The speed of sound is greater in gas medium than in the solid medium.

h. For echo, the distance between the source of sound and reflecting
body should be more than 17m.

4. Match the following:

Transverse wave sound wave

Longitudinal wave multiple reflection

Echo ripples in pond

Reverberation single reflection

5. Give one word

a. The maximum positive displacement in transverse wave.
b. The maximum negative displacement in transverse wave.
c. The part of longitudinal wave where the particles of medium are

closer to each other.

d. The part of longitudinal wave where the particle of medium are far
from each other.

e. Distance between two consecutive crests.

6. Answer the following questions:

a. Define sound. How is it produced?
b. What are the sources of sound? Give example.
c. What is propagation of sound? Can sound travel in vacuum?
d. What are the factors that affects the speed of sound in air?

e. How does temperature of gas affect the speed of sound?

f. In which medium is the speed of sound maximum?

g. What happens when an electric bell is put into an evacuated jar and
made it to ring?

7. Give reasons of the following:

a. You cannot hear the sound in the moon.
b. Speed of sound is least in the gas.

Science and Environment Book - 7 107

8. Differentiate between
a. Longitudinal wave and transverse wave
c. Crest and trough
c. Echo and reverberation

Life Skills

• Visit a sound studio and find out which characteristics of sound doesn't allow it to

travel out of a studio.

Project work

Collect the pictures of ten sources of sound and paste them on a
chart paper. Also write their names and show it to your teacher.

108 Science and Environment Book - 7

9Lesson Magnet

Specific After the completion of the lesson, students will be able to:
Objectives
• differentiate between natural magnet and artificial magnet.
• explain about temporary and permanent magnet.
• explain the uses of magnets.
• explore different methods to make a magnet.

Preliminary Syllabus
Concept • Natural magnet
• Artificial magnet
The first magnet was discovered by a Greek shepherd named • Permanent magnet
Magnes. It is said that the nails in his shoes and the iron tip • Temporary magnet
of this staff got stuck to a large black rock on which he was • Methods to make
standing. Greeks named this strange type of rock 'magnetite'.
The chinese also knew about magnets. Ancient chinese sailors magnet
used magnets for navigation. • Single touch method
• Double touch method
Magnets are made of materials that attract objects made of • Electric method
certain substances like iron, cobalt and nickel.

Magnets come in various shapes and sizes. They can be found
as horseshoe, ring, cylindrical or bar shape.

Not all objects are attracted to magnets. Objects that are
attracted by a magnet are said to be magnetic substances, iron
and nickel. Objects that are not attracted by a magnet are said
to be non-magnetic substances. For eg. wood and plastic.

Science and Environment Book - 7 109

Introduction

A magnet is a substance that can attract certain objects like iron, nickel, etc
and produce magnetic field around itself. It was discovered about 2000 years
ago. The substances that are attracted by a magnet are known as the magnetic
substances and the property by virtue of which a magnet attracts the magnetic
substance is known as magnetism.

Properties of a magnet

1. Magnets attract magnetic substances toward itself.
2. A magnet has two poles i.e. north pole and south pole.
3. A magnet with a single pole does not exist. If a magnet is broken into two

pieces, each piece will behave like an independent magnet, with a north
pole and a south pole.
4. A freely suspended magnet always points towards the geographical north
and south.
5. Magnets have more magnetic force at their poles than in their middle part.
6. Like poles of a magnet repel and unlike poles of magnet attract each other.
7. A magnet has a magnetic field around it.
8. When a bar magnet is rubbed over an iron bar, it changes the iron bar into
a magnet.

Uses of magnets

Magnets are very useful material. Ancient people used natural magnet i.e.
lodestone to find the directions. But at present, magnets have got a variety of
uses.

1. Magnets are used in magnetic toys, stickers, refrigerator's door, etc.
2. Magnets are used for separating iron from ores containing non-magnetic

substances.
3. Electro-magnets are used in generators, motors, loud speakers, telephone,

TV, fans, mixers, electric bells, etc.
4. Electromagnets are used in cranes to lift heavy iron bars and to separate

iron objects from scrap.
5. Eye doctors use magnets to remove tiny iron pieces that have accidentally

110 Science and Environment Book - 7

fallen into the patient's eye.
6. Magnets are used in making magnetic compasses which help sailors and

navigators to know the directions.

Types of magnet:

There are basically two types of magnet. They are described here.

Natural Magnets

Materials which occur naturally and possess
magnetic properties are called natural magnets.
Magnetite (Lodestone) is an example of natural
magnet. It was discovered by shepherd of Asia
minor about 2000 years ago in the place named
Magnesia when suddenly both the nails (in his
shoes) and the metal tip of his stick become firmly stuck to the large black rock
on which he was standing. He named it as lodestone. It was actually the ore
of iron called magnetite. The natural magnets are found in irregular shapes as
they look like ordinary stone or rock and they also have less attractive property.

Artificial magnets

The magnets that are made by man are known as artificial magnets. Artificial
magnets are made by magnetizing different shapes of magnetic materials. A
rectangular iron bar, an iron needle, a blade or an iron nail can be turned into a
magnet of different shape: Bar magnets, cylindrical magnets, dumbbell shaped
magnets, horse shoe magnets are different types of artificial magnet. Also
artificial magnets are more powerful than natural magnet. Artificial magnets
are of two types - permanent and temporary.

Bar magnet U-shaped magnet Magnetic needle Magnetic compass

Science and Environment Book - 7 111

Permanent magnets

The magnets having the ability to remain
magnetized without the influence of nearby
external magnetic field are known as the permanent
magnets. Permanent magnets are made from hard
magnetic materials like steel, iron, cobalt, nickel, etc.
These type of magnets are used in radio speaker,
transistors, tape recorder, etc.

Temporary magnets

The magnets that cannot remain magnetized on
their own is known as temporary magnets. They
lose their magnetism shortly after being removed
from the magnetic field. Temporary magnets are
made from a soft magnetic materials. Electro-
magnet is an example of temporary magnet which
is made by passing electric current through an
insulated copper wire. It behaves as magnet only
until the current flows through the copper wire. These are used in electric fan,
generator, electric bell, etc.

Activity 1

Take a bar magnet, tie it with a thread and
suspend it freely. After some time, the magnet
comes to rest. Now observe the position of the
magnet.

The magnet points towards the geographical
north and south. Then, take another bar magnet
and make its one end closer to the one end of
the suspended magnet. And again bring the other end of the magnet closer
to the same end of the suspended magnet. Observe what happens? What
conclusion can you draw from the above experiment?

112 Science and Environment Book - 7

Differences between natural and artificial magnet

Natural Magnet Artificial Magnet

1. It occurs naturally in specific 1. It is made by man.
places.

2. It has weak force of attraction. 2. It has strong force of attraction.

3. It is of irregular shape 3. It has regular shape.

4. Lodestone is an example of 4. Bar magnet, horse shape magnet,

natural magnet. U magnet are some examples of

parmanent magnet.

Methods of making magnet

Artificial magnets can be made by using different magnetic materials like
iron, steel, cobalt, nickel, etc. Different methods are applied to make magnet of
desired shape and size. The process of converting a magnetic substances into
magnet is called magnetization. The methods of making magnets are rubbing
method and electric method.

Rubbing method: In this method, magnetic property is developed in a magnetic
substance by rubbing it against a magnet. It can be done in two different ways.

a. Single touch method: This is the simplest and
also the most convenient method to make a
small magnet that does not have a very big field
of attraction. All you need is a magnetic object
and a permanent magnet.

Place a paper on a flat surface and place the magnetic object on it. Then rub
the permanent magnet over the surface of the object. Make sure to rub in
one direction only. It is necessary to lift the magnet from the surface of the
object, after every swipe or run. Then swipe it again in the same motion as
before and continue rubbing the magnet over the object at least 50 times. If
the process of swiping the permanent magnet over the magnetic object has
been done correctly, the pins will get attracted towards your magnet and
will stick to it. The more times you rub the permanent magnet over your
magnetic object, the more powerful your temporary magnet will become.

b. Double touch method: This method is very much similar to the first
method. Here, two permanent magnets are taken to magnetize the magnetic

Science and Environment Book - 7 113

materials, operating in
opposite direction. We
have to rub the magnetic
materials at its centre with
the opposite poles of the
permanent magnet. Both
the magnets are then lifted together and again brought in the middle.
This method is repeated 30-40 times. By this method, stronger magnet are
made.

Electric method

In this process, an electromagnet is made out of
magnetic substance (preferably use large sized
iron nail), a copper wire and a battery. Initially
a copper coil must be tightly coiled around the
iron nail at least 10 times. The end of the copper
wire must be left free. Connect the lower end
of the copper wire to the negative point of the
battery and the upper end of the copper wire to
the positive point of the battery. This magnet also
work for sometime after it has been disconnected from the battery. Test the
magnet by using the light - weight pins. The electro magnet is nowadays used
as an industrial magnet. The best example of an industrial electromagnet is a
crane that is used in salvage yards.

Demagnetisation

A magnet can be made to lose its magnetism, i.e. it can be demagnetised by
hammering the magnet. Heating the magnet above a certain temperature also
demagnetises it.

Precautions while handling magnets
We have to take care while handing magnets. We should take the following
precautions.
1. Do not heat magnets: We should not heat magnets because on heating

they lose their magnetism.
2. Do not hammer or drop magnets: Magnets should not be beaten with a

hammer or dropped on the ground because they lose their magnetism.

114 Science and Environment Book - 7

3. Keep magnets away from computers, cassettes and magnetic units:
Cassettes and computers contain magnets as the data and their functions
depend on the principles of magnetism. Keep the magnets away from such
objects as they may affect the magnetic field of the magnets placed inside.

4. Magnets should be stored properly: While storing the magnets, we should
keep the opposite pairs of the magnets on the same side with a block of
wood between the bar magnets. Also, two soft iron keepers should be
placed on both sides of the poles. In this way, we can protect magnetism of
magnets.

Points to Remember

1. The substances which attract the magnetic substances like iron, cobalt,
nickel, etc and always rest in a north - south direction when suspended
freely is known as magnet.

2. The substances that are attracted towards the magnet are called
magnetic substances.

3. The property by which a magnet attracts the magnetic substance
towards itself is known as magnetism.

4. The magnets that are found naturally in the specific places are known
as natural magnets.

5. The magnets that are made by man are known as artificial magnets. Bar
magnet, horse shoe magnet, U-shaped magnet are some examples of
artificial magnets.

6. Those magnets which retain the magnetic property for a long time is
known as permanent magnets.

7. Those magnets which lose the magnetic property shortly after being
taken away from the magnet are known as temporary magnet.

8. In rubbing method, a magnetic substance is converted into a magnet by
rubbing it against a permanent magnet.

9. The rubbing method in which a single permanent magnet is rubbed
against a magnetic substance is called single touch method.

10. The rubbing method in which two permanent magnets are rubbed
against a magnetic substance in two directions simultaneously is called
double touch method.

11. The method of making magnet by passing electric current through an
insulated copper wire is known as electric method of making magnet.

Science and Environment Book - 7 115

Exercise

1. Choose the best answer from the given alternatives.

a. Which of the following is a magnetic substance?

i. iron nail ii. cobalt iii. nickel iv. all of them

b. Which of the following is natural magnet?

i. U- shaped magnet ii. bar magnet

iii. loadstone iv. all of them

c. A magnet is not used in __________.

i. magnetic compass ii. refrigerator door

iii. speaker iv. gas stove

d. Which of the following is a temporary magnet?

i. Magnetic needle ii. bar magnet

iii. electromagnet iv. compass

e. A freely suspended magnet always rest in __________ direction.

i. N-S ii. E-S iii. W-S iv. E-W

f. A magnet can be demagnetised by __________.

i. heating ii. beating iii. keeping in a box iv. Both (i) and (ii)

g. Single - touch method is used to produce __________.

i. aluminium ii. copper iii. gold iv. small magnet

2. Fill in the blanks with suitable words.

a. The property of a magnet by which a magnet attracts the magnetic
substances is called __________ .

b. Lodestone is a __________ magnet.

c. The magnets made by man is known as __________ magnet.

d. Artificial magnet have more __________than natural magnet.

e. A magnet made by passing electric current through a copper wire is
known as __________.

116 Science and Environment Book - 7

f. Single touch method is the __________ method of making magnet.

g. The process of converting magnetic substance into magnet is known
as __________.

3. Put a tick (√) for the correct statement and a cross (×) for the incorrect
one.

a. A magnet can attract the magnetic substance only.

b. A magnetic substances can be converted into magnet by rubbing
against permanent magnet.

c. The poles of a magnet can be separated by breaking it into halves.
d. A magnet can be used for navigation to determine the direction.
e. A bar magnet is a natural magnet.

f. A rubber is a magnetic materials.

g. Magnets when heated, do not lose their magnetic property.

4. Match the following:

Lodestone magnetic substance

Bar magnet natural magnet

Electro magnet artificial magnet

Iron temporary magnet

5. Give one word.

a. A substance that can attract iron, nickel, cobalt.

b. The property of the magnet by which it attracts the magnetic substance.

c. The process of converting magnetic substances into a magnet.

6. Answer the following questions:

a. Define magnet and magnetism.

b. Write three properties of a magnet.
c. Write any three uses of a magnet.
d. How can you show that a given iron bar is a magnet without the aid

of any other equipment?

e. Describe the single touch method of making magnet.

Science and Environment Book - 7 117

f. How can you make a magnet by electric method?
g. 'Electromagnets are extensively used in everyday life'. Give any two

applications of it.
h. What is meant by demagnetization? Write its causes.
i. What are magnetic keepers? How are they used?
7. Give reason of the following:
a. The magnetic poles of a magnet cannot be separated.
b. A magnetic compass is used to find out the direction.
c. An insulated wire should be used while making an electro-magnet.
9. Differentiate between
a. Natural and artificial magnet
b. Permanent and temporary magnet
c. Bar magnet and electromagnet

Think and Discuss

What happens when a magnet falls accidently in (a) water (b) fire?

Project work

Prepare an electromagnet with the help of your teacher.

118 Science and Environment Book - 7

10Lesson Electricity

Specific After the completion of the lesson, students will be able to:
Objectives
• introduce electricity and state its type.
• introduce static electricity with demonstration.
• introduce current electricity and electric circuits.
• write the symbols used in electric circuits.
• demonstrate series and parallel combination of cells and mention their

importance.

Preliminary Syllabus
Concept
• Static electricity
In present scenario, we cannot imagine life without • How does a body get
electricity. Electricity plays a very important role
starting from our common house hold appliances charged?
to large scale industries and from entertainment to • Effects of static electricity
transport and communication. For example, electricity • Lightening and thunderstorm
is used at homes for lighting, operating fans and • Current electricity
heating purposes. In industry, electricity is used to run • Electric circuit
different types of machines and in transport sector,
electricity is used to run electric trains and vehicles. • Closed and open circuit
Besides, electricity can be converted into heat energy, • Symbols used in circuit
light energy, mechanical energy, sound energy etc.
Even the communication of the brain with other organs diagrams
through the nerves takes place by electrical impulses. • Combination of cells
Therefore, electricity is an important form of energy.
• Series connection
• Parallel connection

Science and Environment Book - 7 119

Introduction

Electricity is the form of energy produced due to the continuous flow of electron
through a conductor. Electricity occurs naturally like or is produced by man
by using generator, cells, dynamo, hydropower and different other methods.
It is the form of energy which we use to operate machines and electric devices.
Electricity can be converted into different other forms of energies like heat
energy, mechanical energy, light energy. When the charges are not moving, it
is called static electricity and when the charges are moving, it is called electric
current or dynamic electricity.

Static electricity

When you rub a plastic comb against your hair and take the comb closer to the
small pieces of paper, the papers get attracted towards the comb. It is because
when you comb your hair, the comb becomes negatively charged due to the
transfer of electron from the hair to the comb. It is a form of a static electricity.

Static electricity is often created when two objects
that are not good conductor are rubbed together and
electrons from one of the objects rub off onto the other.
A stationary electric charge built up on an insulating
material is static electricity.

How an object is charged by static electricity?

Every objects in the universe are made up
of atoms. Each atom consists of protons and
electrons. The protons are the positively
charged particles whereas the electrons are the
negatively charged particles. The atom consists
of equal number of electrons and protons. So,
the atoms are electrically neutral. The protons
are strongly held by the nucleus while the
electrons are loosely bound in the atom. So, an
atom can easily lose its electron. When an atom loses its electron, it gains a
positive charge and if it gains an electron from other atom, it gains a negative
charge.

120 Science and Environment Book - 7

When two objects are rubbed against each other, friction is developed and the
heat energy is produced. This heat energy makes the transfer of the electron
from one body to another body. Thus, the static electricity is produced in the
body. Various insulators like plastic, rubber, wool, polyester, etc. develop static
electricity due to friction. Static electricity can be developed in our clothes
made of wool, polyester or acrylic due to the friction with our body. This is
the reason we hear tic-toc sound and even see the flashes when we open our
clothes in a dark room.

Effects of static electricity

There are different effects of static electricity. Lightning and thunderstorm are
some of the natural effects of a static electricity. Have you ever seen a lightning
or a thunderstorm? Do you know, how the lightning and thunderstorm occur?

Lightning is also a flow of electricity. This fact was discovered by Benjamin
Franklin in 1752.

The friction of cloud rubbing together causes the
bottom of the cloud to be negatively charged and the
top to be positively charged. Gradually, the charge
become very large and it induces an opposite (ie.
positive charge) on the ground. As the charge gets
larger and larger, the electron at the bottom of the
cloud repel each other more and more until they
finally discharge. It is called the lightning. A large amount of heat energy is
released during the lightning. This heat energy makes nearby air hot which
can be seen as a fast moving flash in the sky. A loud sound is heard shortly
after the lightning due to the sudden expansion and contraction of the air.
The light and sound actually happen at the same time but the light of the
lightning flash travels faster than the grumbling sound of the thunder because
the velocity of the light is faster than the velocity of the sound.

When the cloud becomes charged, static electricity is induced in it. Once the
electric discharge or the lightning electricity occur, the static electricity becomes
current electricity. Such produced electricity always takes the shortest possible
path and the path usually leads to the ground. Usually, the easiest route to the
ground is the tallest point. Lightning can kill human beings and animals. It can
set building on fire. During lightning, we should not stand under a tree.

Science and Environment Book - 7 121

Thunderstorm

A storm with thunder and lightning and typically rain or hail is called
thunderstorm. A thunderstorm is produced by the cumulonimbus cloud.
Thunderstrom is electrically very strong. Thunder is the sound that comes
from the rapid spread of air along the lightning strike. Thunderstorm can
cause great loss of life and property.

However, thunderstorm do not only causes damage, they can prove to be a
great help to mankind and all the living things. We get lots of water in the
summer. Plants get rain when they need it. Lightning in thunderstorms also
helps to keep the balance between earth and atmosphere. Lightining changes
nitrogen gas in the air into nitrogen compounds. These fall to the ground and
are added to the soil. Nitrogen is one of the main ingredients in the fertilizer.
10% of the nitrogen fertilizer needed for farming is made by lightning.

Lightning conductor

As we all know that lightning can cause the great loss
of life and property. The building can be damaged
if the lightning struck on it. So, to protect our house
from the lightning, a lightning conductor is used. A
lightning conductor is a metal rod or wire fixed to
an exposed part of a building or other tall structure
to divert lightning harmlessly into the ground. The
rod might be an inch (2.5cm) in diameter. It connects a huge piece of copper
or aluminium wire that's also an inch or so in diameter. The wire is connected
to a conductive grid, burried in the ground nearby. If lightning strikes, the
system attempts to carry the harmful electric current away from the structure
and safely into the ground. The system has ability to handle the enormous
electrical current associated with the strike. If the strike contact a material
that isn't a good conductor of electricity, the material will suffer massive heat
damage. The lightning rod system is an excellent conductor and thus allows
the current to flow to the ground without causing any heat damage.

Current electricity

Electricity produced due to the continuous flow of electrons through a
conductor is known as current electricity. Electric current, in simple term is
electrons in motion along a path regardless of the number of electron flowing.
The path may be a conductor such as, copper, aluminium and silver. These

122 Science and Environment Book - 7

conductors consist of large number of free electrons. The free electrons can
be forced to move from one region of the conductor to the other. When these
free electrons flow continuously in a particular direction, current electricity
is produced. When a cell is connected to a bulb with copper wires, the bulb
glows due to continuous flow of electrons through the wire. During this, the
direction of electric current is from the positive terminal of the battery to the
negative terminal of the battery.

The sources of current electricity are cell, battery, dynamo and generator.
Current electricity is one of the most useful things in the modern world. We
use various electronic appliances which utilize current electricity to perform
various types of work. Electrical energy can be converted into different other
types of energy. It is used to operate electrical devices like televisions, radios,
computers, mobiles, telephones, fans, heaters etc.

Electric circuit

The path along which the current flows is known as electric circuit. An electric
circuit consists of current source, switch, conducting wire and an electric
appliance (load). When these components are arranged properly, it can
conduct electric current. These electric current flow from positive terminal to
the negative terminal. There are two types of electric circuit; open circuit and
closed circuit.

Open circuit cells switch connecting wire

The circuit in which the electric current does not flow Open circuit
is known as open circuit. The switch of this electric
circuit is off, so the load does not work in an open
circuit. Thus, if the switch is opened or the wire is
cut, the flow of current is stopped and we say that
the circuit is open or incomplete.

Closed circuit cells switch

The circuit in which the electric current flow is called connecting wire
closed circuit. The switch in a closed circuit is on, so
the load works continuously in a closed circuit.

Closed circuit

Science and Environment Book - 7 123

Activity 1

To make a switch
Materials Required: A paper clip or
safety pins, two drawing pins, a thick
sheet of cardboard, three wires, a dry
cell and a small torch bulb
Procedures:
1. Insert a drawing pin into the ring at one end of the safety pin and fix it

on the cardboard sheet.
2. Fix the other drawing pin on the cardboard sheet. Make sure that the

free end of the safety pin can be turned to touch the other pin. Now
your switch is ready.
3. Now make a circuit by connecting a battery and a torch bulb with this
switch as shown in the figure.
4. Turn the safety pin so that its free
end touches the other drawing pin.
What do you observe?
5. Now, remove the safety pin away
from the other drawing pin. What
do you observe?
You will observe that
1. When the free end of the safety pin touches the other drawing pin, it
makes the circuit complete. Hence, the bulb glows. In this position, the
switch is said to be 'ON'.
2. When we remove the safety pin away from the other drawing pin, it
breaks the circuit. Hence the bulb does not glow. In this position, the
switch is said to be 'OFF'.

Symbols used in an electric circuit

An electric circuit consists of cell, switch, conductive wire and electric
appliances. It is not possible to draw the picture of every components while
showing an electric circuit in the figure. So, all the components of an electric
circuit are represented by their respective symbols. Some of the symbols used
in electric circuit are given below.

124 Science and Environment Book - 7

wire +- connecting wire wire crossed wire joined switch +A-
battery without connection ammeter
or
resistor +G earthing or fuse A.C. source +V-
galvanometer filament lamp voltmeter

Combination of cell

A single cell may not be enough to supply energy in an electric circuit every

time. So, we need to add two or more cells in a single circuit to increase the

voltage or current. These cells need to be combined in a proper arrangement

in order to complete a circuit. The process of connecting two or more cells in

an electric circuit is known as the combination of cells. Cells can be arranged

in two different ways in an electric circuit. They are series combination and

parallel combination. bulb
Series combination +- +- + -

The combination of the cells in which the switch
on

positive terminal of a cell is connected

with the negative terminal of the other Fig. Cells in series combination in an
electric circuit
cell and so on is called series combination.

This type of combination of the cells help to increase the voltage and current.
The voltage increase with the increase in the number of the cells. The total
voltage of this electric circuit is the sum of the voltage of all the individual cells
used in the circuit.

ie. V= V­ 1+V2+V3...

For example: if we use three cells of 1.5V each in a series combination, the total
voltage produced in the electric circuit will be

V= 1.5 +1.5+1.5

= 4.5 V

This combination of the cells help to increase the brightness of the bulb.

Parallel combination

The combination of the cells in which the positive terminal of the cells are
connected to one point and the negative terminal is connected to another point
is known as parallel combination of the cells.

Science and Environment Book - 7 125

Unlike series combination, the voltage +- bulb
does not increase with the increase in the +- switch
number of cells in a parallel combination on
rather the electricity is provided for longer +-
time. It doesnot increase the brightness
of the bulb either. The cells have same Fig. Cells in parallel combination in an
voltage in parallel combination circuit. electric circuit
The total voltage in a parallel circuit is
equal to that of a single cell.

ie. V = V1 = V2 = V3...

For example: if three cells of 1.5 V each are connected in a parallel combination,
the total voltage in the circuit will be 1.5 V.

Points to Remember

1. Electricity is the form of energy produced due to continuous flow of
electrons through a conductor.

2. A stationary electric charge built up on an insulating materials is
called static electricity.

3. Lightning is an example of static electricity.

4. A lightning rod is used to protect the house from the lightning and
thunderstorm.

5. The path along which the electric current flows is known as electric
circuit.

6. The circuit in which the electric current doesnot flow continuously is
known as open circuit.

7. The circuit in which the electric current flows continuously is known
as closed circuit.

8. The process of connecting two or more cells in circuit is known as
combination of cells.

9. The combination of the cell in which the positive terminal of a cell is
connected to the negative terminal of another cell and so on is called
series combination.

10. The combination of the cell in which the positive terminals of the cells
are connected at one point and the negative terminals are connected
to another point is known as parallel combination of the cell.

126 Science and Environment Book - 7

Exercise

1. Choose the best answer from the given alternatives.

a. Which of the following is a source of electricity?

i. cell ii. dynamo iii. generator iv. all of the above.

b. Which of the following device converts electrical energy into
mechanical energy?

i. bulb ii. heater iii. fan iv. T.V.

c. Which of the following is not used to make electric circuit?

i. cell ii. connecting wire iii. load iv. magnet

d. Which of the following component is used to open and close a circuit ?

i. bulb ii. switch iii. wire iv. ammeter.

e. What is the symbol of an electric cell?

i. ii. iii. iv.

+- +-

f. If 4 cells of 1.5 V each are connected in a parallel series, what will be
its voltage?

i. 1.5 V ii. 6 V iii. 3 V iv. 4.5 V.

2. Fill in the blanks with suitable words.

a. _______ electricity is produced due to friction.
c. Neutral atom has same number of _______ and _______ .
b. Lightning rod is used to protect the building from _______.
c. Electricity doesnot flow through _______ circuit.
d. A _______ completes or breaks an electric circuit.
e. The brightness of the bulb can be increased with the increase in the

number of the cells in _______ combination.
f. For a long lasting cells, we have to connect them in a_______ series.

3. Put a tick (√) for the correct statement and a cross (×) for incorrect one.

a. An atom is electrically neutral.
b. An atom loses its electron and gains a positive charge.
c. Current electricity is produced due to continuous flow of electrons in

a circuit.

Science and Environment Book - 7 127

d. A lightning rod is made up of plastic wire.

e. Lightning is the effect of static electricity.

f. Electric current does not flow in closed circuit.

g. The brightness of the bulb increases with the increase of cells in
parallel series.

4. Match the following.

Static electricity Conduct electricity.

Current electricity Due to friction.

Closed circuit Increase brightness.

Open circuit Due to flow of charge.

Series combination Does not conduct electricity.

5. Draw the symbol of the following.

a. cell b. battery c. switch d. ammeter e. bulb

6. Answer the following question.

a. What is static electricity? How is it produced?

b. How does lightning occur?

c. How does a lightning rod protects the house from the lightning?

d. Write any two effects of thunderstorm.

e. Define current electricity. What are the source of current electricity?

f. What is an electric circuit? Draw a circuit diagram for a circuit having
one cell, one bulb and a switch.

g. What is closed circuit? Show it in diagram.

h. What is open circuit? Show it in diagram.

i. What combination of cells are beneficial to get maximum voltage?

j. What combination of cells are beneficial to get current for longer time?

7. Observe the following pictures and answer the following:

–+ -1.5+v -1.5+v -1.5+v
–1.5v +
11.5.5v–v +

Fig. 1 Fig. 2

128 Science and Environment Book - 7

a. Name the each type of combination of cells.
b. What type of above combination has more current?
c. Calculate the voltage in each circuit.
8. Give reason of the following:
a. Sound of the thunder is heard only shortly after the lightning occur.
b. Lightning are useful for human beings.
c. Symbols are used to represent components of electric circuit.
9 Differentiate between.
a. Static and current electricity
b. Open and closed circuit
c. Parallel and series combination of cells
10. Draw the figures of series combination and parallel combination of
cells.

Project work

• Prepare an electric circuit in a parallel combination with the help of your
teacher.

• Prepare an electric circuit of a series combination with four cells.
• Demonstrate the voltage increases with increase in number of cells

in series combination and remains constant in parallel combination of cells.

Science and Environment Book - 7 129

Chemistry11Chapter Matter

Specific After the completion of the lesson, students will be able to:
Objectives
• describe matter and its general properties.
• explain elements and name the first twenty elements with their symbols.
• define compounds and write the molecular formulae of some common

compounds.
• define atom and molecules and describe the method of writing molecular

formula.
• define physical and chemical change and differentiate between them.

Preliminary Syllabus
Concept
• States of matter
Matter refers to anything around us which has mass and • Elements and their symbol
occupies space. Its presence can be felt by any one or more • Atom
of our five senses. Mater comes in enormous variety with • Molecules
different shape, size and texture. The water of the river, • Molecular formula
the air we breathe, the blowing gases of water or even a • Change in matter
particle of sand, each thing is matter. - Physical change
- Chemical change.
This means that, all those things which 'occupy space' and
'have mass' can be classified as matter whereas other things
which have feelings and sensation such as love, manners,
taste, attitude, affection, thoughts, ideas, etc cannot be
considered or classified into matter.

In modern day "Scientists have evolved two types of
classification of matter which is based on their physical
nature and chemical nature.''

130 Science and Environment Book - 7

Introduction

Everything that has mass and occupies space are known as matter. All the
objects in the universe are made up of matter. Air, water, soil, book, pencil,
chair and board, all are matters. But heat, light, electricity, sound, shadow are
not considered as matters as they do not have mass and do not occupy the
space either.
Matter consists of the tiny particles called molecules which are so small that
they can't be seen through our naked eyes. On the basis of the arrangement
of the molecules in a matter, it is classified into three different states. They are
solid, liquid and gas.
Solid
The matter having fixed shape and volume is known as solid. It is because
there is a strong force of attraction between the molecules of the solid. Among
the three states of matter, they have the greatest intermolecular force and the
least intermolecular spaces. Solids are generally hard substances like wood,
stone, brick, metals, but they may be soft as well like paper, cloth, powder.

Properties of solid

• They have fixed shape and volume.
• They are generally hard.
• They do not flow.
• They are incompressible as their molecules are tightly arranged.

Liquid
The matter having fixed volume but no fixed shape is known as liquid. It takes
the shape of the container where it is placed. Unlike solids, the molecules
in liquids are not tightly packed as they have less intermolecular force of

Science and Environment Book - 7 131

attraction. This results in the free movement of the molecules in the liquid
and thus they can flow easily. However, they do have fixed volume. Water, oil,
petrol, kerosene, milk, ink, alcohol are some examples of liquid.

Properties of liquid
1. They don't have fixed shape.
2. They have fixed volume.
3. They flow from higher level to lower level.
4. They cannot be compressed.
5. They always maintain their level.

Gas
The matter having neither fixed shape nor the fixed volume is known as gas.
The molecules in the gas are loosely packed because of the negligible force of
attraction between them. Gases can move freely in any direction and fill the
entire space available to them. Most of the gases are colourless. Thus, they
are invisible but we can feel them. Oxygen, carbon dioxide, water vapour and
hydrogen are some examples of gases.
Properties of gas

1. They do not have definite shape and volume.
2. The gas molecules are free to move anywhere.
3. They are highly compressible.
Types of matter
Matters can be broadly classified into two categories ie. pure substance and
impure substance. The substances that are composed of only one type of
molecule is known as pure substances. The substances that are composed of
two or more than two types of molecules are known as impure substances.
They are salt solution, sugar solution, mixture of husk and rice, etc.

132 Science and Environment Book - 7

Elements
An element is defined as a pure substance that contains only one kind of
atoms. For example, iron is an element made of only one kind of atoms of
iron.
The atom of an element may, or may not join together to form molecules.
Element can exist in the form of atom like Argon (Ar) or in a form of molecule
like Nitrogen (N­2­). There are 118 elements known so far. Out of which 92
elements occur naturally on earth while remaining 26 are prepared artificially.
The elements are found in all three states. Sodium, iron, calcium, gold, silver
etc. are found in solid state. Mercury and bromine are found in liquid state
while oxygen, hydrogen, nitrogen, helium, etc are found to be in gaseous
state.
The elements have their own atomic number. They are arranged according
to their atomic number in a systematic table known as a periodic table. Each
element is represented by their respective symbol. They are generally denoted
by the first letter or first two letter of their name where first letter is capital
and second is small. For example; Hydrogen is denoted by H, Helium by He,
Lithium by Li, Oxygen by O. Some of the elements are also symbolized by
the first letter or first two letters of their Latin name. Gold has been given the
symbol Au from its Latin name 'Aurum'.
Therefore, the short and abbreviated form of the name of an element is known
as symbol. It was introduced by a Swedish chemist, Berzellius in 1914.
Lets study some details about the first 20 elements of the periodic table.

Science and Environment Book - 7 133

Atomic number Elements Symbols

1 Hydrogen H
2 Helium He
3 Lithium Li
4 Beryllium Be
5 Boron B
6 Carbon C
7 Nitrogen N
8 Oxygen O
9 Fluorine F
10 Neon Ne
11 Sodium (Natrium) Na
12 Magnesium Mg
13 Aluminium Al
14 Silicon Si
15 Phosphorus P
16 Sulphur S
17 Chlorine Cl
18 Argon Ar
19 Potassium (Kalium) K
20 Calcium Ca

Compound

A substance resulted by the combination of two or more than two different
elements in a fixed proportion by mass is known as compound. In a compound,
the elements are combined in such a way that the atoms of different elements
are held together by chemical bonds that are difficult to be broken. Water and
carbon dioxide are common examples of compound. Water is a compound
formed by the combination of two atoms of hydrogen and one atom of oxygen
which is written as H2O. Similarly, carbon dioxide is a compound composed of
one atom of carbon and two atoms of oxygen. It is written as CO2.

A compound differs from a mixture in which the bonding among the atoms
of constituent substance doesnot occur. Often a compound looks and behaves
nothing like any of the elements that comprise it. Consider, hydrogen (H) and
oxygen (O). For example, both of these elements are gases at room temperature
but they combine to form a compound of water which is obviously a liquid
at room temperature. Compounds are generally a homogeneous and show
definite physical and chemical properties.

134 Science and Environment Book - 7

Some compounds and their composition.

Water(H2O) = 2 Hydrogen + 1 Oxygen.
Salt (NaCl) = Sodium + Chlorine.

Carbondioxide (CO2) = Carbon + 2 Oxygen.
Baking soda (NaHCO3) = Sodium + Hydrogen + Carbon + 3 Oxygen.
Difference between elements and compound.

Element Compound

1. An element is a pure chemical 1. A compound is a chemical substance
substance made of same type of composed of two or more different

atoms. chemical substances.

2. It cannot be broken down into 2. It can be separated into simpler

simple form. forms by chemical methods.

3. There are 118 elements known. 3. The list of compounds is endless.

4. Hydrogen (H), Iron (Fe), Copper 5. Example: water (H2O).
(Cu), sodium (Na), Helium (He) sodium chloride (NaCl)

are some examples of element. sulphuric acid (H2SO4) are some
examples of compound

Atom

We see different types of matter around us. What actually causes difference
in their property? The difference is because of their composition. Each matter
is composed of different building blocks called atoms. Thus, atoms are
responsible for the distinctiveness of matter from each other.

Atom is the smallest particle of a matter, which cannot be further broken down
into simpler particle but are responsible for all the chemical changes in matter.
Atoms may or may not exist independently.

Basically, atoms of most of the elements are not able to exist independently.
They form molecules. These molecules aggregate in large numbers to form the
matter that we can see, feel or touch. A molecule can be defined as the smallest
particle of an element or compound that is capable of independent existence
showing all the properties of that substance. Atoms of same or different
elements can combine to form molecules. If the atoms are of the same type
then we get the molecule of an element. For example two atoms of Oxygen
form an Oxygen molecule (O2). If the atoms of different elements combine in
a definite proportion, we get the molecule of a compound. For example, one

Science and Environment Book - 7 135

atom of hydrogen and one atom of chlorine combine to form a compound
called hydrogen chloride (HCl).

Structure of an atom

An atom itself is tiniest of the things, but even atoms are further made up of
smaller particles known as subatomic particles. They are protons, electrons
and neutrons. They are also called elementary particles. At the centre of the
atom, there is a hugely concentrated mass known as nucleus. There are a
number of tiny particles orbiting around the nucleus called electrons. The
nucleus consists of protons and neutrons. Around the nucleus, there are many
shells or orbits. Electrons revolve in these shells. The various constituent, sub
atomic particles and their arrangement within an atom is called structure of
the atom.

Protons

Protons are positively charged particles located at

the centre of the nucleus. They have mass almost

equal to hydrogen atom which contribute to the
mass of an atom. They are denoted as p+.

Neutrons

They are located at the nucleus of the atom. They are electrically neutral
particles having mass almost equal to proton. They also contribute to the mass
of the atom. A neutron is denoted by n°.

Electron

The electrons are negatively charged particles and have negligible mass. They
revolve around the nucleus of an atom like the planets revolve around the sun
in the solar system. In an atom; there are same number of protons in the nucleus
as there are electrons orbiting around it. This means the number of positively
charged particle is equal to the number of negatively charged particle. Thus,
an atom has no overall electric charge and are electrically neutral. An electron
is denoted by e–.

Thus, we can summarize the above information as

Particle Mass Electric change Symbol
Proton 1 amu positive p+

136 Science and Environment Book - 7

Electron 1/1837 amu Negative e-
Neutron 1 amu Neutral n°

Atomic number (Z)

The atomic number of an element is the number of protons in the nucleus of
an atom of the element. In a neutral atom, the number of protons is equal to
number of electrons. So atomic number can also be defined as the number of
electrons or protons present in it.

Atomic number (Z) = No. of protons = No. of electrons

ie. Z = p+ = e-.

Every element, has its own atomic number. The modern periodic table is
ordered by increasing atomic numbers.

Atomic mass (A)

The sum of the number of protons and neutrons present in the nucleus of
an atom is called its atomic mass. It is also called atomic weight or mass
number which is generally denoted by A. The mass of the electron in an atom
is extremely small. So, it is neglected during the calculation of atomic mass.
Therefore, Atomic mass = number of protons + number of neutrons.

ie. A = p+ + n°

For any element, atomic mass is shown as superscript and atomic number as
subscript.

An element is generally denoted by ZXA where,
z = atomic number
A = atomic mass
X = symbol of an element.

An atom has 19 electrons, 19 protons and 20 neutrons. What is its atomic
number and atomic mass?

Solution:
Number of electrons (e-) = 19
Number of protons (p+) = 19
Number of neutrons (n) = 20
Atomic number (z) = number of electrons = number of protons
So, atomic number = 19.

Science and Environment Book - 7 137

Then,
Atomic mass = number of protons + number of neutrons
= 19 + 20
= 39

Therefore, atomic number and atomic mass of the element is 19 and 39
respectively.

Arrangement of electrons in an atom

We know that the electrons revolve around the

nucleus of an atom. In nature, there are 92 main

elements, with the atomic number 1 to 92. We can

also say these elements have 1-92 protons in the P
nucleus and 1-92 electrons orbiting around the N M L K n

nucleus. n=1

All the electrons do not revolve along the same n=2
path. A definite number of electrons revolve in n=3

n=4

same path or orbit. The shells number 1,2,3,4...are designated as K,L,M,N...

respectively. The orbit near the nucleus is called lower energy level and the

orbit distant from the nucleus is called higher energy level. The distribution of

electrons in different shells is called electronic configuration.

The distribution of electrons in a shell is done according to the following rules.

1. Maximum number of electrons that can be accomodated in a shell is given
by 2n2 rule, where n = shell number. For example:

For K shell, n = 1, maximum number of electrons in
K-shell = 2n2 = 22 = 2× 1 = 2 electrons.

For L shell, n=2, maximum number of electrons in
L-shell = 2n2= 2(2)2= 2×4 = 8 electrons.

For M-shell, n=3, maximum number of electrons in
M-shell =2n2= 2(3)2= 2×9= 18 electrons.

For N-shell, n=4, maximum number of electrons in
N-shell = 2n2= 2(4)2= 2×16= 32 electrons.

2. The outermost shell cannot accomodate more than 8 electrons.

3. The orbit just inner to outermost orbit cannot have more than 18 electrons.

138 Science and Environment Book - 7

Electronic configuration of first 20 elements

SN. Elements Symbol Atomic Proton Electron Neutrons Atomic

number (n =A-P) mass Electronic Configuration

(Z) (A)

K LM N

1. Hydrogen H 1 1 1 0 1 1

2. Helium He 2 2 2 2 4 2

3. Lithium Li 3 3 3 4 7 21

4. Berylium Be 4 4 4 5 9 22

5. Boron B 5 5 5 6 11 23

6. Carbon C 6 6 6 6 12 24

7. Nitrogen N 7 7 7 7 14 25

8. Oxygen O 8 8 8 8 16 26

9. Fluorine F 9 9 9 10 19 27

10. Neon Ne 10 10 10 10 20 28

11. Sodium Na 11 11 11 12 23 2 81

12. Magnesium Mg 12 12 12 12 24 2 82

13. Aluminium Al 13 13 13 14 27 2 83

14. Silicon Si 14 14 14 14 28 2 84

15. Phosphorus P 15 15 15 16 31 2 85

16. Sulphur S 16 16 16 16 32 2 86

17. Chlorine Cl 17 17 17 18 35 2 87

18. Argon Ar 18 18 18 22 40 2 88

19. Potassium K 19 19 19 20 39 2 88 1

20. Calcium Ca 20 20 20 20 40 2 88 2

The number of electrons and protons are same as the atomic number. But the
number of neutron does not follow any rule.

The following elements have equal number of protons and neutrons, ie. p = n.
He, C, N, O, Ne, Mg, Si, S, Ca
Number of neutrons in hydrogen is zero while argon has 22 neutrons.

Rest of the elements upto atomic number 20 have number of neutrons 1 more
than the number of protons ie. n = p + 1

Science and Environment Book - 7 139

Electronic configuration of some of the elements

1. Hydrogen

Symbol =H

Atomic no. =1

No. of electron (e-) = 1 p=1
n=0
No. of proton (p+) = 1
K LM N
No. of neutron (n) = 0 Shell 1 –– –
Electron
2. Helium p=2
Shell n=2
Symbol = He Electron
K LM N
Atomic no =2 2 1– –

No. of electrons (e-) = 2 p=3
n=3
No. of protons (p+) = 2
K LM N
No. of neutrons (n) = 2 2 1– –

3. Lithium p=7
n=7
Symbol = Li
K LM N
Atomic no. =3 2 5– –

No. of electrons(e-) = 3

No. of protons (p+) = 3

No. of neutrons (n) = 4

4. Nitrogen Shell
Electron
Symbol =N

Atomic no. =7

No. of electrons(e-) = 7

No. of protons (p+) = 7

No. of neutrons (n) = 7

Shell
Electron

140 Science and Environment Book - 7

Valency

The combining capacity of an element is called valency. We studied that the
number of electrons that atom can hold in each shell are 2, 8, 18, 32 respectively.
The outermost shell may have 1 to 8 electrons. This outermost shell is known
as valence shell. The electrons present in valence shell takes part in chemical
reactions. These electrons determine the combining capacity of an elements
and thus retains its valency.

For example, the electronic configuration of Na is

Shell K L MN

electrons 2 8 1

It has one electron in its valence shell. So, its valency is 1.
Similarly, the electronic configuration of chlorine is

Shell K L MN

electrons 2 8 7

It has 7 electrons in its valence shell. There is one les electron to complete its
M-shell with 8 electrons. Its combining capacity is also 1. So, it attributes its
valency as 1.

In the given examples, both elements have valency one. But there is a difference
that sodium can donate one electron of valence shell where as chlorine can
accept only one electron in its valence shell. Elements which can donate
electron are electropositive and known as cation. Whereas the elements which
can accept electron are electronegative and known as anions.

The valency of element having atomic number 1 to 20 is given in the following
table.

Science and Environment Book - 7 141

S.N. Element Symbol Valency
1. Hydrogen H 1
2. Helium He 0
3. Lithium Li 1
4. Beryllium Be 2
5. Boron B 3
6. Carbon C 4
7. Nitrogen N 3
8. Oxygen O 2
9. Fluorine F 1
10. Neon Ne 0
11. Sodium Na 1
12. Magnesium Mg 2
13. Aluminium Al 3
14. Silicon Si 4
15. Phosphorus P 3
16. Sulphur S 2
17. Chlorine Cl 1
18. Argon Ar 0
19. Potassium K 1
20 Calcium Ca 2

Representation of some radicals. Valency
1
Name of radical Representation 1
Ammonium HHHNNNOCSSHOOHOOO432-34+--3--- 1
Nitrate CPSSOOOO43432232---- 1
Nitrite 1
Bisulphate 1
Bisulphite 1
Bicarbonate 2
Hydroxide 2
Sulphate 2
Sulphite 3
Carbonate
Phosphate

142 Science and Environment Book - 7

Molecular formula

In a compound, the molecule containing different atoms combine in a definite
ratio. The molecule of the compound is represented by placing symbols of
various constituents elements side by side indicating their valencies in the
lower side.

While writing the formula of a chemical compound, it is important to remember
that a compound must be electrically neutral. In other words, the positive and
negative valency of the ions i.e. of cations and anions must add upto zero.

We know the molecular formula of water contains two atoms of hydrogen and
one atom of oxygen. The molecular formula of water should, therefore contain
symbols of hydrogen and oxygen. It should also show the number of atom of
each element in a molecule. We write the formula of water as H2O. Lets learn
the steps to write a molecular formula taking an example of calcium chloride.

Step 1: Write the symbol of calcium ion and chloride ion. The symbol of positive
ion must be written in the left and the negative ion in right side.

Ca Cl

Step 2: Write the valency of each ion below the symbol as shown below.
Ca Cl
21

Step 3: Mark an arrow from the valency of the negative ion towards the positive
ion and from the valency of the positive ion to negative ion.

Ca Cl
21
Step 4: Shift the valency in the direction of the arrow to cross them between
the positive and negative ions. The crossed valencies are placed to the lower
right of each symbols. The radical containing more than one atoms must be
enclosed in a bracket and the valency is placed outside the bracket to the lower
right.

Ca1 Cl2
ie. CaCl2
Therefore, the molecular formula of calcium chloride is CaCl2.

Science and Environment Book - 7 143

In the case of two numbers having common factor, the number must be
reduced to get a simple ratio. For example the chemical formula for sulphur
dioxide is written as SO2 and not as S2O4. The table given below shows few
steps for writing some more formula.

NaCl Na+1 Cl-1 Na Cl Na1Cl1 NaCl
KCl 11
CO2 K1+
H2O 1 Cl1- K Cl K1Cl1 KCl
MgCl2 C4+ 1 11

H1+ O2- CO C2O4 CO2

Mg2+ 42

O2- HO H2O1 H2O

12

Cl1- Mg Cl Mg1Cl2 MgCl2
21

Importance of molecular formula.

1. It gives information about the elements that forms the compound.
2. It shows the proportion of different atoms of the elements combined with

each other.
3. It also shows the number of molecules of the compound.

Change in state of matter

The state of matter can be changed by different natural or chemical processes.
For example, an ice is in solid state, it changes into water when heated. The
water changes into water vapour on further heating. Some of the changes are
temporary while some changes are permanent. The reversible changes are
called temporary change whereas the irreversible changes are permanent
change. The melting of ice is temporary as water can be converted back to ice
by freezing but burning of wood is permanent change as the burnt wood or
the ashes can't be converted back to wood. Change in matter can occur in two
ways, physical change and chemical change.

Physical change

A physical change is a temporary change during
which no new substances are formed. Physical
change are usually temporary, reversible and the
composition of the substance remains unchange.

144 Science and Environment Book - 7

For example melting of ice, vaporisation of water.

Chemical change

The permanent irreversible change in which physical and chemical property
of matter are changed is called chemical change. A
chemical change results in the formation of new
substance. Thus, it is a permanent change and
irreversible. For example: the burning of wood,
formation of curd from milk, rusting of iron,
electrolysis of water.

Difference between physical change and chemical
change.

Physical change Chemical change

1. Only physical properties are 1. Both physical and chemical

changed in the physical change. properties are changed in a

chemical change.

2. It is a temporary change. 2. It is a permanent change.

3. It is a reversible change. 3. It is an irreversible change.

4. New substance are not formed 4. New substances are formed during

during a physical change. a chemical change.

Activity 1

To show that conversion of ice into water and water into ice is a physical
change.
Material required: ice cube, beaker, refrigerator
Procedure:

• Take a lump of ice and keep it in a beaker, leave the beaker in open area

for a while. What do you observe?

• Now, keep the beaker and its contents inside refrigerator for 3-4 hours.

What do you observe?
Make a report on your observation based on types of changes.

Science and Environment Book - 7 145

Points to Remember

1. Anything that has mass and occupies space are known as matter.
2. Matters exist in three states ie. solid, liquid and gas.
3. A pure substance is made up of only one type of molecule.
4. An impure substance is made up of two or more than two types of

molecules.
5. Elements are the pure substances that cannot be broken down into

simpler form by any chemical means.
6. Symbol is the short and abbreviated form of the name of element.
7. Compounds are the substances formed by the combination of two or

more different elements in fixed proportion.
8. An atom is the smallest particle of an element which can take part in

chemical reaction.
9. A molecule is the smallest particle of element which is capable of

independent existence.
10. An atom consists of positively charged protons, negatively charged

electrons and electrically neutral neutrons.
11. Atomic number of an element is the number of protons or the number

of electrons present in its atom.
12. Atomic mass is the sum of protons and neutrons present in the nucleus

of an atom.
13. Electronic configuration is the arrangement or distribution of electrons

in different shells of an atom.
14. The combining capacity of an element is called valency.
15. Molecular formula is the symbolic representation of a molecule.
16. The change in physical properties of a substance while preserving its

chemical properties is called physical change.
17. The change in both physical and chemical properties of a substance is

called chemical change.

146 Science and Environment Book - 7