Science 8

7. The image formed by a concave mirror is real, inverted and of the
same size as object. What is the position of object ? Draw the ray
diagram.

G. Conceptual questions:
1. Ayush, an eighth grader has no knowledge of swimming but goes
for swimming in a pool. He sees the pool to be shallow. Is it safe for
him to jump into the pool? Explain with reasons.
$ ÀVKHUPDQ VHHV D ELJ ÀVK LQ D SRQG DQG WKURZV D VSHDU VWUDLJKW DW WKH
ÀVK &DQ WKH VSHDU NLOO WKH ÀVK" ([SODLQ ZLWK UHDVRQ

H. Study the diagram and answer the questions that follow:

1. What kind of mirror is used? Y
F
:KDW DUH &2 2) $% ) DQG ;< B

called? Object

3. Write down the relation between P A C O
OC and OF.

4. Complete the given diagram. ;
5. Write down the position and

nature of the image formed.

I. Write down the characteristics of the image formed by a plane
mirror with the help of a diagram.

J. Complete the following diagram.

AM
o air
glass

N

Take a plane mirror and observe different kind of images formed by it. Then
enlist the characteristics of a plane mirror based on your observations.

97 Times' Crucial Science & Environment Book - 8

8CHAPTER Sound

Guglielmo Marconi

Guglielmo Marconi was an Italian inventor and electrical engineer
NQRZQ IRU KLV SLRQHHULQJ ZRUN RQ ORQJ GLVWDQFH UDGLR WUDQVPLVVLRQ DQG

IRU KLV GHYHORSPHQW RI 0DUFRQL·V ODZ DQG D UDGLR WHOHJUDSK V\VWHP

Estimated Periods: 8
Objectives: At the end of the chapter, the students will be able to:

'HILQH VRXQG DQG VRXUFHV RI VRXQG
,QWURGXFH VSHHG IUHTXHQF\ DQG ZDYHOHQJWK RI VRXQG ZDYH
6KRZ WKH UHODWLRQ DPRQJ VSHHG IUHTXHQF\ DQG ZDYHOHQJWK
'HILQH HFKR DQG UHYHUEHUDWLRQ DQG WHOO WKHLU HIIHFWV
'LIIHUHQWLDWH EHWZHHQ HFKR DQG UHYHUEHUDWLRQ

What is sound?
Can sound transmit in vacuum?
Where does sound travel with higher speed: in water or in air?
What is echo? How does it affect the sound? Discuss.

Introduction

When a string of a guitar is plucked, we observe that the string starts vibrating
and sound is produced. When an object vibrates, it causes movement (vibration)
of the nearby molecules (of media such as air, water or solid objects). These
molecules collide with other molecules close to them, causing them to vibrate
as well. Those other molecules, in turn, hit their neighbouring molecules and so
on. Such a continual collision of molecules transfers energy in the form of wave,
called sound wave. The sound wave keeps going ahead until the molecules run
out of energy. As the sound wave travels through a medium (e.g. air), there is
a series of molecular collisions; however, the air molecules themselves do not
travel with the wave. Due to collision, each molecule just moves away from its
resting position and eventually returns to it after transferring energy to other
molecules.

Thus, sound is a form of energy which is produced due to vibration of an
object. Any object which vibrates is a source of sound. For example, when you
touch a speaker in the front part when it is producing sound, you can feel the

Times' Crucial Science & Environment Book - 8 98

YLEUDWLRQV :KHQ \RX SXW \RXU ÀQJHUV DJDLQVW \RXU WKURDW ZKLOH VSHDNLQJ \RX
can feel vibrations. When vibration of the object stops, the production of the
sound also stops.

Guitar Madal Radio Drum

Wave

Wave is a disturbance in a medium which carries energy from one point to
another. It is produced due to vibration of molecules of a medium.

For example, when sound is produced, it causes the vibration of air molecules
and production of sound wave. Sound waves, light waves, radio waves, etc are
examples of waves. Wave is of two types:

(a) Transverse wave (b) Longitudinal wave

Transverse wave

Transverse wave is the wave in which molecules of the medium move up and
down perpendicular to the direction of energy.

Crest

AB

dƌĂŶƐǀĞƌƐĞ ǁĂǀĞ Trough

,Q WKH ÀJXUH WKH HQHUJ\ SDVVHV IURP SRLQW $ WR SRLQW % DQG SDUWLFOHV RI WKH
medium move up and down perpendicular to the direction of energy.
The upward raised part of the wave is called crest and the downward lowered
part is called trough.
For examples, light wave, radio wave, wave produced in water when a stone
is thrown on it, etc.

Longitudinal wave

Longitudinal wave is the wave in which the particles of the medium vibrate to
and fro in the same direction as the direction of energy.

99 Times' Crucial Science & Environment Book - 8

Compression

AB

ZĂƌĞĨĂĐƟŽŶ
>ŽŶŐŝƚƵĚŝŶĂů ǁĂǀĞ

,Q WKH ÀJXUH WKH HQHUJ\ SDVVHV IURP SRLQW ɇ$· WR ɇ%· DQG WKH SDUWLFOHV RI WKH
medium move in ’to’ and ’fro’ in the same direction. Sound wave, wave produced
in spring when a jerk is given, etc are examples of longitudinal wave. When a
VRXQG LV SURGXFHG WKH DLU PROHFXOHV YLEUDWH ɇWR· DQG ɇIUR· LQ WKH VDPH GLUHFWLRQ
as the direction of sound. In this process, air molecules remain tightly packed
in some regions called compressions and remain loosely packed in some regions
called rarefactions.

Differences between transverse wave and longitudinal wave

Transverse wave Longitudinal wave

1. The wave in which the particles of 1. The wave in which the particles of
the medium vibrate up and down WKH PHGLXP YLEUDWH ɇWR· DQG ɇIUR· LQ
perpendicular to the direction of the same direction as the direction of
energy is called transverse wave. energy is called longitudinal wave.

2. It has crests and troughs. 2. It has compression and rarefaction.

3. Light wave, radio wave, etc are 3. Sound wave, wave produced in
examples of transverse wave. a spring, etc are the examples of
longitudinal wave.

ĐƟǀŝƚLJ ϴ͘ϭ To show the formation of the compression and rarefaction.

Materials required: Compression

Slinky spring

Procedure: ZĂƌĞĨĂĐƟŽŶ

7DNH D VOLQN\ VSULQJ DQG À[ RQH HQG RI LW WR D ZDOO

2. Stretch the spring from another end and vibrate ’to’ and ’fro’. What do
you observe ?

Observation:

Two types of regions are distinctly formed in the spring. The region where the
rings are tightly packed is compression and the region where the rings are
loosely arranged is rarefaction.

Some terms used in wave

Complete wave

The part of a wave which contains a compression and a rarefaction or a crest
and a trough is called a complete wave.

Times' Crucial Science & Environment Book - 8 100

Wave length
Length of a complete wave is called wavelength. It is also equal to the distance
between two successive compressions or rarefactions. In case of transverse
wave, it is equal to the distance between two successive crests or troughs. It is
denoted by /DPEGD nj Its SI unit is metre (m).

Frequency
The number of complete waves produced in a second is called frequency. It is
denoted by ɇf ’. The SI unit of frequency is Hertz and is denoted by Hz.

Suppose, a wave is produced in 1 second in which the number of complete
waves is three. Here the frequency of the above wave is 3 Hertz.

1000 Hertz = 1 Kilo hertz

1000 Kilo hertz = 1 Megahertz

Think and Solve

The frequency of a sound wave is 100Hz. What does it mean?

Velocity
The distance travelled by a wave in one second is called the velocity of the
wave. Its SI unit is m/s. Speed of wave can be calculated by the formula,

9HORFLW\ IUHTXHQF\ I ð ZDYHOHQJWK nj
? Y I ð nj
The unit of frequency should be Hertz and that of wavelength should be metre
for the unit of speed to be m/s in the above formula. Speed of sound is different
in different media. Speed of sound in air is 332m/s. It means that sound travels
332m in 1 second.
Speeds of sound in different media are as follows:

Solid Substance Speed of Sound in m/s
Liquid
Gas Iron 5950
Glass 3980
Water 1531
Alcohol 1207
Air 332
Hydrogen 1284
Oxygen 316

The speed of sound is the highest in solid. It is the least in gaseous medium.

101 Times' Crucial Science & Environment Book - 8

5HÁHFWLRQ RI VRXQG

Like other forms of energy, sound also returns to the same medium when it
strikes on any obstacle. The phenomenon of returning of sound to the same
PHGLXP ZKHQ LW VWULNHV D VROLG REVWDFOH LV FDOOHG UHÁHFWLRQ RI VRXQG

ĐƟǀŝƚLJ ϴ͘Ϯ 7R GHPRQVWUDWH WKH UHÁHFWLRQ RI VRXQG

Materials required:
Two plastic pipes, a plane mirror, a
clock, a wooden board, a table, etc.
Procedure:
1. Place a plane mirror in a

vertical position on a table.
2. Place a wooden board in

YHUWLFDO SRVLWLRQ SHUSHQGLFXODU WR WKH SODQH PLUURU DV VKRZQ LQ ÀJXUH
3. Keep two plastic pipes on the table one in each side of the wooden board

so that each pipe makes about 30° angle with the wooden board.
4. Place a clock at the end of one pipe.
5. Keep your ear at the end of another pipe.

What do you feel ?
Observation:
<RX FDQ KHDU WKH ɇWLF· ɇWLF· VRXQG RI WKH FORFN 7KLV LV EHFDXVH WKH VRXQG SURGXFHG
E\ WKH FORFN LV UHÁHFWHG IURP WKH SODQH PLUURU $QG LW UHDFKHG WKH HDU
Conclusion:
6RXQG LV UHÁHFWHG ZKHQ LW VWULNHV RQ D VROLG VXUIDFH.

Propagation of sound and sound wave
ĐƟǀŝƚLJ ϴ͘ϯ To observe the formation of waves
Materials required:
Tuning fork, a bowl with water, etc.
Procedure:
1. Take a bowl with water and place

it on a table.
2. Take a tuning fork and hit its

forks on a rubber pad.
3. Bring the vibrating tuning forks close to the water and touch the water.
What do you see?
You can see ripples produced in water surface spreading out from the point
where you touch the water with the prong. These ripples are called waves.
They carry sound from the source to other places.

Times' Crucial Science & Environment Book - 8 102

As in the water, sound wave is produced in the air also. When an object
vibrates in air, the molecules of the air move in to and fro motion. Due to this,
the air molecules are tightly packed in some regions and loosely arranged in
other regions. The region where the air molecules are tightly packed is called
compression and the region where the air molecules are loosely arranged is
called rarefaction. The compressions and rarefactions are formed alternatively
in the air. The compressions and rarefactions collectively form wave. This
wave carries sound energy from the source to other places.

Compression

AB

ZĂƌĞĨĂĐƟŽŶ
>ŽŶŐŝƚƵĚŝŶĂů ǁĂǀĞ

Sound wave is a longitudinal wave. A longitudinal wave is wave in which the
particles of the medium move to and fro motion in the direction of the wave.
In sound wave, the particles of the medium move in to and fro motion forming
compressions and rarefactions.

Sound energy transfers from the source to the other places in the form of
wave. The process of transmission of sound from one place to another place is
called propagation of sound.

Sound wave travels through material medium only. It cannot travel in vacuum.
Therefore, it is called mechanical wave.

ĐƟǀŝƚLJ ϴ͘ϰ To show that material medium is required for the propagation

of sound.

Materials required:

Bell jar, an electric bell, vacuum pump, vaseline, Electric line
etc.

Procedure:

7DNH D EHOO MDU DQG À[ DQ HOHFWULF EHOO LQVLGH Wire
WKH EHOO MDU DV VKRZQ LQ WKH ÀJXUH

2. Put the apparatus on a table and connect a Bell jar
vacuum pump to the lower part of the jar.

3. Connect the electric bell to a battery with a Hammer
switch.

4. Make the apparatus air-tight by putting Gong
vaseline in the connections.

5. Turn the switch on and remove air from
the jar by operating a vacuum pump. What
GR \RX ÀQG"

103 Times' Crucial Science & Environment Book - 8

Observation:
At the beginning, you can hear the sound of the bell distinctly. The sound
becomes fainter slowly and it gets vanished at last even though the hammer
is hitting the gong.
Conclusion:
At the beginning there was air inside the bell jar and the sound passed through
the air. When the air was removed from the jar, the sound could not come outside
due to lack of material medium. Thus, sound cannot be heard. This activity proves
that material medium is required for the transmission of sound.

Propagation of sound in solid

Sound can travel through solids. Speed of sound is the highest of all in solid
medium.

ĐƟǀŝƚLJ ϴ͘ϱ To show that sound can travel through solid.

Materials required:
A bench or desk
Procedure:
Put your ear on a desk as shown in the
ÀJXUH 7HOO RQH RI \RXU IULHQGV WR WDS WKH
desk gently at another end.
Observation:
You can hear the sound of tapping.
Conclusion:
This activity shows that sound can travel through solids.

Propagation of sound in liquid
ĐƟǀŝƚLJ ϴ͘ϲ To show that sound can travel through liquid.

Materials required:
A bucket full of water, a bell, a hammer, etc.
Procedure:
Take a bucket full of water. Hold a bell with a wire so
that the bell lies inside the water of bucket as shown
LQ WKH ÀJXUH +LW WKH EHOO ZLWK D KDPPHU LQVLGH WKH
water. Can you hear the sound of the bell?
Observation:
You can hear the sound of the bell.
Conclusion:
Sound can travel through liquids.

Times' Crucial Science & Environment Book - 8 104

Various kinds of sounds are being produced in each second. However, we
cannot hear all types of sound waves. We can hear the sound waves within
certain range of frequency.

The sound having frequency higher than
20,000 Hz is called the ultrasonic sound.
It is commonly known as ultrasound. We Boat or ship

cannot hear ultrasonic sounds. However, dƌĂŶƐŵŝƩĞƌ Detector

some animals like dog, dolphin, monkey Water surface

and deer can hear ultrasonic sounds. On Incident ZĞŇĞĐƚĞĚ d
the other hand, the sound with frequency wave wave

less than 20 Hz is called the infrasonic
sound. We cannot hear this sound too.
Our ears can hear the sound waves having Sea bed

frequency within 20 Hz to 20,000 Hz. Such ZĞŇĞĐƟŽŶ ŽĨ ƵůƚƌĂƐŽƵŶĚ ĨƌŽŵ ƚŚĞ ƐĞĂďĞĚ
sound is called audible sound. So, whatever sound we hear in our daily life is
the audible sound.

There are several uses of the ultrasonic sound. One of such important uses of
WKH XOWUDVRQLF VRXQG LV WR ÀQG WKH GHSWK RI WKH VHD $ VSHFLDO HTXLSPHQW QDPHG
SONAR (Sound Navigation and Ranging) is used for this purpose. It is carried in
the ship. This instrument sends the ultrasonic sound directly towards the bed of
WKH VHD 7KHQ WKH UHÁHFWHG VRXQG IURP WKH VHD EHG LV UHFHLYHG E\ WKH LQVWUXPHQW
The time taken by the sound to return from the sea bed is recorded. Using the
known value of speed of sound in water, the depth of the sea can be calculated.

Echo

,I ZH VKRXW ORXGO\ LQ IURQW RI D KLOO RU LQ D ODUJH UHÁHFWLQJ REMHFW ZH ZLOO
hear our own sound after some time. This sound which we hear later is echo.
,W LV UHÁHFWHG IRUP RI RULJLQDO VRXQG E\ WKH KLOO RU UHÁHFWLQJ VXEVWDQFH The
UHÁHFWHG sound is called echo. Following conditions are required for the echo
to occur.
7KH GLVWDQFH EHWZHHQ WKH VRXUFH DQG UHÁHFWLQJ VXUIDFH PXVW EH DW OHDVW

17m.
7KH UHÁHFWLQJ VXUIDFH VKRXOG EH ULJLG DQG ODUJH
3. The sound must be loud enough to be heard.

Reverberation

When you speak in a big empty hall, you feel that your sound becomes louder.
+HUH UHÁHFWLQJ VXUIDFHV L H ZDOOV DUH FORVHU IURP WKH VRXUFH RI VRXQG 6R
WKH UHÁHFWHG VRXQG JHWV PL[HG WR WKH RULJLQDO VRXQG DQG WKH RULJLQDO VRXQG LV
heard for longer time. This effect is called reverberation. Reverberation is the

105 Times' Crucial Science & Environment Book - 8

SURORQJDWLRQ RI RULJLQDO VRXQG GXH WR PL[LQJ RI UHÁHFWHG VRXQG WR WKH RULJLQDO
sound. For reverberation to occur, distance between the source of sound and
UHÁHFWLQJ VXUIDFH VKRXOG EH OHVV WKDQ P
In an auditorium or cinema hall, the excessive reverberation is not desirable.
To reduce reverberation, the roofs and walls of the auditorium or cinema
hall are covered with sound absorbing materials. Sound can be absorbed
by curtain, chair, clothes, beds, etc. So reverberation does not occur in well
furnished rooms.
/LWWOH UHYHUEHUDWLRQ LV EHQHÀFLDO LQ PXVLFDO VRXQGV ,W PDNHV WKH VRXQG OLYHO\

Echo Reverberation

7KH UHÁHFWHG VRXQG LV FDOOHG 1. The prolongation of original sound
echo. GXH WR PL[LQJ RI UHÁHFWHG VRXQG WR WKH
original sound is called reverberation.

)RU GLVWLQFW HFKR WKH UHÁHFWLQJ )RU UHYHUEHUDWLRQ WKH UHÁHFWLQJ
surface should be more than 17m surface should be nearer than 17m
away from the source of sound. from the source of sound.

Application of echo

7R ÀQG WKH GHSWK RI VHD RU RFHDQ

(FKR FDQ EH XVHG WR ÀQG WKH GHSWK RI VHD RU RFHDQ Fathometer is the
instrument used to measure the depth of ocean.

The sound is sent from the surface of water towards the bed of ocean. The
sound strikes the bed and returns to the surface. If the time required for
the sound to return to the surface is t, speed of sound in water is v, then
depth of the ocean d is calculated by the following formula:

d = v×t
2

2. To locate positions by animals

Echo is used by animals like whale, bat, dolphin, etc to locate positions.

3. To identify the internal part of human body

Ultrasound is sent to the internal parts of human body through a machine.
,W JHWV UHÁHFWHG VKRZLQJ WKH FRQGLWLRQ RI LQWHUQDO SDUWV RI KXPDQ ERG\

Solved Numerical Problem 8.1
If a sound produced by a man has wavelength of 0.5m and its frequency
is 660Hz, what is the speed of the sound?
Given,
:DYHOHQJWK nj P

Times' Crucial Science & Environment Book - 8 106

Frequency (f) = 660Hz
Speed (v) = ?
We have,
9 I ð nj ð P V
Therefore, speed of the sound is 330m/s.

Solved Numerical Problem 8.2
Echo is heard after 4 seconds from the time of sending of sound from
the surface of the sea. What is the depth of the sea if speed of sound
in water is 1500 m/s?

Given,

Speed of sound in water (v) = 1500m/s

Time to hear echo (t) = 4 seconds

Depth of sea (d) = ?

We have,

d = v×t = 1500 × 4 = 3000m
2 2

Therefore, depth of the sea is 3000m.

tuning fork ͗ ƚǁŽͲ ƉƌŽŶŐĞĚ ƐƚĞĞů ĚĞǀŝĐĞ ƵƐĞĚ ďLJ ŵƵƐŝĐŝĂŶƐ
auditorium : a part of theatre or concert hall
electric bell ͗ Ă ďĞůů ǁŚŝĐŚ ǁŽƌŬƐ ǁŝƚŚ ĞůĞĐƚƌŝĐŝƚLJ
bell jar : bell shaped glass cover
ƐůŝŶŬLJ ƐƉƌŝŶŐ : pre- compressed helical spring

1. Sound is a form of energy produced due to the vibration of an object.
2. Sound travels through a material medium.
3. Sound propagates from one place to another place in the form of wave.
4. Wave is a disturbance in a medium which carries energy from one point to another.
5. Transverse wave is a wave in which the particles of the medium vibrate up and down

perpendicular to the direction of energy.
6. Longitudinal wave is a wave in which the particles of the medium vibrate to and fro

in the same direction as the direction of energy.
7. Crest and trough are the parts of transverse wave.
8. Compressions and rarefactions are parts of longitudinal wave.

107 Times' Crucial Science & Environment Book - 8

9. Wavelength is the length of a complete wave.

10. The number of complete waves produced in a second is called frequency.

11. The distance travelled by a wave in one second is called the speed of wave.

12. The phenomenon of returning of sound to the previous medium after striking a
solid surface is called reflection of sound.

13. The reflected sound is called echo.

14. The prolongation of original sound due to mixing of the reflected sound to the
original sound is called reverberation.

Exercise

A. State True or False and correct the false statements.
1. Sound can travel through vacuum.
2. Speed of sound is the highest in solid medium.
3. Sound wave, light wave and radio wave are transverse waves.
4. Compressions and rarefactions are the parts of longitudinal wave.
:DYH OHQJWK LV GHQRWHG E\ /DPEGD nj
6. The unit of frequency is m/s.

B. Answer these questions in short.
1. What is sound? How is it produced?
2. Can sound travel in vacuum?
3. What is wave? Give some examples.
4. What is transverse wave? Give some examples of transverse wave.
5. What is longitudinal wave ? Give some examples.
'HÀQH L &RPSUHVVLRQ LL 5DUHIDFWLRQ LLL &UHVW LY 7URXJK
7. What is wavelength? Mention its unit.
8. What is frequency? Mention its unit.
9. Whatisvelocityofwave?Mentionformulatocalculatethe velocityofwave.
:KDW LV UHÁHFWLRQ RI VRXQG"
11. What is echo? Mention some uses of echo.
12. What is reverberation ? How is reverberation minimized ?

Times' Crucial Science & Environment Book - 8 108

C. Differentiate between:
1. Echo and reverberation
2. Longitudinal wave and transverse wave

D. Give reasons:
1. When you touch a ringing bell, it stops producing sound.
2. Sound wave is called longitudinal wave.
3. Sound becomes louder in an empty room.
4. Ceilings and walls of a cinema hall are covered by sound absorbing
materials.

E. Numerical Problems:
1. Find the velocity of sound in air if its frequency is 100Hz and
wavelength is 3.2m?
2. What is the wavelength of sound of frequency 400Hz moving in
steel with a velocity of 5000m/s ?
3. Find the depth of the sea in which echo is heard after 4 seconds.
(Given: the velocity of sound in water is 1500m/s)

F. Speeds of sound in three different media A, B, and C are given in
the table
1. Which medium is solid? Why? Medium Velocity of sound
A 330m/s
2. Which medium is liquid? Why?
B 5000m/s

C 1500m/s

Answers 2. 12.5m 3. 3000m

E. 1. 320m/s

Visit appropriate place or building to observe echo and reverberation.
Also observe the effect of increase or decrease of distance between the
VRXUFH RI VRXQG DQG UHÁHFWLQJ VXUIDFH LQ WKHVH SKHQRPHQD :ULWH GRZQ
what you observed.

109 Times' Crucial Science & Environment Book - 8

9CHAPTER Magnetism

Alexander Graham Bell

$OH[DQGHU *UDKDP %HOO ZDV D 6FRWWLVK ERUQ VFLHQWLVW
inventor, engineer, and innovator who is credited with

SDWHQWLQJ WKH ÀUVW SUDFWLFDO WHOHSKRQH

Estimated Periods: 7
Objectives: At the end of the chapter, the students will be able to:

GHILQH D PDJQHW DQG HQOLVW LWV SURSHUWLHV
H[SODLQ WKH PROHFXODU WKHRU\ RI PDJQHWLVP
H[SODLQ PDJQHWLF LQGXFWLRQ
WHOO WKH FDXVHV RI GHPDJQHWL]DWLRQ DQG LWV SUHYHQWLYH PHDVXUHV

What is a magnet?
What is a loadstone? Why is it named so?
Can magnets be produced artificially?
What are the properties of a magnet? Discuss.

Introduction

Magnets attract or repel each other with a force called magnetism. The
magnetic property begins from an atom of a substance. Each atom has protons
and neutrons in its nucleus and electrons in the outer shells. The electrons
are the particles that carry negative charge. Protons carry positive charge
and neutrons have no charge. When electrons revolve round the nucleus, their
movement generates an electric current and causes each electron to act like a
microscopic magnet. However, in most substances, equal numbers of electrons
spin in opposite directions. Such a movement cancels out their magnetism.
Hence, the materials such as paper, cloth, wood, copper, etc do not show
magnetic property. In substances such as iron, cobalt, and nickel, most of the
electrons spin in the same direction. This makes the atoms in these substances
show magnetic property. Magnets are made out of such magnetic substances.
A magnet is known to people since the ancient times. The ancient people used
loadstones, naturally magnetized pieces of iron ore, as magnets. The word
magnet in Greek means stone from Magnesia, a part of ancient Greece where
loadstones were found. The loadstone is also called magnetite.It is claimed
that magnets were in use in Greece and China since 2500 years ago. Ancient

Times' Crucial Science & Environment Book - 8 110

Chinese navigators used magnetite to know the directions, especially during
the cloudy weather.
7KH QDWXUDO PDJQHWV L H ORDGVWRQHV GR QRW KDYH GHÀQLWH VKDSH DQG VL]H 7KH\
KDYH ZHDN PDJQHWLF VWUHQJWK %XW SRZHUIXO DUWLÀFLDO PDJQHWV FDQ EH PDGH
in required shape and strength. Bar magnet, U-shaped magnet, Horse-shoe
PDJQHW GLVN PDJQHW ULQJ PDJQHW F\OLQGULFDO PDJQHW HWF DUH VRPH DUWLÀFLDO
magnets.

Natural magnet Horse-shoe magnet Bar magnet U-shaped magnet

Magnets are very useful materials. They are being used extensively in different
devices. A magnet is used in radio, computer, transistor, television, speaker,
motor pump, electric generator, etc. A magnet can be used to lift heavy pieces
of iron in industries. Doctors can use magnet to remove iron pieces if they
enter into our eyes.

Properties of magnets

A magnet is a substance which attracts magnetic substances and always
comes to rest showing N-S directions when suspended freely. Magnets have
the following properties:

1. A magnet attracts magnetic substances, such as iron, cobalt and nickel.

2. A freely suspended magnet always comes to rest pointing north-south
directions.

3. The like poles of a magnet repel while the unlike poles attract.

4. The magnetic poles always exist in pair. The north and south poles of a
magnet can never be separated.

5. A magnet is strong at the poles and weak in the middle.

$ PDJQHW KDV D PDJQHWLF ÀHOG 7KH VSDFH DURXQG D PDJQHW XS WR ZKLFK
LWV HIIHFW FDQ EH H[SHULHQFHG LV FDOOHG PDJQHWLF ÀHOG

7. The magnetic force is transferred to the magnetic substance which is in
contact to the magnet.This process is called magnetic induction.

Molecular theory of magnetism

The molecular theory of magnetism was developed by a British physicist Sir
Alfred Ewing. The theory states that each molecule of a magnet or magnetic

111 Times' Crucial Science & Environment Book - 8

substance is an independent magnet which is called a molecular magnet. This
theory explains the mechanism of magnetization and demagnetization.
According to this theory, the molecular magnets are arranged in the form of closed
rings or chains in a magnetic substance. In a ring, the north pole of a molecular
magnet is attracted to the south pole of another molecular magnet and so on.
Thus, there are no free magnetic poles in a magnetic substance and it does not act
as a magnet. It is the demagnetized state of a magnetic substance.

N N N NNN N SSS NNN N SSS NNN N SSS NNN N SSS NNN N SSS NNN N SSS NNN N SSS
S S S

N
S
NN NN
S S SS
N
S S S S S S S S

N
S

N
S

N
S

ůŽƐĞĚ ĐŚĂŝŶ Žƌ ƌŝŶŐ ĨŽƌŵ ŽĨ ŵĂŐŶĞƟĐ KƉĞŶ ĐŚĂŝŶƐ ŽĨ ŵĂŐŶĞƟĐ ŵŽůĞĐƵůĞƐ ŝŶ
ŵŽůĞĐƵůĞƐ ŝŶ Ă ĚĞŵĂŐŶĞƟnjĞĚ ƐƵďƐƚĂŶĐĞ ŵĂŐŶĞƟnjĞĚ ƐƵďƐƚĂŶĐĞ

When the molecular magnets are arranged in the form of open chains i.e.
parallel to each other, the substance gets magnetized. Thus, one end of the
magnet has free north poles of molecular magnet and the another end has free
south poles. A magnet shows magnetic properties due to these free ends.

At the middle of a magnet, the molecular magnets lose their force by attracting
each other. Hence, a magnet has less magnetic force in the middle and more
at the ends.

Since a magnet is made of molecular magnets and each magnetic molecule has
N and S-poles, the poles of a magnet can never be
separated.
N
S

Evidences in favour of molecular theory of
magnetism

The molecular theory of magnetism is considered
true because the following evidences support it.

a. A magnet is strong at poles and weak in the middle.

b. The poles of a magnet cannot be separated.

c. A magnet can be made from a magnetic substance only. It is because
the non-magnetic substances do not have magnetic molecules and they
cannot be turned into magnets.

d. A magnet gets demagnetized when it is hammered or heated.

Magnetic Induction

When an iron nail is brought in contact with a magnet, the iron nail is attracted
by the magnet. It behaves as a magnet and attracts another iron nail. The

Times' Crucial Science & Environment Book - 8 112

QH[W LURQ QDLO WRR DFWV DV D PDJQHW DQG DWWUDFWV WKLUG RQH %XW ZKHQ WKH ÀUVW
iron nail is separated from the magnet, other nails fall down.

It is clear from the above activity that a magnetic substance gains temporary
magnetism when it is very near or in contact with a magnet. This process is
called magnetic induction.

The property of a magnet due to which its magnetic property is transferred
temporarily to a magnetic substance which is attracted by it is called magnetic
induction

0DJQHWLF ÀHOG

A magnet can affect another magnet or the magnetic substance within a certain
UHJLRQ 7KLV UHJLRQ LV FDOOHG PDJQHWLF ÀHOG 7KXV the space around a magnet
XSWR ZKLFK WKH PDJQHWLF LQÁXHQFH FDQ EH H[SHULHQFHG LV FDOOHG PDJQHWLF ÀHOG
7KH PDJQHWLF ÀHOG LV UHSUHVHQWHG E\ PDJQHWLF OLQHV RI IRUFH 7KH PDJQHWLF
lines of force have the following properties:

a. Magnetic lines of force are the continuous closed curves which originate
from the north pole of a magnet and end on the south pole.

b. The magnetic lines of force are more at the poles than in the middle.

c. The magnetic lines of force never intersect each other.

ĐƟǀŝƚLJ ϭ͘ϭ To draw the magnetic lines of force. N

Materials required:

A cardboard, a white sheet of paper, a bar
magnet, a magnetic compass, thumb pins, etc.

Procedure: S
1. Spread a white sheet of paper over a

FDUGERDUG DQG À[ LW XVLQJ WKXPE SLQV

2. Place a bar magnet in N-S direction at the middle of the paper and draw
its outlines.

3. Place a magnetic compass close to the N-pole of the bar magnet. The
S-pole of the compass is attracted by N-pole of the magnet but the N-pole
of the compass points at certain direction.

4. Mark the direction shown by N- pole of the compass using a pencil.

5. Now, move the compass little towards the south pole and repeat the same
process.

6. Repeat the process several times until the compass reaches the S-pole of
the magnet.

7. Draw a curved line joining all these points.

8. Repeat the whole process many times to get a number of magnetic lines
of force.

113 Times' Crucial Science & Environment Book - 8

Demagnetization

The molecular magnets remain in an open chain i.e. parallel to each other in a
magnet. If such open chains are disturbed by any means, molecular magnets get
arranged in the form of closed chains and the magnet gets demagnetized. Thus,
the process of losing magnetic property by a magnet is called demagnetization.
A magnet gets demagnetized by the following ways:
1. Dropping a magnet repeatedly or striking it against a hard surface.
2. Heating a magnet.
3. Bringing the similar poles of a magnet together by force.
4. Storing the magnet without using magnetic keepers.
5. Regular hammering of a magnet, etc.

Methods of conserving magnetic strength

A magnet can be kept safe for a long time in the following ways:

1. A magnet should not be heated or placed near Horseshoe
a hot object. magnet
bent from a
2. A magnet should not be hammered with a hard strip of steel
object or beaten hard. Magnetic
domains
3. A magnet should not be rubbed with other line up to
objects.
create
4. It should not be dropped frequently onto the magnetic
hard surface or ground.
ÀX[

5. A magnet should be protected from rusting.

6. Magnets should be stored by placing unlike Iron keeper
poles of magnets together. Magnetic keepers becomes magnetized and helps
should be used to connect the unlike poles the magnet stay magnetized
while storing.

Terrestrial magnetism 1800 Km
Geographical North
It has been proved from several South pole of Magnetic
experiments and researches that the the earth’s 17° equator
earth has a magnetic property. The magnet Geographical
magnetic property of the earth is Magnetic equator
called terrestrial magnetism. meridian
It has been imagined that there is a
huge magnet inside the earth whose
north pole lies towards the earth’s North pole of
south pole and the south pole lies the earth’s
Geographical South magnet

Times' Crucial Science & Environment Book - 8 114

towards the earth’s north pole. But, the geographical poles and the magnetic
poles do not lie at the same place. There is a gap between the geographical
meridian and magnetic meridian.
The terrestrial magnetism is supported by the following evidences:
1. A freely suspended magnet comes to rest pointing N-S direction because

its S-pole is attracted by the earth’s N pole and the N-pole is attracted by
the earth’s S-pole.
2. A freely suspended magnet does not show exact geographical poles at
rest. It is due to the fact that the magnetic poles and geographical poles
of the earth do not coincide and the poles of suspended magnet are
attracted by the magnetic poles, not by the geographical poles.
3. A freely suspended magnet beyond the equator leans at rest because one
pair of opposite magnetic poles lies closer and the other pair lies farther
beyond the equator.
4. A magnetic substance buried inside the earth pointing N-S direction gets
magnetized after some time.

navigator ͗ Ă ƉĞƌƐŽŶ ǁŚŽ ĞdžƉůŽƌĞƐ ďLJ ƐĞĂ

inductor ͗ ƉƌŽĚƵĐĞƌ ŽĨ ŵĂŐŶĞƟĐ Žƌ ĞůĞĐƚƌŝĐĂů ĞīĞĐƚ

molecular magnet : molecule based magnet

1. A magnet is a substance which attracts magnetic substances; and the property of
a magnet is called magnetism.

2. The magnets are of two types- natural magnets and artificial magnets.

3. Molecular theory of magnetism states that each molecule of a magnet or magnetic
substance is an independent magnet which is called a molecular magnet.

4. The molecular magnets of a magnetic substance are arranged in the form of closed
chains.

5. The molecular magnets of a magnet are arranged in the form of open chains.

6. The space around the magnet where the magnetic influence can be experienced is
called magnetic field.

7. The magnetic property of the earth is called terrestrial magnetism.

8. The process of developing temporary magnetism in a magnetic substance which is
in contact or very near to a magnet is called magnetic induction.

115 Times' Crucial Science & Environment Book - 8

Exercise

A. Answer these questions in very short.
1. How are molecular magnets arranged in a magnetic substance?
2. How are molecular magnets arranged in a magnet ?
3. Give an example of natural magnet.
:KDW LV PDJQHWLF ÀHOG "
5. Write down a property of magnetic lines of force.
6. Do the magnetic meridian and geographical meridian exactly
coincide?
7. How does hammering destroy magnetism?

% 'HÀQH 2. Molecular magnet
1. Magnetism 4. Terrestrial magnetism
3. Magnetic induction

C. Give reasons:
1. A magnet is strong at poles and weak in the middle.
2. The poles of a magnet cannot be separated.
3. An iron piece can be made magnet but not a copper piece.
4. A freely suspended magnet points N-S directions at rest.
5. A magnet gets demagnetized if it is dropped repeatedly.
6. A magnetic compass does not show exact geographical poles.

D. Answer these questions.
1. What is a magnet ? Write down the properties of a magnet.
2. What is terrestrial magnetism ? How can you support it ?
3. What is molecular theory of magnetism ? How can you support it ?
4. What role is played by molecular magnets to magnetize and
demagnetize the substances?
5. Why doesn’t a freely suspended magnet show exact geographical poles
at rest?

Times' Crucial Science & Environment Book - 8 116

E. Study the diagram and answer the questions that follow:N
:KDW SURFHVV LV LOOXVWUDWHG E\ WKH ÀJXUH" S
'HÀQH WKH SURFHVV
3. What happens if the nail in contact with the
magnet is separated? Why?

Take a piece of soft iron and make it magnet by single touch method. Then
study its magnetic properties by performing different experiments such as
suspending it freely, testing it for sepution, putting it on iron dust, etc. Write
\RXU REVHUYDWLRQV DQG ÀQGLQJV

117 Times' Crucial Science & Environment Book - 8

10CHAPTER Electricity

Alessandro Volta

He is known for the discovery of
Methane and invention of electric cell.

Estimated Periods: 7
Objectives: At the end of the chapter, the students will be able to:

GHILQH HOHFWULFLW\ DQG LWV W\SHV
H[SODLQ DQG GHPRQVWUDWH WKH VWUXFWXUH DQG XVHV RI VLPSOH FHOO DQG GU\ FHOO
H[SODLQ WKH GRPHVWLF HOHFWULILFDWLRQ DQG LWV LQVWUXPHQWV
LQWURGXFH IXVH DQG 0&% ZLWK WKHLU XVH

What do you mean by charge? What are its types?
How is electricity produced?
What are the sources of electricity?
What are the uses of electricity? Discuss..

Introduction
Every form of matter consists of atoms, which may be of similar or different
kinds. An atom contains protons and neutrons in the nucleus while electrons
in the shells revolving around the nucleus. The electrons carry negative
charge and are responsible for the electrical property of matter. The electrical
property of a body is due to the increase or decrease in the number of electrons
RU WKHLU ÁRZ 7KLV SURSHUW\ LV UHJDUGHG DV HOHFWULFLW\
Electricity is the form of energy which is possessed by a body due to the
FKDQJH LQ QXPEHU RI HOHFWURQV RU GXH WR WKH ÁRZ RI HOHFWURQV LQ WKH ERG\
The electricity can be of two types-static electricity and current electricity.
The form of electrical energy which is possessed by a body due to the change
in number of electrons is called static electricity. Such electricity cannot be
transferred from one point to another. It is possessed by the insulators such
as glass, plastic, rubber, fur, etc. On the other hand, the form of electrical
HQHUJ\ ZKLFK LV SURGXFHG GXH WR WKH ÁRZ RI HOHFWURQV LQ D FRQGXFWRU LV FDOOHG
current electricity. It is transferred from one point to another with the help of
a conducting wire.

Times' Crucial Science & Environment Book - 8 118

Sources of electricity

$ GHYLFH ZKLFK VXSSOLHV HQHUJ\ IRU WKH ÁRZ RI HOHFWULFLW\ LQ D FLUFXLW LV FDOOHG D
source of electricity. There are two main types of sources of current electricity.
They are cell and dynamo or generator.

Cell (Electrical cell)

A cell is a device which produces current electricity by converting chemical
energy into electrical energy. There are several types of electrical cells. But we
shall discuss about simple cell and dry cell in this chapter.

Simple cell

A simple cell is called so due to its structure.
It consists of two metallic plates dipped in
acid solution in a beaker. The metallic plates
are zinc and copper while the acid is dilute
sulphuric acid s(Her2vSeOs4)a.s The zinc plate acts +
as cathode and a negative terminal –
Copper Zinc

whereas the copper plate acts as anode and
serves as positive terminal of the battery.
When a galvanometer is connected in the Water with
circuit with the help of conducting wires, the dilute H2SO4

JDOYDQRPHWHU LQGLFDWHV WKH ÁRZ RI FXUUHQW ^ŝŵƉůĞ ĐĞůů
electricity through the conducting wire. A
simple cell can produce a maximum potential difference of 1 volt.

Working of a simple cell

Metal plates are the electrodes and dilute sulphuric acid is the electrolyte in
a simple cell. Dilute sulphuric acid dissociates to form hydrogen cations (H+)
and sulphate anions (SO4ïï ) in an aqueous solution.

H2SO4 ń 2H+ + SO4ïï
In this dissociation, hydrogen gains positive charge by losing electrons while
sulphate gains negative charge by gaining the electrons (lost by hydrogen).

At cathode (Zinc plate)

TlwehifttehinszuitnlhpcehtazoitnfeocriompnlasZtne(SSmOOa44ïkïa)enaidtrretehaleetatnrseaegcatthetiedvfetroetweeraemrledincstarzloi,nni.cse..pTclahatetehe.oxTdcehe.sesseofioenlesctrreoancst

Zn + SO4ïï ń ZnSO4 + 2eï
At anode (Copper plate)

The H+ ions are attracted towards the copper plate where they gain electrons

119 Times' Crucial Science & Environment Book - 8

and form molecules of hydrogen. Since the Cu plate loses electrons, it acts as
positive terminal, i.e. anode.

2H+ + 2eï ń H2
,I WKH WZR PHWDO SODWHV DUH FRQQHFWHG E\ D FRQGXFWLQJ ZLUH WKH HOHFWURQV ÁRZ
from negative to positive terminal i.e. from zinc to the copper plate through
the external circuit. This arrangement now serves as a simple cell.

Defects of a simple cell

A simple cell cannot work well due to its two defects: local action and
polarization.

Local action

The zinc plate used in the simple cell is not pure and contains impurities
such as iron, copper, carbon, etc. The impurities act as anode within the zinc
plate and hence local current is produced. This current increases the internal
resistance of the cell and it becomes defective. Such defect is called local action.

The local action can be removed by using pure zinc plate or by using mercury
coated zinc plate (amalgamated zinc). The coated mercury brings the pure
zinc on the surface by dissolving it and mercury itself covers the impurities. It
prevents the formation of local current.

Polarization

The defect of a simple cell due to the formation of layer of hydrogen bubbles on
the copper plate is called polarization. The bubbles of hydrogen gas, being the
LQVXODWRU UHVLVW WKH ÁRZ RI HOHFWURQV DQG LQFUHDVH WKH LQWHUQDO UHVLVWDQFH RI
the cell.

Polarization is removed by the use of
depolarizers such as manganese dioxide
(cMopnpOer2)s, ulppohtaatsesi(uCmuSOd4i)c,hertocm. ate (K2Cr2O7),
Cu + – Zn

The improved form of simple cell in which Bubbles of
LMencOla2ncihse used as a depolarizer is called ŚLJĚƌŽŐĞŶ ŐĂƐ
cell.

Dry Cell

A dry cell is a source of electricity in which a carbon rod is used as an electrode
and a paste of ammonium chloride is used as electrolyte.

A dry cell consists of a carbon rod with brass cap and a paste of electrolyte in a
zinc container. In order to make a dry cell, a carbon rod with a brass cap at its

Times' Crucial Science & Environment Book - 8 120

XSSHU HQG LV SODFHG LQ D PXVOLQ EDJ 7KH EDJ LV WKHQ ÀOOHG ZLWK D PL[WXUH RI
mFRaQnWDgLaQnHeUs ReYdHiUo xDi dFeDU(GMEnRODU2G) a 7ndKHc aUrHbPoDnLQpoLQwJd SerD.UTW hRiIs WbKaHg ]iLsQFth FeDnQ pLlVa ÀceOOdHGin ZaLWzKin Dc
paste of electrolyte. The paste HisWFm DaQdGe DbGyGLmQiJx iZnDgWaHUm m7KoHn iXuSmSHcUh lHoQrGid ReI (WNKHH 4]CLQl)F,
SODVWHU RI 3DULV VWDUFK ÁRXU
can is sealed with a waxy substance and then covered with a plastic cap.
+
Brass cap
The zinc can is then wrapped with paper. It may be
then wrapped with polythene or soft metal sheet.
Carbon rod

The dry cell produces electricity due to the - Carbon dust + MnO2
chemical reaction between ammonium chloride ƌLJ ĐĞůů Muslin bag
and zinc. When two terminals of the cell are Paste of NH4Cl
FRQQHFWHG ZLWK D ZLUH WKH HOHFWULFLW\ ÁRZV LQ WKH Paper cover
external circuit. The strength of a dry cell is 1.5V. Zinc container

Working of a dry cell

Ammonium chloride ce(lNl.HIt4Cdli)ssoicsiateths eas: main
electrolyte used in dry

NH4&O ń1+4+ + Clï
TtohweaNrdHs4z+iinocnssuarrfeacaet.tracted towards the carbon rod and Clïions are attracted

At zinc surface

The Clïions react with Zinc and form ZnCl2. This reaction produces free
electrons.

Zn + 2Clï ń =Q&O2 + 2eï
At carbon rod

The NanHd4+Hio2.ns get electrons from the carbon rod and produce the molecules of
NH3

2 NH4+ + 2eï ń 1+3 + H2

If the two terminals of the battery are connected with a conducting wire,
HOHFWURQV ÁRZ IURP ]LQF FDQ WR WKH FDUERQ URG 7KXV FXUUHQW HOHFWULFLW\ LV
produced. When whole of the ammonium chloride reacts with zinc, the cell
stops producing electricity.

Role of MnO2 in a dry cell

Mhyadnrgoagennesgeasditooxfiodrem(MwnatOe2r) acts as depolarizer in the dry cell. It reacts with
and manganese trioxide and removes polarization.

2 MnO2 + H2 ń 0Q2O3 + H2O

121 Times' Crucial Science & Environment Book - 8

Advantages of dry cell over simple cell

$ VLPSOH FHOO KDV D OLTXLG HOHFWURO\WH DQG D EULWWOH YHVVHO 6R LW LV GLIÀFXOW WR
carry from one place to another. Its strength is only 1 volt. But a dry cell uses
a paste of ammonium chloride in a durable zinc container. So, it is easier to
carry, transport and use it. The strength of a dry cell is 1.5 volt. A dry cell can
be produced in required shape and size.

Combination of cells

The cells are combined to form battery. In general, the cells are combined in
order to get required voltage or current for the circuit.

1. Series combination
The combination of cells in which the positive terminal of one cell is
connected with the negative terminal of another cell is called series
combination of cells.

In this combination of cells, voltage increases when the number of cells
increases.

|||
| |
– + – +– +
| |
^ĞƌŝĞƐ ĐŽŵďŝŶĂƟŽŶ ^ĞƌŝĞƐ ĐŽŵďŝŶĂƟŽŶ ;ĚŝĂŐƌĂŵŵĂƟĐͿ

Therefore, the brightness of the bulb also increases. But the cells do not
become long lasting in this type of combination.

2. Parallel combination of cells

The combination of cells in which the positive terminals of all the cells
are connected to one point and the negative terminals are connected to
another common point is called parallel combination.

–+

–+ WĂƌĂůůĞů ĐŽŵďŝŶĂƟŽŶ
;ĚŝĂŐƌĂŵŵĂƟĐͿ
|||

WĂƌĂůůĞů ĐŽŵďŝŶĂƟŽŶ

Times' Crucial Science & Environment Book - 8 122

In this combination, the brightness of the bulb does not change if the
number of cells changes. But, the bulb can glow for a longer time i.e. the
life of cells is long lasting.

House wiring system

Electricity is generated in the power house. It is produced at high voltage. The
high voltage current is transported up to our house through the conducting
wires. The high voltage is decreased by a transformer and is supplied to our
house. A current of 220 Volts is supplied to our house.

The electricity is supplied through two wires to our house. The main wire
that supplies current to the house is called live or phase wire and the other
is called neutral wire. In our house, a fuse is connected to the live wire before
connecting it with the main meter of the house. Such fuse is called Corporation
Fuse. A switch called main switch is connected to the wire that comes out from
the meter. The main switch is enclosed inside a metal box called switch box.
The main switch helps to cut off or supply all the current to the house. A wire
is connected from the main switch which carries the unwanted current leaked
in the box to the earth. This is called earthing. There is another fuse called
Consumer's fuse which is connected to the main switch. A wire comes out
from the main meter. It is connected to the distribution board. The electricity
is distributed to different rooms and places of the house from the distribution
board. This whole system is called house wiring system. The wires used in the
house wiring system are insulated wires.

During wiring in a house, all electrical appliances are connected in parallel
combination. A separate switch is connected for each appliance in the house
wiring system.

Some Electrical Devices

The electrical devices use electricity to perform function. The devices which
convert electrical energy into other forms of energy are called electrical devices.
Some electrical devices frequently used in our daily life are described below:

1. Electric lamp

Electric lamp is a device that converts
electrical energy into light energy. There
DUH WZR W\SHV RI HOHFWULF ODPSV ÀODPHQW
ODPS RU EXOE DQG ÁXRUHVFHQW ODPS
7KH ÁXRUHVFHQW ODPS LV EHWWHU WKDQ WKH
ÀODPHQW ODPS EHFDXVH LW SURGXFHV PRUH
light and less heat. Nowadays, Compact Electric bulb CFL

Fluorescent Lamp (CFL) is in common use. The CFL is superior to other
types of lamps because it consumes less power and produces more light.

123 Times' Crucial Science & Environment Book - 8

2. Computer
A computer is an electronic device that accepts data and instructions
and processes them into useful results. A computer needs electricity to
perform tasks. A computer is used to write books, to store and retrieve
information, to design printing and publishing materials, etc. The
computer has an extensive use in e-mail and internet.

3. Heater

Heater is a device that converts electrical energy into heat
energy. There are several forms of heaters such as room
heater, food cooking heater, immersion heater (heater to
boil water), etc. Electric kettle, electric iron, electric jug,
rice cooker, etc also work by converting electricity in to
heat.

4. Electric bell Halogen heater

Electric bell is a device that converts electrical energy into sound energy.
$Q HOHFWULF EHOO FDQ EH XVHG LQ KRPH RIÀFH VFKRRO FROOHJH HWF ,Q D
KRXVH WKH VZLWFK RI WKH HOHFWULF EHOO LV À[HG VDIHO\ LQ WKH ZDOO RXWVLGH WKH
house. If anyone presses the switch, the bell rings inside the house so
that people staying inside open the door.

5. Radio and television

Radio is an important means of mass communication. We listen to a radio
for news, music, interviews, government notices, advertisements, etc. A
radio uses battery or current electricity to operate. Similarly, television
is an important audio-visual means of communication which runs by
using electricity. We watch news, views, sports, movies, advertisement,
wildlife, etc in a television.

6. Telephone and mobile phone

A telephone and mobile phone are the means of communication. The
receiver of a telephone converts electrical energy into sound energy and
the sound energy into electrical energy. Thus, the conversation using a
telephone is possible. A telephone needs electricity to run. Similarly, a
mobile phone works by using a battery as a source of electricity.

Fuse

A fuse is a thin wire having low melting point. It is used as a safety device in
DQ HOHFWULF FLUFXLW ,W PHOWV DQG EUHDNV WKH FLUFXLW LI WKHUH LV RYHUÁRZ RI HOHFWULF
current. Generally, a fuse is made of an alloy of tin and lead. A fuse is placed
in a box while using.

Times' Crucial Science & Environment Book - 8 124

ĨƵƐĞ ĐĂƌƚƌŝĚŐĞ ĨƵƐĞ ƐLJŵďŽů ŽĨ ĨƵƐĞ

6RPHWLPHV RYHUÁRZ RI HOHFWULF FXUUHQW RFFXUV LQ WKH HOHFWULF FLUFXLW :KHQ
H[FHVVLYH FXUUHQW ÁRZV WKURXJK WKH FLUFXLW WKH IXVH JHWV KHDWHG XS DQG PHOWV
TXLFNO\ 7KLV VWRSV WKH ÁRZ RI FXUUHQW LQ WKH FLUFXLW DQG VDYHV IURP DFFLGHQWV
,I IXVH LV QRW XVHG WKH RYHUÁRZ RI FXUUHQW PD\ KHDW WKH ZLUH H[FHVVLYHO\ DQG
FDXVH ÀUH 6XFK ÀUH FDQ EXUQ WKH ZKROH KRXVH

It is necessary to use a fuse of suitable capacity in a circuit. Generally, a fuse of
FDSDFLW\ OLWWOH PRUH WKDQ WKDW RI WKH FXUUHQW ÁRZLQJ LQ WKH FLUFXLW LV XVHG 2QFH
the fuse melts and breaks the circuit, the fuse gets damaged permanently.
Then it should be replaced with a new fuse.

Miniature Circuit Breaker (MCB)

Miniature circuit breaker is an advanced form of fuse which breaks the circuit
without being damaged itself. An MCB is easier and safer to use. When there
LV RYHUÁRZ RI HOHFWULFLW\ LQ WKH FLUFXLW WKH VZLWFK JRHV RII LWVHOI DQG EORFNV WKH
ÁRZ RI FXUUHQW 7KLV SUHYHQWV WKH DFFLGHQWV GXH WR RYHUÁRZ RI FXUUHQW 7KH
advantage of MCB over the traditional fuse is that it is not damaged itself
while breaking the circuit. When the switch is turned on by pushing it up, it
VWDUWV ZRUNLQJ DJDLQ ,I WKH RYHUÁRZ RI FXUUHQW LV FRQWLQXLQJ WKH VZLWFK GRHV
not get turned on. In this way, the circuit is protected by the use of MCB.

ĚĞƉŽůŝnjĞƌ ͗ Ă ĐŚĞŵŝĐĂů ǁŚŝĐŚ ƌĞŵŽǀĞƐ ĚĞƉŽůĂƌŝnjĂƟŽŶ

amalgamated ͗ ĐŽǀĞƌĞĚ ďLJ ŵĞƌĐƵƌLJ

galvanometer : an instrument used to measure small amount of current

1. The electrons carry negative charge and are responsible for the electrical property
of matter.

2. The electricity can be of two types- static electricity and current electricity.

3. The form of electrical energy which is possessed by a body due to the change in
number of electrons is called static electricity.

4. The form of electrical energy which is produced due to the flow of electrons in a
conductor is called current electricity.

125 Times' Crucial Science & Environment Book - 8

5. A cell is a device which produces current electricity by converting chemical energy
into electrical energy.

6. A simple cell cannot work well due to two defects: local action and polarization.

7. The defect of a simple cell to produce local current due to the presence of
impurities is called local action.

8. The defect of a simple cell due to the formation of a layer of hydrogen bubbles on
the copper plate is called polarization.

9. The improved form of simple cell in which MnO2 is used as a depolarizer is called
Leclanche cell.

10. A dry cell is a source of electricity in which a carbon rod is used as an electrode
and a paste of ammonium chloride is used as electrolyte.

Exercise

A. Answer these questions in very short.
1. How does a matter show electrical property?
'HÀQH VRXUFH RI HOHFWULFLW\
3. What is the role of sulphuric acid in a simple cell?
4. Name the metal plates used as cathode and anode in a simple cell.
5. How are electrons produced in a simple cell?
6. Name the two types of defects of a simple cell.
7. What is depolarizer? Give an example.
8. What is the strength of a simple cell?
9. What is the strength of a dry cell?
'HÀQH /HFODQFKH FHOO

B. Write down differences between:
1. anode and cathode
2. simple cell and dry cell
3. series and parallel combination of cells

C. Give reasons.
1. Manganese dioxide is used in a dry cell.
2. Local action can be removed by using amalgamated zinc.

Times' Crucial Science & Environment Book - 8 126

D. Answer these questions:
1. What is polarization? How does it occur? How is polarization
removed from a simple cell.
2. Explain the working of a simple cell.
3. Explain the structure of a dry cell with neat and labelled diagram.
4. Explain the working of a dry cell.
5. What is series combination of cells? Where is it used? What is its
advantage?

E. Study the diagram and answer the questions:
:KDW LV VKRZQ LQ WKH GLDJUDP" 'HÀQH LW
2. Draw the diagram and label its parts.
3. Name the electrolyte used in such cells.

Take some dry cells, copper wire and a bulb of a torch light. Connect the cells
in series and parallel and observe the brightness in bulb in each case. Write
down your observation.

127 Times' Crucial Science & Environment Book - 8

11CHAPTER Matter

Antoine Lavoisier
He is known for studying the properties of Hydrogen and
oxygen. He is sometimes known as the father of chemistry.

Estimated Periods : 7
Objectives: At the end of the chapter, the students will be able to:

H[SODLQ WKH FKDUDFWHULVWLFV RI HOHFWURQ SURWRQ DQG QHXWURQ
H[SODLQ YDOHQF\ DQG WHOO WKH ZD\V RI LGHQWLI\LQJ YDOHQF\ RI ILUVW HOHPHQWV
GHILQH DWRPLF QXPEHU DQG DWRPLF PDVV RI WKH HOHPHQWV JLYH D VKRUW LQWURGXFWLRQ RI

SHULRGLF WDEOH

Can you name the three states of matter? What are they?
Why are solids hard? Why can’t they flow?
What are the substances made of?
How are the smallest particles of substance represented? Discuss.

Introduction

We study about different forms of matter and its properties in chemistry.
Chemistry is the branch of science in which we study about composition,
properties and change of matter.
Matter is anything that has mass and occupies space. Matter is composed of
a number of extremely small particles called atoms. Hence, an atom is the
smallest particle of an element which can take part in a chemical reaction. An
element consists of only one kind of atoms. But, an atom of one element differs
from that of another element. For example, the atoms of sodium are all alike
but they differ from those of iron.
An atom alone may or may not exist freely. For example, the atoms of hydrogen,
oxygen and chlorine cannot exist freely. They combine with other atoms to
form molecules. But the atoms of helium, argon, etc exist freely. A molecule is
the smallest particle of an element or compound which can exist freely.

Times' Crucial Science & Environment Book - 8 128

The molecules of an element are made up of the atoms of the same element. For
example, hHy2dirsogaemn.olBeucut ltehoef hydrogen which contains two atoms of the same
element, molecule of a compound consists of two or more
atoms owf hdiicfhferceonnttaeilnesmtewnotsa.tFomorseoxfahmypdlreo,gHen2Oainsdaomneolaetcoumle of the compound,
water, of oxygen.

Differences between atom and molecule

Atom Molecule

1. It is the smallest particle of an element 1. It is the smallest particle of an
that can take part in a chemical reaction. element or compound that is
Atoms may or may not exist freely. capable of free and stable existence.

2. Atoms can combine to give rise to a 2. A molecule may be decomposed
molecule. into atoms.

3. For example, H is an atom of hydrogen. 3. For example, H2 is a molecule of
hydrogen.

Structure of an atom

An atom is composed of very tiny particles called electron, proton and neutron.
These particles are collectively known as sub-atomic particles, elementary
particles or fundamental particles of an atom.

Electron

Protons Nucleus
Neutrons

Shell

Structure of an atom

Protons and neutrons are located in the nucleus. The nucleus lies at the centre
RI WKH DWRP 7KH HOHFWURQV UHYROYH DURXQG WKH QXFOHXV LQ À[HG RUELWV FDOOHG
shells or energy levels.

Electron

An electron is a negatively charged particle that
revolves around the nucleus of an atom. Each
electron carries a unit negative charge (i.e. the
charge of an electron is ï ). The electron is denoted
by Hï. An electron is the lightest particle among
the sub-atomic particles. The elliptical path along
which an electron revolves around the nucleus is ůůŝƉƟĐĂů ƐŚĞůůƐ ĂƌŽƵŶĚ ƚŚĞ ŶƵĐůĞƵƐ
called shell or orbit. The mass of an electron is about 1/1837 times the mass of
a hydrogen atom.

129 Times' Crucial Science & Environment Book - 8

Proton

Protons are the positively charged sub-atomic particles present in the nucleus
of an atom. A proton is denoted by p+. The charge of a proton is +1. It is exactly
the opposite electric charge as that of an electron. The number of protons in
the nucleus determines the total quantity of positive charge in the atom. The
mass of a proton is equal to the mass of a hydrogen atom.

Neutron

A neutron is a neutral sub-atomic particle present in the nucleus of an atom.
A neutron is denoted by n°. The mass of a neutron is nearly equal to the mass
of a hydrogen atom. All elements except hydrogen contain neutron.
Comparative study of electron, proton and neutron

Sub-atomic particle Symbol Mass Charge Location
+1 Nucleus
Proton p+ 1 amu ï Shell
Electron Hï 1/ 1837 amu 0 Nucleus
Neutron n° 1 amu

Atomic mass unit (amu)

As the mass of the sub-atomic particles is extremely small, it is quite
inconvenient to express their mass in terms of milligram, gram, etc. The
atomic mass unit is the unit of measuring extremely small masses. The mass
of electron, proton and neutron is expressed in terms of atomic mass unit
(amu). In this unit, the mass of a hydrogen atom is taken as equal to 1 amu.
This 1 amu is the mass of a proton because hydrogen has no neutron and
electron present in it has a negligible mass.

1 amu = 1.661×10–24 gram
1 gram = 6×1023 amu
It means that the mass of 6×1023 protons is equal to 1 gram. The mass of a
proton and a neutron is nearly same. But the electrons are extremely light.
The mass of 1837 electrons is equal to the mass of a proton. Hence, the mass
of an electron is expressed as 1/1837 th the mass of a proton.
1p = 1n = 1837e = 1 amu

Electric charge

Electrons and protons have a special property called electric charge. Electrons
carry a negative charge while protons carry a positive charge. However, an
atom is electrically neutral because the negative charge of its electrons is
exactly neutralized by the positive charge of its protons in an atom.

Times' Crucial Science & Environment Book - 8 130

The electric charge is measured in the unit Coulomb. In short form, Coulomb
is written as Coul or C only. The absolute charge of an electron is –1.6×10–19C.
Hence,

1 Coulomb = 6.25×1018 electrons

Atomic number (Z)

Altogether 118 elements are discovered so far. But, each of these elements has
a unique atomic number. No two elements have the same atomic number. The
atomic number is determined by the number of protons present in the nucleus.
Hence, WKH DWRPLF QXPEHU PD\ EH GHÀQHG DV WKH QXPEHU RI SURWRQV SUHVHQW LQ
the nucleus of an atom. Since the number of protons is equal to the number of
HOHFWURQV LQ D QHXWUDO DWRP WKH DWRPLF QXPEHU LV DOVR GHÀQHG DV WKH QXPEHU
of electrons present in a neutral atom.
Hence,

Atomic number = No. of protons = No. of electrons
? Z = p or e
For example, hydrogen contains one proton in its atom. So, its atomic number
(z) is 1. Similarly, sodium contains 11 protons in its nucleus. So, its atomic
number is 11.
The atomic number determines the properties of an element.
Atomic mass (A)
As already discussed, the mass of an electron in an atom is negligible. Hence,
the mass of an atom is determined by the combined mass of protons and
neutrons present in the nucleus. It is the relative weight and doesn’t have
unit. Thus, WKH DWRPLF PDVV RI DQ HOHPHQW LV GHÀQHG DV WKH VXP RI PDVVHV RI
protons and neutrons in an atom. It is also known as atomic weight.

Atomic mass = No. of protons + No. of neutrons
? A=p+n
The actual mass of a hydrogen atom or a proton is 1.6 × 10ï grams, which is
extremely small. So, for the sake of convenience, the mass of hydrogen atom is
taken as equal to 1. The mass of any other element is determined by comparing
with the mass of a hydrogen atom.

S.N. Element Symbol No. of No. of No. of Atomic
electrons protons neutrons Mass
1. Hydrogen H 1
2. Helium He 1 1 0 4
3. Lithium Li 2 2 2 7
3 3 4

131 Times' Crucial Science & Environment Book - 8

4. Beryllium Be 4 4 5 9

5. Boron B 5 5 6 11

6. Carbon C 6 6 6 12

7. Nitrogen N 7 7 7 14

8. Oxygen O 8 8 8 16

9. Fluorine F 9 9 10 19

10. Neon Ne 10 10 10 20

11. Sodium Na 11 11 12 23

12. Magnesium Mg 12 12 12 24

13. Aluminium Al 13 13 14 27

14. Silicon Si 14 14 14 28

15. Phosphorus P 15 15 16 31

16. Sulphur S 16 16 16 32

17. Chlorine Cl 17 17 18 35

18. Argon Ar 18 18 22 40

19. Potassium K 19 19 20 39

20. Calcium Ca 20 20 20 40

Representation of an element

An element can be represented in terms of its symbol, atomic number and
atomic mass. It can be written as:

A X or ZXA
Z
Where, X = Symbol of an element

Z = Atomic number of the element

A = atomic mass of the element

For example, if an iesle2m0eanntdisitsreaptroemseicntmedasassis42004C0a. ,Ititamlsoeasnhsowthsatthtahtecaaltcoimumic
number of calcium
has 20 protons, 20 electrons and 20 neutrons.

Molecular weight

A molecule of a compound consists of two or more atoms of different kinds. The
molecular weight of a compound is determined by adding the atomic weights
of all atoms present in the molecule. Hence, the sum of atomic weights of all
the atoms of a molecule is called molecular weight.

For example,

Molecular weight of water (H2O) = 2 × H + 1 × O
= 2 × 1 + 1 × 16 = 18 amu

Times' Crucial Science & Environment Book - 8 132

Similarly,
Molecular weight of ammonia (NH3) = 1 × N + 3 × H
= 1 × 14 + 3 × 1 = 17amu

(OHFWURQLF FRQÀJXUDWLRQ

7KH HOHFWURQV RI DQ DWRP UHYROYH DURXQG WKH QXFOHXV LQ FHUWDLQ VSHFLÀHG SDWKV
called orbits or shells. These shells are also known as energy levels. The shells
near the nucleus have low energy and are called lower energy levels whereas
the shells that lie away from the nucleus are called higher energy levels.
These energy levels or shells are represented by the letters, K, L, M, N,…, etc
or the numbers 1, 2, 3, 4, ………, etc. Electrons are distributed in these shells
according to certain rules. The systematic distribution of electrons in different
HQHUJ\ OHYHOV RU VKHOOV LV NQRZQ DV HOHFWURQLF FRQÀJXUDWLRQ.
The different shells around the nucleus have different electron-holding
capacity. Neils Bohr and Bury (1921 AD) proposed a scheme for the distribution
of electrons in different shells; which is given below:
1. The maximum number of electrons that can be accommodated in a shell

is determined by 2n2 rule, where n is the number of shell.
For example:

+7 6 5 4 3 2 1 K LMNOPQ

2
8
18
32
50
72

98

)RU . VKHOO Q EHFDXVH LW LV WKH ÀUVW VKHOO IURP WKH QXFOHXV

So, maximum no. of electrons, 2n2 = 2 × (1)2 = 2

Similarly, for L-shell maximum number of electrons, 2n2 = 2 × (2)2 = 8

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

For N-shell, maximum number of electrons, 2n2 = 2 × (4)2 = 32 and so on.

But, this is not the single rule governing the distribution of electrons.
The 2n2 rule is limited by some other rules.

133 Times' Crucial Science & Environment Book - 8

2. The outermost shell of an atom cannot have more than 8 electrons and
the shell just inner to it cannot contain more than 18 electrons.

,W LV QRW QHFHVVDU\ IRU D VKHOO WR EH FRPSOHWHO\ ÀOOHG EHIRUH D QHZ VKHOO VWDUWV
WR EH ÀOOHG It is determined by the relative energy of the sub-shells.

7KH DWRPLF VWUXFWXUHV RI ÀUVW HOHPHQWV ZLWK WKHLU HOHFWURQLF
FRQÀJXUDWLRQ LV JLYHQ EHORZ

p=1 p=2 p=3 p=4
n=0 n=2 n=4 n=5

,LJĚƌŽŐĞŶ Helium >ŝƚŚŝƵŵ Beryllium

p=5 p=6 p=7 p=8
n=6 n=6 n=7 n=8

Boron Carbon Nitrogen Oxygen

p=9 p = 10 p = 11 p = 12
n = 10 n = 10 n = 12 n = 12

Fluorine Neon ^ŽĚŝƵŵ DĂŐŶĞƐŝƵŵ

p = 13 p = 14 p = 15 p = 16
n = 14 n = 14 n = 16 n = 16

Aluminium Silicon WŚŽƐƉŚŽƌƵƐ ^ƵůƉŚƵƌ

p = 17 p = 18 p = 19 p = 20
n = 18 n = 22 n = 20 n = 20

ŚůŽƌŝŶĞ Argon WŽƚĂƐƐŝƵŵ Calcium

6LJQLÀFDQFH RI HOHFWURQLF FRQÀJXUDWLRQ

7KH HOHFWURQLF FRQÀJXUDWLRQ RI HOHPHQWV LV WKH EDVLV IRU WKH V\VWHPDWLF VWXG\
of elements. It helps to determine valency of an element. It also determines
the position of an element in the periodic table.

The maximum electron-holding capacity of K-shell is 2. An element having
single shell becomes stable if it contains 2 electrons. This is known as duplet.
For example, hydrogen has single electron in K-shell. So, it tries to gain one
more electron to attain duplet. Since one electron is required by hydrogen to

Times' Crucial Science & Environment Book - 8 134

make it stable, the valency of hydrogen is 1(one). The valency of helium is zero
(Why?). Similarly, the valency of other elements can be determined.
For the elements having 1-4 electrons in outermost shell,

Valency = No. of electrons in outermost shell (Except helium)
For example; carbon has 4 electrons in its outermost shell, so its valency is 4.
For elements having 5-8 electrons in the outermost shell,

Valency = 8 - No. of electrons in outermost shell.
Nitrogen has 5 electrons in its outermost shell.

So, valency of nitrogen = 8 - 5 = 3 and so on.

S.N. Element Symbol (OHFWURQLF FRQÀJXUDWLRQ Valency
K LMN
1. Hydrogen H 1 1
2. Helium He 2 0
3. Lithium Li 21 1
4. Beryllium Be 22 2
5. Boron B 23 3
6. Carbon C 24 4
7. Nitrogen N 25 3
8. Oxygen O 26 2
9. Fluorine F 27 1
10. Neon Ne 28 0
11. Sodium Na 281 1
12. Magnesium Mg 282 2
13. Aluminium Al 283 3
14. Silicon Si 284 4
15. Phosphorus P 285 3
16. Sulphur S 286 2
17. Chlorine Cl 287 1
18. Argon Ar 288 0
19. Potassium K 2881 1
20. Calcium Ca 2882 2

The elements helium, neon and argon have their outermost shell completely
ÀOOHG 6R WKH\ GR QRW UHTXLUH DQ\ HOHFWURQV IRU JDLQLQJ GXSOHW RU RFWHW 7KXV
the valency of these elements is zero. The state of having eight electrons in the
outermost shell of an atom is called octet. These elements do not react with

135 Times' Crucial Science & Environment Book - 8

RWKHU HOHPHQWV GXH WR WKHLU VWDEOH HOHFWURQLF FRQÀJXUDWLRQ +HQFH WKH\ DUH
known as inert gases.

7KH HOHPHQWV ZKRVH YDOHQFH VKHOOV DUH FRPSOHWHO\ ÀOOHG DQG GR QRW SDUWLFLSDWH
in chemical reactions are called inert gases.

&ODVVLÀFDWLRQ RI HOHPHQWV 3HULRGLF 7DEOH

In the ancient time, very few elements were known to the human beings.
So, they could be studied individually. But later on, it was quite tedious and
GLIÀFXOW WR VWXG\ WKH HOHPHQWV LQGLYLGXDOO\ EHFDXVH RI WKHLU ODUJH QXPEHU 6R
scientists realized to categorize the elements for making their study easy.
Many attempts were made by several chemists to classify the elements. But
WKH VLJQLÀFDQW RQH ZDV WKDW GHYHORSHG E\ D 5XVVLDQ VFLHQWLVW Dmitri Mendeleev
LQ $ ' +H FODVVLÀHG DOO WKH NQRZQ HOHPHQWV RI WKDW WLPH LQ WKH RUGHU
of increasing atomic mass. He found that the elements with similar properties
appeared at regular intervals.

7KH V\VWHPDWLF FODVVLÀFDWLRQ RI HOHPHQWV LQ GHÀQLWH URZV DQG FROXPQV RQ WKH
basis of similarities in properties is known as periodic table. There are two
forms of periodic table:

a) Mendeleev’s periodic table b) Modern periodic table

Mendeleev’s periodic table

7KH V\VWHPDWLF FODVVLÀFDWLRQ RU DUUDQJHPHQW RI HOHPHQWV LQ D WDEXODU IRUP
on the basis of their atomic weight is called Mendeleev’s periodic table. This
table is based on a periodic law which states that the physical and chemical
properties of the elements are periodic function of their atomic weights.

The law means that if the elements are arranged in the increasing order of
their atomic weight, the properties of the elements are repeated. Mendeleev
found that the elements with similar properties happened to fall in the same
group one after another.

0HQGHOHHY·V RULJLQDO IRUP RI 3HULRGLF 7DEOH

Group Group Group Group Group Group Group Group

I II III IV V VI VII VIII

Period 1 H
Period 2
Period 3 Li Be B C N O F
Period 4
Na Mg Al Si P S Cl

K Ca 1* Ti V Cr Mn Fe Co

Cu Zn 2* 3* As Se Br Ni

Times' Crucial Science & Environment Book - 8 136

Period 5 5E Sr Y Zr Nb Mo 4* 5X 5K
Period6
Ag Cd In Sn Sb Te I Pd

Ca Ba La Hf Ta W 5H Cs Ir

Au Hg Th Pd Bi Po At Pt

Names given by Mendeleev:

1*: Eka-boron, 2*: Eka-aluminium, 3*: Eka-Silicon, 4*: eka-Manganese

0HQGHOHHY FODVVLÀHG HOHPHQWV LQWR VL[ KRUL]RQWDO URZV DQG HLJKW YHUWLFDO
columns in his original periodic table. The horizontal rows in the periodic
table are known as periods whereas the vertical columns of the periodic table
are known as groups. In Mendeleev’s periodic table, the elements with similar
properties were in the same group and the elements with gradual change
in properties were in a period. While arranging elements on the basis of
increasing atomic mass, Mendeleev left some gaps for undiscovered elements
and proposed names for them. It is because the known elements did not match
properties for placing them in those gaps. The names proposed by Mendeleev
for undiscovered elements were Eka-boron, Eka-aluminium, eka-silicon and
Eka-manganese.

The zero group consisting of the inert gases such as helium, neon, argon, etc
were not discovered at the time of Mendeleev. So, there was no place for the
inert gases in the Mendeleev’s original periodic table.

Advantages (merits) of Mendeleev’s periodic table

,W ZDV WKH ÀUVW V\VWHPDWLF FODVVLÀFDWLRQ RI HOHPHQWV LQ WKH IRUP RI WDEOH

2. It promoted the discovery of new elements by leaving gaps.

3. It helped in the systematic and easier study of the properties of several
elements at a time

Demerits of Mendeleev’s periodic table

1. Hydrogen is a non-metal but it is kept along with metals in group I.
Hence, the position of hydrogen is not properly explained.

2. Chemically dissimilar elements have been placed in the same group.
For example, copper, silver and gold are placed in group I with the soft
metals like lithium, sodium and potassium. Their properties are quite
different.

3. Certain elements having high atomic mass have been placed before those
with lower atomic mass. For example, argon (atomic mass:40) has been
placed before potassium (atomic mass: 39) in the Mendeleev’s periodic table.

4. There was no separate position for metals and non-metals.

137 Times' Crucial Science & Environment Book - 8

5. There was no place for inert gases in the Mendeleev’s original periodic
table.

Modern periodic table
$OWKRXJK 'PLWUL 0HQGHOHHY LV WKH SLRQHHU LQ WKH V\VWHPDWLF FODVVLÀFDWLRQ RI
elements, his periodic table has more demerits than merits. The demerits of
Mendeleev’s periodic table led to the conclusion that atomic mass cannot be the
IXQGDPHQWDO EDVLV IRU WKH FODVVLÀFDWLRQ RI HOHPHQWV $IWHU WKH GHWDLOHG VWXG\
of physical and chemical properties of elements, Henry Moseley and others
proposed that atomic number is more fundamental property of an element
than atomic mass. So, they developed a new periodic table known as modern
periodic table.
7KH V\VWHPDWLF FODVVLÀFDWLRQ RU DUUDQJHPHQW RI HOHPHQWV LQ D WDEXODU IRUP RQ
the basis of their atomic number is called Modern Periodic Table. The modern
periodic table is based on modern periodic law which states that the physical
and chemical properties of elements are periodic function of their atomic
number.
The demerits of Mendeleev’s periodic table were solved by the modern periodic
table. They are as follows:
1. Hydrogen is placed in group I because it has one electron in the valence

VKHOO DQG LWV RXWHU HOHFWURQLF FRQÀJXUDWLRQ LV VLPLODU WR WKDW RI DONDOL
metals.
2. Chemically similar elements lie themselves in the same group while
arranging on the basis of atomic number.
3. The wrong position of elements due to atomic weight is solved while
arranging the elements in the increasing order of atomic number.

Times' Crucial Science & Environment Book - 8 138

7KH 3HULRGLF 7DEOH RI (OHPHQWV /RQJ )RUP

139 Times' Crucial Science & Environment Book - 8

&KDUDFWHULVWLFV RI 0RGHUQ 3HULRGLF 7DEOH

The modern periodic table consists of nine groups and seven periods. They are
explained in brief as follows:

Periods

The horizontal rows in the periodic table are called periods. The periods are
numbered as 1, 2, 3, 4, 5, 6 and 7. The elements of the same period have
the equal number of shells. But their chemical properties are quite different
because of different number of electrons.
(DFK SHULRG KDV D À[HG QXPEHU RI HOHPHQWV 7KH QXPEHU RI HOHPHQWV LQ HDFK
period is equal to the maximum number of electrons that can be accommodated
LQ HDFK VKHOO )RU H[DPSOH . VKHOO KROGV PD[LPXP RI HOHFWURQV DQG WKH ÀUVW
period has 2 elements. Similarly, L shell holds a maximum of 8 electrons and
the 2nd period has 8 elements and so on.

Period No. of elements Type
Period 1 2 Very short
Period 2 8 Short
Period 3 8 Short
Period 4 18 Long
Period 5 18 Long
Period 6 32 Very Long
Period 7 32 Very Long

There is gradual change in the properties of elements lying in the same period.

Groups

The vertical columns of the periodic table are called groups. There are
altogether 18 groups. The groups are named in numerals as groups 1, 2, 3, 4,
........., 16, 17 and 18. The group 18 is also known as Zero (0) group. Each group
consists of a single column of elements. The group zero (0) consists of single
column of non-reactive gases.

Group No. of valence Elements Special name
electrons
H, Li, Na, K, etc. Alkali metals (except hydrogen)
11 Be, Mg, Ca, etc. Alkaline earth metals
B, Al, Ga, etc. Boron family
22 C, Si, Ge, etc. Carbon family
N, P, As, etc. Nitrogen family
13 3

14 4

15 5

Times' Crucial Science & Environment Book - 8 140

16 6 O, S, etc. Oxygen family
17 7
18 8 (2 in He) F, Cl, Br, I, etc. Halogens

He, Ne, Ar, Kr, Noble or inert gases
etc.

In this unit, we study about the elements of some groups in the periodic table.

&ODVVLÀFDWLRQ RI ÀUVW HOHPHQWV

7KH ÀUVW WZHQW\ HOHPHQWV LQFOXGH WKH HOHPHQWV KDYLQJ DWRPLF QXPEHU WR
You can make a mini-periodic table by classifying these elements.

Group 1 2 13 14 15 16 17 18
Period
H He
1
2 Li Be B C N O F Ne
3
4 Na Mg Al Si P S Cl Ar

K Ca

The elements of group 1 have single electron in their valence shell. These
elements can easily lose one electron during a chemical reaction and gain
a positive charge. Hence, these elements are the reactive elements. Except
hydrogen, all other elements are metals and they form soluble bases by
reacting with water. So, these metals are called alkali metals.
7KH JURXS FRQVLVWV RI WKH HOHPHQWV VXFK DV ÁXRULQH ) FKORULQH &O
bromine (Br), etc. It is the group of very reactive non-metals. These elements
are very reactive because they contain seven electrons in their valence shell
and can easily gain one electron from other elements to gain a negative charge.
The elements of group 1 and 17 combine to form stable compounds. For example,
hydrogen combines with chlorine to form hydrochloric acid (HCl), sodium reacts
with chlorine to form sodium chloride or common salt (NaCl), etc.

Importance of modern periodic table

1. It makes the study of elements easier and systematic.
2. It helps to explain the properties of elements on the basis of their atomic

number.
3. It helps in the discovery of new elements by predicting their position and

properties.
4. It helps to compare the atomic properties of the elements.

)LQGLQJ WKH JURXS DQG SHULRG QXPEHU RI DQ HOHPHQW

141 Times' Crucial Science & Environment Book - 8

The group or period on which an element lies can be determined with the help
RI HOHFWURQLF FRQÀJXUDWLRQ RI DQ HOHPHQW )RU JHQHUDO UXOH

Group of an element = No. of valence electrons ( Group 1 and 2)

Group of an element = 10 + No. of valence electrons (For other groups)

Period of an element = Total number of shells in an atom

/HW XV WDNH DQ H[DPSOH RI SRWDVVLXP ,WV HOHFWURQLF FRQÀJXUDWLRQ LV

Here, group of potassium = No. of electrons in valence shell

=1

= Group 1.

Period of potassium = Total no. of shells

=4

= 4th period.

Hence, potassium lies in group 1 and 4th period.

extremely : to a very great deree
decomposed : broken down
ƐŝŐŶŝĮĐĂŶĐĞ : importance
elementary : basic
negligible : very low

1. An atom is the smallest particle of an element which can take part in a chemical
reaction.

2. A molecule is the smallest particle of an element or compound which can exist
freely.

3. Electron, proton and neutron are known as sub-atomic particles.

4. The electrons revolve around the nucleus in fixed orbits called shells or energy levels.

5. The atomic number may be defined as the number of protons present in the nucleus
of an atom.

6. The atomic mass of an element may be defined as the sum of masses of protons and
neutrons in an atom.

7. The sum of atomic weights of all the atoms of a molecule is called molecular weight.

8. The systematic distribution of electrons in different energy levels or shells is
known as electronic configuration.

9. The systematic classification of elements in definite rows and columns on the basis

Times' Crucial Science & Environment Book - 8 142

of similarities in properties is known as periodic table.

10. There are two laws regarding the periodic table. They are: Mendeleev’s periodic
law and modern periodic law.

11. Mendeleev’s periodic law states that the physical and chemical properties of
elements are periodic function of their atomic weights.

12. Modern periodic law states that the physical and chemical properties of elements
are periodic function of their atomic number.

Exercise

A. Answer these questions in very short.
1. What is chemistry?
2. What is an atom?
3. What is the relative charge of an electron?
4. What does a nucleus contain?
5. Write down the atomic number of magnesium and argon.
+RZ PDQ\ HOHPHQWV ZHUH FODVVLÀHG E\ 'PLWUL 0HQGHOHHY LQ KLV
original periodic table?
7. How many elements are there in 2nd period?
8. In which group and period does oxygen lie?
9. Name the group which contains three columns of elements but no
sub-group.
10. What is the basis of modern periodic table?

% 'HÀQH ZLWK H[DPSOHV

1. Atomic number 2. Atomic mass 3. Molecule
6. Alkali metals
4. Inert gases 5. Halogens

7. Octet

& *LYH UHDVRQV
1. The valency of sodium is one.
2. Argon is an inert gas.
3. The elements of group 1 are called alkali metals.
4. Argon is placed before potassium in modern periodic table.

143 Times' Crucial Science & Environment Book - 8

5. Hydrogen is placed in group 1 in the modern periodic table.
6. An atom is electrically neutral.
7. Mendeleev left some gaps in his periodic table.

' :ULWH GRZQ GLIIHUHQFHV EHWZHHQ
1. Atom and molecule
2. Atomic weight and molecular weight
3. Mendeleev’s periodic table and modern periodic table
4. Group and period

( $QVZHU WKHVH TXHVWLRQV

1. What are the fundamental particles of an atom?

2. Compare the properties of electron, proton and neutron.

3. What is 2n2 rule? Explain with examples.

4. State Mendeleev’s periodic law. Write down the merits of
Mendeleev’s periodic table.

5. What are the defects of Mendeleev’s periodic table?

6. Write down the features of modern periodic table.

7. Write down the uses of periodic table.

:KDW LV HOHFWURQLF FRQÀJXUDWLRQ" :ULWH GRZQ WKH UXOHV WKDW JRYHUQ
HOHFWURQLF FRQÀJXUDWLRQ RI DQ HOHPHQW

9. Write down any two compounds that are formed by the chemical
combination of elements of group 1 and 17 elements.

10. Give two examples each of very reactive metals and non-metals.

11. What do you understand from the symbol 1399K?

) &DOFXODWH WKH PROHFXODU ZHLJKW RI WKH IROORZLQJ PROHFXOHV

i. H2O ii. CO2 iii. NH3
iv. CH4 v. CaCO3 vi. O2

* 'LDJUDPPDWLF TXHVWLRQV

1. Name the elements having the following nuclei:

1p 20 p 18 p 12 p 19 p
20 n 22 n 12 n 20 n

Times' Crucial Science & Environment Book - 8 144

2. Study the diagram and answer the questions:

11 p 17 p
12 n 18 n

AB
i. Name the elements A and B.
ii. Write down the group and period of A and B.
iii. What is the valency of element B? Why?
iv. Write down the name of compound formed if A and B combine.
+ &RQFHSWXDO TXHVWLRQV
1. An element with 22 neutrons has atomic mass 40. What is the

number of protons of that element? Name the element and draw its
atomic structure.
2. Draw the atomic structure of the element which contains 11 protons.
Write down the name of the element. What is its valency? Why?
3. An element contains nine protons and 10 neutrons. Calculate its
atomic mass.

145 Times' Crucial Science & Environment Book - 8

12CHAPTER Language of
Chemistry

Jöns Jacob Berzelius

The Swedish chemist Jöns Jacob Berzelius (1779-1848) was one
RI WKH ÀUVW (XURSHDQ VFLHQWLVWV WR DFFHSW -RKQ 'DOWRQ
V DWRPLF
theory and to recognize the need for a new system of chemical

V\PEROV +H ZDV D GRPLQDQW ÀJXUH LQ FKHPLFDO VFLHQFH

Estimated Periods : 7
Objectives: At the end of the chapter, the students will be able to:

GHILQH V\PERO DQG WHOO WKH V\PEROV RI VRPH FRPPRQ HOHPHQWV
H[SODLQ YDOHQF\ DQG UDGLFDOV
GHILQH PROHFXODU IRUPXOD DQG ZULWH GRZQ WKH PROHFXODU IRUPXODH RI VRPH FRPPRQ

FRPSRXQGV
GHILQH FKHPLFDO HTXDWLRQ DQG FKHPLFDO UHDFWLRQ DQG ZULWH VRPH FKHPLFDO UHDFWLRQV LQ

WHUPV RU ZRUGV DQG IRUPXODH

How many elements are there?
Can we represent these elements by their symbols?
Can compounds be represented by symbols?
What do you mean by molecular formula? Discuss..

Introduction

We study about elements and compounds in chemistry. The number of
elements is less, i.e. 118 but there are millions of compounds which are formed
by the chemical combination of these elements. Some of the compounds are
formed by the combination of only two elements and contain few atoms. But
some of the compounds are so complex that they contain hundreds of atoms
RI GLIIHUHQW HOHPHQWV 6R LW ZRXOG KDYH EHHQ TXLWH WHGLRXV DQG GLIÀFXOW WR
represent those compounds if they were not represented in terms of symbols
and formulae. Hence, the use of chemical symbols and formulae has made the
study of chemistry easy and systematic. The use of symbols, formulae and
structures to represent the molecules or chemical reactions is called language
of chemistry.

Symbol

The short and abbreviated form for the full name of an element is known as
symbol. The symbols of elements are mainly taken from their English names

Times' Crucial Science & Environment Book - 8 146