Modern Concept Science and Environment - 7 93
ACTIVITY 2
1. Take a plane mirror and erect it on a stand. Plane mirror
2. Put a pencil perpendicular to the mirror surface. Q Plastic pipe
3. Take two short pieces of a pipe and incline them equally Plastic pipe
from the normal in front of the mirror. P R
4. Put a burning candle at the edge of one tube and observe Eye Burning
candle
it from the opening of another tube. Do you observe the
flame of the candle trough another tube?
Types of Reflection
There are two types of reflection. They are regular reflection and irregular reflection.
a) Regular Reflection:
In a regular reflection, light is thrown back in a well-defined
direction. If a parallel beam of light falls on a reflecting surface
and the reflected light is also a parallel beam, then the reflection
is known as a regular reflection. Regular reflection is possible Regular reflection
on smooth surfaces. For example,there is a regular reflection on a still water surface,
polished metallic surface, etc.
b) Irregular Reflection MEMORY PLUS
If a parallel beam of light In absence of irregular reflections, Irregular reflection
falls on a reflecting surface object would appear either bright or
and the reflected light is not dark and they are not easily seen.
a parallel beam, then the reflection is known as an irregular reflection. This type of
reflection takes place when the surface is not smooth. For example, wall, tree, etc.
FACT WITH REASON
Why does a reflection in a plane mirror form image?
Reflection in a plane mirror forms image because of the regular reflection.
A smooth surface shines. Why?
A regular reflection takes place on a smooth surface. When the reflected light from a smooth
surface enters our eyes, the image of the objects seen is very sharp and distinct. Thus a smooth
surface shines.
A clear and sharp image is formed on calm water. Why?
A clear and sharp image is formed on calm water due to the regular reflection of light from the
water surface. If water surface is disturbed, an irregular reflection occurs and the sharp image
gets blurry.
A rough surface does not shine
An irregular reflection takes place on a rough surface. When the reflected light from a rough surface
enters our eyes, the image of the objects seen is blurry. Thus a rough surface does not shine.
94 Light
7.4 Image
An image of an object is formed in a smooth surface when light falls on it. The images
formed are of two types. They are real image and virtual image.
a) Real Image
An image which can be obtained on a screen is called a real image. Such kind of
image is formed when the rays of light actually meet or intersect after reflection. For
example, the image formed on our retina, image formed by the lens in camera, etc.
FACT WITH REASON
Our face is seen upside down if you look very close in metal spoon or bowl. Why?
If we look very close in steel spoon or bowl, our face will be seen upside down because it is
real image.
b) Virtual Image
An image which cannot be obtained on a screen is MEMORY PLUS
called a virtual image. It is formed when the light
rays appear to meet after reflection. For example, the A real image is always inverted and a
virtual image is always erect.
image formed inside of a plane mirror, image formed in water, etc.
7.5 Mirror
A smooth polished surface from which regular reflection can take place is called a mirror.
Characteristics of Mirror
i) The surface of a mirror shines.
ii) There is a formation of image in the mirror due to a regular reflection from it.
Types of Mirror
Mirrors are of two types. They are plane mirror and spherical mirror.
Plane Mirror
The mirror which has flat and smooth reflective surface is called plane mirror.
Characteristics of the Image Formed by a Plane Mirror
Image formed by a plane mirror is laterally inverted.
The phenomenon due to which the image of an object
turns through an angle of 1800 about a vertical axis is
called a lateral inversion. Due to the lateral inversion,
the left side of an object looks to be on the right side in
the image formed by a mirror and vice versa. The main Image formed by a plane mirror
characteristics of the image are:
Modern Concept Science and Environment - 7 95
i) Image is virtual.
ii) It is erect.
iii) It is of the same size as the object.
iv) Distance between image and mirror is the same as the distance between object
and mirror.
FACT WITH REASON
On the hospital vans, the word AMBULANCE is written in the form of its mirror image. Why?
A plane mirror forms a laterally inverted image. The mirror image word,
'AMBULANCE' written on a hospital van, can be seen in its correct form from the
rear-view mirror of another vehicle ahead. It helps the drivers of another vehicle
to give side quickly.
Applications of Plane Mirror MEMORY PLUS
i) It is commonly used as a looking glass in daily Plane mirrors are used in solar
life. cookers.
ii) Plane mirrors are used as reflectors in optical instruments like periscope,
kaleidoscope, etc.
7.8 Periscope
A periscope is an optical device that allows us to see over walls. It is based on the laws
of reflection.
Construction A 45° Plane mirror (M1)
45°
It is made from a tube or card-board Tree to be seen B
bent twice at right angles. Such double 45°
bending makes two openings at both Plane mirror (M2)
upper and lower ends facing opposite 45°
of one another. Two plane mirrors are 45°
adjusted at the end of each opening in
such a way that the light rays fall on 45°
them at an angle of 45o as shown in the
given figure. Periscope
Working
The light rays falling at an angle of 45o on the mirror facing the upper opening get
reflected downward. These rays again fall at an angle of 45o on the mirror facing the
lower opening. Finally, the light rays from the upper opening enter to the eyes of an
observer through the lower opening.
96 Light
Uses MEMORY PLUS
A periscope is used to observe objects at a height, The word periscope is derived from two
which is being obstructed. Greek words. “Peri” means “around,”
a) Periscope is used in submarines : Submarines and “scopus” means “to look”.
can run inside water in a submerged condition. The ships and other bodies over
the surface of water are observed from a submarine through a periscope.
FACT WITH REASON
Periscope is used in submarines. Why?
Periscope is used in submarines because it reflects light and helps to observe objects at the surface of water.
b) Periscope is used for security purpose : Soldiers sitting in a trench observe the
activities of enemies over the ground through a periscope.
7.8 Kaleidoscope
A kaleidoscope is a simple instrument based on the laws of reflection. It has been a
popular toy for years. MEMORY PLUS Patterns formed
in kaleidoscope
Construction Kaleidoscopes were invented in 1816 by
David Brewster a Scottish inventor.
In a kaleidoscope three
rectangular strips of plane mirror are used. These three
strips are of an equal size. These strips are placed with
their edges touching as shown in the figure. The angle
between edges is 600. Mirrors are wrapped with a paper Tape Mirror
and tied by a piece of a thread. A few bits of colored- Cardboard
glass are placed in the space between the mirrors.
Kaleidoscope
Working
When we look through the viewing hole at one end of a kaleidoscope and rotate the
tube, we see a changing variety of color patterns. Such designs are produced when the
mirrors in kaleidoscope reflect images of the colored-glass.
Use : Kaleidoscope is used as a toy.
STEPS EXERCISE
STEP 1
1. Tick () the correct statement and cross () the incorrect one.
a) Candle is a source of light.
b) Light travels in a curve line.
c) Objects are visible due to light reflected from them.
Modern Concept Science and Environment - 7 97
d) A regular reflection occurs from a plane mirror.
e) The surface of a mirror show irregular reflection.
f) A periscope is a device used to see objects from the sea water.
g) A kaleidoscope is a toy made by three plane mirrors.
2. Fill in the blanks with appropriate words.
a) Objects are not visible without ……. .
b) The main natural source of light is …… .
c) Out of all the light sources available, some are ……. while the others are
non-luminous
d) There is ……. reflection from still surface of water.
e) Angle of …… and angle of ……. are equal during reflection of light.
f) …… surface shines.
g) The image formed by a plane mirror are always of ……. size.
STEP 2
3. Answer in one word.
a) What is the term for the line which is perpendicular on the reflecting surface?
b) Which type of refraction occurs from a rough surface?
c) Which type of reflection occurs from a smooth metallic surface?
d) What is the term for the face seen in the mirror?
e) How many types of reflection of light are there?
f) What is the nature of image formed when we look into a plane mirror?
4. Differentiate between:
a) ray of light and beam of light
b) incident ray and reflected ray
c) angle of incidence and angle of reflection
d) regular reflection and irregular reflection
e) real image and virtual image
5. Give reason. i)
a) A smooth surface shines. ii)
b) A rough surface doesn’t shine.
6. Answer the questions with the help of the given figure.
a) Which type of reflection is shown in the figures?
98 Light
b) Write the name of the devices which are
shown in the given figures and write their
uses.
STEP 3 i)
ii)
7. Answer the following
a) Define light.
b) What are the sources of light? Write with examples.
c) What is a ray of light? Show in diagram.
d) What does the arrow in a ray indicate?
e) What do you understand by a beam of light? Show in the diagram.
f) What is reflection of light? Show in the diagram.
g) What is an image? Write down its types and define each.
h) Write the characteristics of a mirror.
i) The angle between an incident ray and the mirror is 40o.
i) What is the angle of reflection?
ii) What is the angle between the incident and reflected rays of light?
j) Define
i) virtual image ii) real image iii) reflection
iv) angle of incidence iv) angle of reflection
k) Write the laws of reflection of light.
l) What is a periscope?
m) What is the use of a periscope on a submarine?
n) What is a kaleidoscope? Write down its use.
8. Draw the diagram
a) to show the regular and irregular reflections of light.
b) to show the reflection of light from a plane mirror. Also write the
characteristics of the image formed by the plane mirror.
c) to show the construction and working of a periscope.
d) to show the construction and working of a kaleidoscope.
UNIT Estimated teaching periods TheoMryoderPnraCcotniccaelpt Science and Environment - 7 99
31
8
Sound
Syllabus issued by CDC Playing guitar
Introduction to sound
Wave and its types
Propagation of sound
Speed of sound in different media
LEARNING OBJECTIVES
At the end of this unit, students will be able to:
define the sound wave (longitudinal wave) and explain its propagation.
explain the factors that affect the propagation of sound.
demonstrate that the speed of sound in different media is different.
Key terms and terminologies of the unit
1. Sound: Sound is a form of energy which produces sensation of hearing
in our ears.
2. Cause of sound: Sound is caused by a rapid to and fro movement of particles
of matter about their mean position. This movement is also
known as vibration.
3. Sources of sound: The different objects which produce sound from their
vibration are called sources of sound.
4. Wave: Wave is the periodic disturbance in a medium, which carries
energy from one place to another without transferring matter.
5. Transverse wave: The wave in which vibration of the particles of the medium is
perpendicular to the direction of the propagation of the wave
is called transverse wave.
6. Longitudinal wave: The wave in which vibration of the particles of the medium
is parallel to the direction of the propagation of the wave is
called longitudinal wave.
7. Propagation of sound: The transmission of sound from one place to another is called
the propagation of sound.
100 Sound
8. Medium: Any material through which a wave propagates is called a
9. Speed of sound: medium.
Propagation of sound per unit time is called the speed of
sound.
8.1 Introduction
We hear sound with our ears. Sound is a form of energy, which produces the sensation
of hearing in our ears. It creates a sensation in our auditory nerves. In everyday
life, we encounter different types of sounds. For example, sounds from television,
barking dogs, loudspeakers, chirping birds, vehicle horns, vacuum pumps, mixers
and blenders, etc. In school, we hear the sound of bell ringing. The sound from our
vocal cord helps us to communicate with one another.
FACT WITH REASON
Why do we need sound?
In humans, the sound is produced from vocal cord which we need to communicate with
others. It is also required to hear other different kinds of sound.
Bell Madal Tuning fork Whistle
8.2 Sound and Vibration
ACTIVITY 1
Sound produced by vibrations
1. Take a steel plate and a spoon. Strike the plate by spoon. Place your finger on the plate.
2. Plunk a string of a guitar. Touch it by your finger.
3. Strike a drum by a drum beater. Place your hand over the membrane of the drum.
Do you feel vibrations in all the above cases? It shows that vibrating bodies produce sound.
Have you ever touched a ringing bell? We feel vibrations of a ringing bell when we
touch it. A rapid to and fro movement about a mean position is known as vibration.
Vibration in a body is the cause of a sound. Sound is produced from vibrating solid,
liquid or gaseous substances.
Modern Concept Science and Environment - 7 101
All kinds of vibrations may produce sound. For example, a vibrating tuning fork,
vibrating ruler, a vibrating membrane of a drum, vibrating air column in flute,
vibrating vocal cord, etc.
FACT WITH REASON
Sound from a ringing bicycle bell ceases when it is caught by our hand
Vibration in a body is the cause of its sound. When a ringing bell is caught by our hand, the
vibration in it stops and the sound stops coming out.
8.3 Sources of Sound
The different objects which produce sound from their vibration are called sources of
sound. For example, Sitar, guitar string, flute, tabala, Bell, MADAL, etc.
FACT WITH REASON
Why is a flute called source of sound?
Flute is called a source of sound because the vibration of air column in it produces sound.
8.4 Waves
When a pebble is thrown into still water, circular ripples form and spread out in all
directions on the surface of water. These ripples carry energy from stone to all parts
without actual movement of the water from one point to another. A ripple carries
energy from one location to another without transferring water.
To check whether water moves along with ripples A wave in water
or not, we can observe a floating object like a cork or
a leaf placed on the surface of water. As the ripples
move in all possible directions on the water surface,
the leaf on the water surface vibrates up and down
but does not have lateral translatory motion.
Thus, a wave is the periodic disturbance in a medium which carries energy from one
place to another without transferring matter. It is the fine path through which energy,
light or sound gets transmitted from one place to another.
Types of Waves
When a wave travels through matter, the particles of the matter vibrate. On the basis
of the direction of vibration of the particles of a medium there are two types of waves,
viz. transverse waves and longitudinal waves.
102 Sound
Transverse Wave
ACTIVITY 2
1. Take a rope and attach it to a door knob as shown
in the given figure.
2. Shake the rope up and down.
3. Observe the up-and-down motion of the rope creating high points and low points.
The periodic motion set in the rope is called a transverse wave. The wave in which
vibration of the particles of the medium is
perpendicular to the direction of the propagation of MEMORY PLUS
the wave is called transverse wave. For example, Light is an example of a transverse wave.
ripples on water surface, light wave, a wave pattern
set in a stretched string, etc. The motion of particles is an up-and-down motion in a
transverse wave. When waves propagate through water, the molecules of water
vibrate just up and down at their respective position. Waves move forward without
movement of water.
Wavelength
Displacement Crest Crest Crest
Trough Trough Direction of propagation
Wavelength
Graphical representation of a transverse wave
FACT WITH REASON
Disturbance in water surface is a transverse wave. Give reason.
In case of a disturbance in water, the water molecules move up and down in a periodic
motion to form crest and trough. So, it is a transverse wave.
Longitudinal Wave
ACTIVITY 3
1. Take a slinky and ask one of your friend to hold one of its ends. You hold the other
end.
2. Stretch the slinky and give it a sharp push towards your friend.
What do you notice? If you move your hand pushing and pulling the slinky alternatively,
what will you observe? If you mark a dot on the slinky, you will observe that the dot
on the slinky will move back and forth parallel to the direction of the propagation of
the disturbance.
Modern Concept Science and Environment - 7 103
What do you notice? If you move your hand pushing and pulling the slinky alternatively,
what will you observe? If you mark a dot on the slinky, you will observe that the dot
on the slinky will move back and forth parallel to the direction of the propagation of
the disturbance. The wave in which vibration of the particles of the medium is parallel
to the direction of the propagation of the wave is called longitudinal wave.
For example, a sound wave, a wave pattern set in a stretched spring, etc. In a longitudinal
wave, the motion of particles is a back-and-forth. When a stretched spring is released
then waves propagate through it in the form of compression and rarefaction.
Compressions and rarefactions
FACT WITH REASON
The sound waves that we listen are longitudinal waves. Why?
Vibrations in the air consist of areas of high pressure called
compressions and areas of low pressure called rarefactions. So the
sound waves that we listen are longitudinal waves.
Differences between Transverse Waves and Longitudinal Waves
Transverse Waves Longitudinal Waves
1. The wave in which vibration of 1. The wave in which vibration of the
the particles of the medium is particles of the medium is parallel
perpendicular to the direction of the to the direction of the propagation
propagation of the wave is called of the wave is called longitudinal
transverse wave. wave.
2. These waves can be generated in 2. These waves can be generated in
solids and liquids. solids, liquid and gases.
3. A complete transverse wave consists 3. A complete longitudinal wave
of a crest and a trough. consists of a compression and a
rarefaction.
4. It travels with a high speed. 4. It travels with a low speed.
5. It can travel through a vacuum. 5. It cannot travel through a vacuum.
104 Sound
8.5 Propagation of Sound
The transmission of sound from one place to another is MEMORY PLUS
called the propagation of sound. For example, sound In comparison to the light, sound has
propagates through air and reaches up to our ears. very low velocity.
Medium When the speed of any object exceeds
the speed of sound then it is said to be
Any material though which a wave propagates is travelling at supersonic speed.
called a medium. For example, solid, liquid and gas are the media. A material medium
is essential for the propagation of a sound wave. Sound cannot propagate without a
medium such as in a vacuum.
Transmission of Sound in Different Media
ACTIVITY 4
To observe the propagation of sound through a solid
1. Materials required: desk, pencil
2. Procedure: The scratch by a pencil that one friend makes at one end of a desk is
listened by another friend at another end of the desk.
3. Observation: Scratching is more clearly audible through the desk than that through the air.
4. Conclusion: Sound propagates faster through the desk than through the air. Hence,
sound propagates faster in solids than in gases.
ACTIVITY 5
To observe propagation of sound through liquid Hammer
Bucket
1. Materials required: a bucket, water, bell, a hammer
Water
2. Procedure: Take a bucket and fill it with water. Immerse a Bell
bell into the water and hit the bell with the hammer.
3. Observation: The sound of ringing bell is heard outside of
water. The sound comes out through water.
ACTIVITY 6
To observe that sound does not propagate through vacuum.
1. Materials required: bell jar, electric bell, airtight cork.
2. Procedure: Take a bell jar and fit an electric bell in it through an
airtight cork as shown in the given figure. Fit the vacuum pump
at the bottom of the bell jar. Connect the bell to the battery and
switch. Switch on the bell and observe the sound outside. Next
time repeat the process by removing the air inside the bell jar.
Modern Concept Science and Environment - 7 105
3. Observation: The sound of the bell is heard outside when there is air inside the bell jar.
When vacuum is created inside the bell jar, no sound is heard.
4. Conclusion: Sound cannot travel through vacuum. It needs a material medium for
propagation.
Sound can travel through solids, liquids and gases. When the molecules of a solid,
liquid and gas vibrate,they push the nearby molecules. This makes compressions and
rarefactions possible and the sound wave propagates in different media.
Sound cannot propagate through a vacuum. On the surface of the moon, there is a
vacuum due to the lack of atmosphere. Thus, communication on the surface of the
moon is not possible as it is on the earth. Electronic devices are used for communication
on the surface of the moon.
FACT WITH REASON
Sound doesn't propagate on the surface of the moon. Why?
A material medium is essential for propagation of sound. But due to the vacuum, that is the lack
of air, on the surface of the moon, sound does not propagate there.
Communication on the surface of the moon isn’t possible as it is on the earth. Why?
Moon has no atmosphere. Moon has a vacuum on its surface. Sound needs a material medium to
travel. It cannot travel in a vacuum. So, communication on the surface of the moon isn’t possible
as it is on the earth.
Electronic devices are used for communication on the surface of the moon. Why?
Sound waves cannot travel through a vacuum on the surface of the moon. Electronic devices receive
communication signals in the form of electromagnetic waves, which can travel through a vacuum.
8.5 Speed of Sound
The propagation of sound per unit time is called the speed of sound. The speed of
sound is different in different mediums.
Speed of Sound in Different Media
The speed of sound wave depends on the material medium in which it is travelling.
Speed of Sound in Solid
Molecules are tightly packed in solids. Shocks of wave transmit faster in solids. So, the
propagation of sound in solids is faster than that in gases and liquids. For example,
the speed of sound in glass at 25oC is 3980 m/s.
FACT WITH REASON
Why does sound travel fastest in solid?
Sound travels fastest in solid because solid molecules are closely packed that transmits wave faster.
106 Sound
Speed of Sound in Liquid
Molecules in liquid are a bit loosely packed. It takes more time for the transmission
of wave shocks in liquids. So the propagation of sound in liquids is faster than that in
gases but slower than that in solids. For example, the speed of sound in sea water at
25oC is 1531m/s.
Speed of Sound in Gas
Molecules in gases are loosely packed. It takes more MEMORY PLUS
time for sharing the wave shock in gases than that in
solids and liquids. So the speed of sound in gases is 1. Sound propagates 332m in one
less than that in solids and liquids. For example, the second through air.
speed of sound when travelling through air at 20oC
is 343m/s. 2. Astronauts use electric devices to
talk with each other on the moon.
FACT WITH REASON
Speed of sound in gas is lowest. Why?
Speed of sound in gas is lowest because gas molecules are loosely packed so it takes more
time to transfer wave.
Factors Affecting Speed of Sound in Gas
Density of gas, temperature, humidity etc. affect the speed of sound in gases.
i) Density : The speed of sound in gas is inversely proportional to the square root
of its density.
Mathematically,
Speed (v) α 1
√ density
The speed of sound becomes greater in air when its density decreases.
ii) Temperature : The speed of sound is directly proportional to the square root of
temperature,
i.e., v α √ temperature
This means, the speed of sound increases with increase in temperature. For
every 1oC rise in temperature, the speed of sound increases by 0.6 m/s.
FACT WITH REASON
We hear more clearly on a hot day than on a cold day. Why?
The speed of sound increases with the increase in temperature. For example, speed of sound
waves in air at 0oC is 331m/s and the speed of sound at 20oC is 343m/s. So we hear more
clearly on a hot day than on a cold day.
Modern Concept Science and Environment - 7 107
iii) Humidity
The greater the moisture content of the air, the greater the humidity. Moist air is less
dense than the dry air. The decrease in density increases the speed of sound in air.
Therefore, an increase in humidity in air increases the speed of sound and decrease in
humidity in air decreases the speed of sound.
FACT WITH REASON MEMORY PLUS
Humidity is the amount of water
We hear more clearly in rainy season than in summer. Why? vapor present in the air.
An increase in humidity in air increases the speed of sound and decrease in humidity in air
decreases the speed of sound. So we hear more clearly in rainy season than in summer.
Speed of sound increases with the increase in the humidity of air. Why?
Density of moist air is less than that of dry air. When density of air decreases, the speed of
sound increases. Thus, the speed of sound increases with the increase in humidity.
iv) Direction of Wind
The speed of sound increases in the direction of wind, i.e. Speed of sound = speed of
sound in still air + speed of wind
If the wind blows in the opposite direction in which the sound travels, the speed of
sound decreases.
STEPS EXERCISE
STEP 1
1. Tick () the correct statement and cross () the incorrect one.
a) Sound propagates in the form longitudinal waves.
b) Objects should vibrate continuously to produce sound.
c) Vibrating guitar string produces a sound.
d) Sound transmits in vacuum.
e) Speed of sound in solids is more than that in liquids and gases.
f) Humidity decreases the speed of sound.
2. Fill in the blanks with appropriate words
a) Light wave is ……. wave.
b) …… is produced from vibrating solid, liquid and gaseous substances.
c) …… is essential for propagation of sound.
d) Sound cannot travel in …….
e) …… are used for communication on the surface of the moon.
f) The speed of sound is the highest in............
108 Sound
STEP 2
3. Answer in one word
a) Which type of wave is a sound wave?
b) In which medium does sound have the highest speed?
c) Which type of wave is generated in water when a stone is dropped in it?
d) In which medium does sound have the lowest speed?
e) In the form of which wave does our sound propagate through air?
4. Differentiate between:
a) transverse wave and longitudinal wave
b) transmission of wave in water and transmission of wave on spring
5. Give reason.
a) Sound doesn’t propagate on the surface of the moon.
b) Speed of sound is the highest in a solid medium.
c) Speed of sound in a liquid medium is less than that in a solid medium.
d) Speed of sound is the lowest in a gaseous medium.
e) We hear more clearly on a hot day than on a cold day.
f) We hear more clearly in rainy season than in summer.
6. Answer the questions with the help of the given figure
a) Identify the type of waves shown in the given figures and write the name
of sources from which those waves are generated.
Displacement Wavelength Crest Crest
Crest
Trough Trough Direction of propagation
Wavelength
i) ii)
STEP 3
7. Answer the following
a) What is sound? How is sound produced?
b) What is meant by source of sound? Write with examples.
c) What is wave? What are its types? Also define them.
d) What is a transmission of sound? Explain the transmission of sound in
different media.
e) Define a medium?
f) What is the speed of sound?
g) The speed of sound in air is 332 m/s, what does it mean?
h) Write the factors which affect the speed of sound in a gas.
UNIT Estimated teaching periods TheorMyoderPnraCcotniccaelpt Science and Environment - 7 109
31
9
Magnet
Syllabus issued by CDC Magnet
Introduction to magnet (ii) Permanent and temporary magnets
Types of magnets: (ii) Electrical method
(i) Natural and artificial magnets
Utilities of magnets
Methods of making magnets
(i) Rubbing method
LEARNING OBJECTIVES
At the end of this unit, students will be able to:
introduce magnet and differentiate between natural magnet and artificial magnet.
state the utilities of magnets.
explain and demonstrate the methods of making magnets.
Key terms and terminologies of the unit
1. Magnet: The natural or artificial substance which attracts
magnetic substances like iron, nickel etc. is called a
magnet.
2. Magnetic substances: Those substances which are attracted by a magnet are
called magnetic substances.
3. Non-magnetic substances: Those substances which are not attracted by a magnet
are called non-magnetic substances.
4. Natural magnet: Naturally occurring substances, like lodestone, which
possess the property of attracting magnetic substances is
called a natural magnet.
5. Lodestone: Lodestone is a black ore of iron called magnetite which
6. Artificial magnet: has molecular formula Fe3O4.
Magnets which are made in laboratories or factories are
called artificial magnets.
7. Temporary magnet: Those magnets in which magnetic property remains for a
short time are called temporary magnets.
110 Magnet
8. Permanent magnet: Those magnets in which magnetic property remains for a
9. Magnetization: long time are called permanent magnets.
10. Single-touch method:
11. Double-touch method: The process of making magnets from soft iron materials is
called magnetization.
12. Magnetic induction:
13. Electromagnet: The magnetization done by stroking a pole of a permanent
magnet repeatedly on an iron bar in one direction is called
a single touch method.
The magnetization done by stroking opposite poles of the
two permanent magnets in outward directions from the
middle of an iron bar at the same time is called double-
touch method.
The process in which a magnetic material is made to
behave like a magnet in the presence of a magnet without
direct contact is called magnetic induction.
A temporary magnet made by passing an electric current
around a magnetic substance through an insulated wire is
called an electromagnet.
9.1 Introduction
A substance that attracts magnetic substances like iron, nickel etc. is called a magnet.
Magnet rests in the north-south direction when freely suspended. Magnets attract
materials like iron, nickel, cobalt, etc. Substances that are attracted by a magnet are
called magnetic substances. Magnets also attract alloys too. Two such alloys are steel
and alnico. Steel is an alloy of iron and carbon. Similarly, alnico is an alloy of
aluminium, nickel and cobalt. Substances which are
not attracted by a magnet are called non-magnetic MEMORY PLUS
substances. For example, wood, plastic, brass,
aluminium, copper, etc. Magnetic materials can be Lodestone contains magnetite, a
natural magnetic material.
used to make magnets, non-magnetic materials
cannot.
Discovery of a Magnet
The discovery of the magnet is credited to the shepherd
boy, Manganus. There is no exact date or year known for his
discovery of the magnet. Manganus used to live in a town
of Magnesia in Asia Minor (now Turkey). One day, while
roaming on Mt. Ida, he found that his iron-strapped sandals
Lodestone
Modern Concept Science and Environment - 7 111
got stuck to the mountain and created difficulty for him to walk. This was due to the
black stone with iron in it. It had the power to attract iron. He observed that the long
thin stone taken from the mountain was always pointing in a certain direction when
it was suspended freely. Due to this direction indicating property of the stone, it was
named as lodestone, which means ‘leading stone’.
FACT WITH REASON
Lodestone rests in a certain direction when hanged freely. Why?
A lodestone suspends in a certain direction when hanged because it has magnetic properties.
9.2 Magnetism
The various properties of a magnet are called magnetism. For example, a magnet has
two opposite poles, the north pole and south pole.
Properties of a Magnet MEMORY PLUS
1. Attractive property: A magnet has attracting The poles of a magnet cannot be
property to the magnetic substances like iron, separated.
steel, cobalt, nickel, etc.
2. Directive property: A magnet, when suspended freely by a thread, it rests in the
north-south direction.
3. Property of inseparable magnetic pole: When a magnet is broken, each piece is
a complete magnet.
4. The magnetic strength is maximum at the poles of a bar magnet.
5. Property of attraction exists between two unlike poles and repulsion between
like poles.
FACT WITH REASON
If a bar magnet is broken between two, the result is two smaller magnets. Why?
Magnets are made up of molecular magnets. Their poles cannot be separated. No matter how
many times a magnet is broken, new magnetic poles develop at each ends.
9.2 Natural Magnet
The naturally occurring substance (like the lodestone), which
possesses the property of attraction to the magnetic substances, is
Lodestone
112 Magnet
called a natural magnet. Lodestone is a black ore of iron called magnetite. Its chemical
formula is Fe3O4. The name magnetite is derived from the place where it was first
found, i.e. Magnesia in Asia Minor. It is the only known natural magnet to man.
Properties of Lodestone
i) It attracts small pieces of iron.
ii) A lodestone suspended freely with the help of a thread rests in the north-south
direction.
Use of Lodestone in Ancient Times
It was used by the Chinese sailors to guide their ships. For this purpose, magnets were
made in a desired shape and size from the lodestone.
However, the natural magnets have no practical value in the modern days because of
the following reasons:
i) They have a weak magnetic force, which is not MEMORY PLUS
enough to be used in modern devices.
The region around a magnet up
ii) They are found in irregular structures, in quite to which the magnet can affect is
odd shapes. called the magnetic field.
FACT WITH REASON
Lodestone is a natural magnet. Why?
Lodestone is natural magnet because it is formed naturally and has magnetic properties.
9.3 Artificial Magnets
Magnets which are made in laboratories or factories are called artificial magnets. They
are of various shapes and sizes. Some artificial magnets are bar magnet, horseshoe
magnet, U-shaped magnet, cylindrical magnet, magnetic needle, etc.
Bar magnet Horseshoe magnet Compass U-shaped magnet
Modern Concept Science and Environment - 7 113
Need of Artificial Magnet
Nowadays, powerful magnets are used in a number of devices such as a loud speaker, radio,
television, hydro-power station, etc. Following are the reasons of using artificial magnets.
i) They can be made very powerful, which is not possible in the case of a natural magnet.
ii) They can be made in any desired shape and size.
Types of Artificial Magnets
On the basis of temporary and permanent magnetic properties, there are two types of
artificial magnets. They are temporary magnets and permanent magnets.
Temporary Magnets
Those magnets in which magnetic property remains for a short time are called
temporary magnets. They are formed from the substances like soft iron. For example,
when an iron nail is attracted by a magnet, magnetism gets induced in the iron nail.
The magnetic properties of the nail disappear immediately.
Permanent Magnets
Those magnets in which the magnetic property MEMORY PLUS
remains for a long time are called permanent 1. A bicycle dynamo uses permanent
magnets. They are made of substances like steel or magnet.
alnico. These substances possess a high power of 2. The earth itself acts a big magnet.
magnetism after the magnetizing source is removed. The magnetic property of the earth
Now-a-days, permanent magnets of different shapes is called the terrestrial magnestim.
and sizes are being made. These magnets have wide
applications because of being light, strong and permanent.
FACT WITH REASON
Bar magnet is called a permanent magnet. Why?
A bar magnet is called a permanent magnet because its magnetic property remains in it for
a long time.
Differences between Temporary and Permanent Magnets
Temporary Magnets Permanent Magnets
1. Those magnets in which magnetic 1. Those magnets in which the magnetic
property remains for a short time property remains for a long time are
are called temporary magnets. called permanent magnets.
Artificial magnets cannot be
2. Temporary magnets like an 2. magnetized to a very high strength.
electromagnet can be magnetized
to a very high strength.
114 Magnet
3. They are used to make generators, 3. They are used to make speaker,
radio, etc.
electric motors, fans etc.
4. They are made from soft iron. 4. They are made from steel or alnico.
Differences between Natural Magnets and Artificial Magnets:
Natural Magnets Artificial Magnets
1. Magnets formed naturally are called 1. Man-made magnets are called
natural magnets. artificial magnets.
2. Natural magnets are not found 2. Artificial magnets can be made
everywhere on the earth. everywhere.
3. Natural magnets are not found 3. Artificial magnets are made in
in a definite shape. For example different shapes and sizes such as,
lodestone. bar, horseshoe, U-shape, etc.
9.4 Magnetization
The process of making magnets from soft iron MEMORY PLUS
materials is called magnetization. Artificial magnets
can be made by using different methods such The process of loosing the magnetic
as single touch method, double-touch method, property of a magnet is called
induction method, electrical method, etc. demagnetization.
Methods to Make Magnets
By Using a Magnet (or Stroking Method)
This method uses a strong magnet to stroke a magnetic material. The stroking action
aligns the molecular magnets in one direction. S
a) Single Touch Method: In this method,
one pole of a strong bar magnet is stroked
over a magnetic substance like an iron bar N
in one direction from one end to another N S
end many times. AB
Procedure:
Single touch method
a) Place one of the poles of a bar magnet (for example, the north pole) over
one end of the iron bar.
b) On reaching the end B of the iron bar, lift the bar magnet and bring the
same north pole at point A of the iron bar.
Modern Concept Science and Environment - 7 115
This process is repeated many times to develop magnetic properties on the
iron bar. Magnetization done by stroking a pole of a permanent magnet
repeatedly on an iron bar in one direction is called a single touch method.
Poles of Magnet Made by Single Touch Method
In the single touch method, a pole opposite to the removed pole of the bar
magnet develops at that end. In the given figure end A becomes the north pole
and end B becomes the south pole.
FACT WITH REASON
How does stroking method changes a magnetic substance into magnet?
Stroking method changes a metal into magnet by rearranging the molecular magnets in a
straight line.
b) Double (or Divided) Touch Method
ACTIVITY 1
1. Place a steel knitting needle on a table.
2. Take two bar magnets and place them at the center. Rub the needle at the center
with the unlike poles of two magnets at the same time.
3. Repeat this process 30 times and bring the knitting needle near iron filings. Does
the needle attract the iron filings?
In this method, two opposite N S
poles of equally strong bar
magnets are rubbed many times SN S
Double (or divided) touch method B
in opposite directions from the
middle of a magnetic substance. N
A
Procedure:
a) Place the opposite ends of the two magnets at the center, C of an iron bar.
b) Stroke the poles of these two magnets towards the ends of the iron bar.
This process is repeated many times to develop magnetic properties on the
iron bar. The magnetization done by
stroking opposite poles of the two MEMORY PLUS
permanent magnets in outward
directions from the middle of an iron bar The double touch method is more
at the same time is called double-touch efficient than the single touch
method.
method.
116 Magnet
Poles of Magnet Made by Divided Touch Method
In the divided touch method, a pole opposite to the removed pole of the bar
magnet develops at the end of iron bar. In the
given figure, end A becomes the north pole MEMORY PLUS
and end B becomes the south pole.
For both methods, the permanent
Magnetic Induction magnets must be lifted high enough
after stroking the iron bar each time.
The process in which a magnetic material is made
to behave like a magnet in the presence of a magnet without direct contact is called
magnetic induction. In other words, the phenomenon by which an ordinary piece of a
magnetic substance acquires magnetic properties temporarily due to the presence of
another magnet near to it is called a magnetic induction.
FACT WITH REASON
A nail attached to a magnet also behaves like a magnet. Why?
A nail attached to a magnet also behaves like a magnet because of transfer of magnetic
property from magnet to nail.
By Using Electricity Magnetic induction
This is done by placing a magnetic substance inside a coil
of insulated wire connected with a source of electricity.
The magnet made by this method is known as an
electromagnet.
Electromagnet
ACTIVITY 1
1. Take a long coil of insulated copper wire and wind it around an iron rod.
2. Connect the two ends of the coil to a battery through a switch.
3. Close the switch and bring some paper clips made of steel close to the iron rod.
Do they stick to the rod?
4. What happens to the clips when you put the switch off? Do they fall down from
the iron rod?
A temporary magnet made by passing electric current around a magnetic substance
through an insulated wire is called an electromagnet.
FACT WITH REASON
Electromagnet is called a temporary magnet. Why?
Electromagnet is called a temporary magnet because it loses its magnetic property as soon as
electric current through the coil stops.
Modern Concept Science and Environment - 7 117
Method to Make an Electromagnet
i) Make a number of turns of insulated wire around a magnetic substance (iron nail).
ii) Connect the two ends of the wire with two terminals of a battery.
Characteristics of an Electromagnet
i) The strength of an electromagnet can be changed according to the requirement.
ii) It can be made in different shapes and sizes.
iii) An electromagnet is a temporary magnet.
FACT WITH REASON
Why are electromagnets widely used?
An electromagnet is a temporary magnet. It can be made as we required. Its strength can be
changed according to our requirement. So, electromagnets are widely used.
Factors Affecting the Strength of the Magnetic Field of an Electromagnet
a) Number of turns: The strength of the magnetic field can be increased by
increasing the number of turns.
b) Current: The greater is the current flowing in the wire, the more is the magnetic
field strength.
c) Soft iron core: Magnetic lines of force get MEMORY PLUS
concentrated in a soft iron core. It increases Hans Christian Oersted discovered
the magnetic field strength. electromagnetism in 1820.
Applications of an Electromagnet Electromagnet are used for separating
iron from non-magnetic materials.
1. Electromagnets are used to make electrical
appliances, such as electric bell, electric fan, electric motor, etc.
2. Electromagnets are used in electric generators to produce a strong magnetic field.
3. They are used in magnetic separation of iron ores from earth substances.
4. Strong permanent magnets are made with the help of an electromagnet.
Things to Remember in Making an Electromagnet
1. We should wrap the insulated wire around the magnetic substance in one direction.
2. We should use insulated wire in wrapping the magnetic substance.
FACT WITH REASON
An insulated wire is wrapped around the magnetic substance to make electromagnet. Why?
The insulated wire wrapped around the magnetic substance does not let the electric current to
pass through the magnetic substance. If the current passes through the magnetic substance, then
it does not get magnetized. It also makes a short circuit and the wire gets heated.
We should wrap the wire around the magnetic substance in one direction only. Why?
Random wrapping of an electric wire around the magnetic substance cancels the magnetic field.
It also reduces the strength of the electromagnet formed.
118 Magnet
Differences between a Bar Magnet and an Electromagnet
Bar Magnet Electromagnet
1. A bar magnet is a permanent magnet. 1. An electromagnet is a temporary
Its magnetism remains there for a magnet. Its magnetic property lasts
long period of time. as long as current flows through it.
2. It produces a weak magnetic force. 2. It produces a strong magnetic force.
3. Its strength cannot be changed. 3. The strength of an electromagnet can
be changed.
4. The north-south polarity of
permanent magnet is fixed. a 4. The north-south polarity of an
electromagnet can be changed by changing
5. It is made of steel. the direction of current in its coil.
5. It is made of iron.
9.5 Applications of Magnets
1. The directive property of a magnet is used in finding directions in aircraft, ship, etc.
2. In a magnetic separation method, the small MEMORY PLUS
scattered iron particles on the ground are
collected by a magnet. Magnets are used to store data in
computers.
3. High power magnets are used to lift heavy objects.
The magnetic strip of ATM cards
4. A magnetic compass is used to indicate stores the data. Do not ever scratch
directions. it because this will damage the card.
5. Magnets are used to make a large number of electronic devices like radio,
transistor, speaker, telephone, etc.
6. Generators use magnets to generate electricity.
7. Magnetic strips are used in credit cards, debit cards and other forms of identity cards.
STEPS EXERCISE
STEP 1
1. Tick () the correct statement and cross () the incorrect one.
a) Lodestone is made up of magnetite.
b) Magnet attracts pieces of paper.
c) Natural magnet is found everywhere.
d) A freely suspended lodestone rests along east-west direction.
Modern Concept Science and Environment - 7 119
e) Artificial magnets are more powerful than natural magnets.
f) An unmagnetized bar of steel can be magnetized by rubbing a bar magnet
along its length.
g) Electromagnet is a permanent magnet.
2. Fill in the blanks with appropriate words
a) Energy in …… is called magnetic energy.
b) Like poles ……. , and unlike poles ……. .
c) …… is a natural magnet.
d) Magnet attracts …... .
e) Artificial magnets can be made into different …… .
f) Electromagnet is a …… magnet.
STEP 2
3. Answer in one word.
a) What is the term for the man-made magnet?
b) Which type of substance can be changed into a magnet?
c) Where was lodestone discovered?
d) What is the term for the magnetic property in a magnet?
e) Who discovered natural magnet for the first time?
4. Differentiate between:
a) magnetic substances and non-magnetic substances
b) natural magnets and artificial magnets
c) temporary magnets and permanent magnets
d) bar magnet and electromagnet
5. Give reason.
a) A lodestone is a natural magnet.
b) A bar magnet is called a permanent magnet.
c) Insulated wire must be used in making electromagnet.
6. Answer the questions with the help of the given figure
a) Write the name of the method shown to make a magnet. .
120 Magnet
S
NS
N S SN
N
B N S
A A B
b) In the method to make magnets, what happens
i) when the poles of the cell are reversed.
iii) when the insulation of the coil of wire is removed.
iv) when the turns are made more and closer to one another.
STEP 3
7. Answer the following questions.
a) What is a magnet?
b) What is magnetism?
c) Write the properties of a magnet.
d) What was the use of lodestone in the ancient times?
e) Write the advantages of artificial magnets over natural magnets.
f) What is an electromagnet? Write its characteristics.
g) Describe the methods to make magnets.
h) Explain the single touch method to make a magnet.
i) Explain divided touch method to make a magnet.
j) Explain the method to make an electromagnet.
k) Write the methods to increase the strength of an electromagnet.
l) Write four uses of electromagnets.
8. Draw the diagram
a) to show three artificial magnets.
b) to show the method to make a magnet by the single touch method.
c) to show the method to make a magnet by the divided touch method.
d) to show the method to make an electromagnet.
UNIT Estimated teaching periods TheoMryoderPnraCcotniccaelpt Science and Environment - 7 121
31
10
Electricity
Syllabus issued by CDC Electricity
Static electricity
Effects of static electricity
Current electricity
Electric circuit and its types
Symbols used in electric circuit
LEARNING OBJECTIVES
At the end of this unit, students will be able to:
introduce and demonstrate static electricity and explain its effects.
draw electric circuits and introduce the symbols used in electric circuits.
demonstrate the methods of connection of cells and explain its importance.
Key terms and terminologies of the unit
1. Charge: Charge is the electric property of the tiny atomic particles,
namely, electron and proton.
2. Electricity: Electricity is the energy produced on a body by accumulation
of charge on it or by the flow of charge through it.
3. Static electricity: The energy developed on an insulators due to the change in
the number of electrons while rubbing their surfaces is called
static electricity.
4. Lightning: Lightning is the electric spark which occurs due to sudden flow
of huge amount of charge in the charged clouds.
5. Current electricity: The energy produced due to the continuous flow of electrons
through a conductor is called the current electricity.
6. Electric circuit: A continuous path made by connecting an electric source,
electric load, switch, etc. with the help of a conducting wire is
called an electric circuit.
7. Source of electricity: The device which produces electricity is called a source of
electricity.
122 Electricity
8. Conductor: The substance through which electrons can flow is
called a conductor.
9. Load: The device which runs from electrical energy is called
an electric device or load.
10. Switch: A device which is used to open or close an electric
circuit is called a switch.
11. Open circuit: The electric circuit through which the current cannot
flow due to an interrupted path is called an open
circuit.
12. Closed circuit: The electric circuit through which current can flow in
an uninterrupted path is called a closed circuit.
13. Conventional current: In an electric circuit the direction of electric current
is taken from the positive terminal to the negative
terminal. This is called the conventional direction of
electric current.
14. Combination of cells: Grouping of two or more cells is called a combination
of cells.
15. Series combination of cells: The grouping of cells in which the negative terminal of
the first cell is connected to the positive terminal of the
second cell, the negative terminal of the second cell is
connected to the positive terminal of the third cell and
so on is called a series combination.
16. Parallel combination of cells: The grouping of cells in which the positive terminals
of all cells are connected to one point and the negative
terminals are connected to another point is called a
parallel combination.
10.1 Introduction
In today’s world, electricity is an important source of energy for us. It is the most
useful and convenient form of energy. Electricity is used in our homes for lighting
bulbs, heating rooms, running fans, etc. It is easy to convert electricity into different
forms of energy. Electricity can be stored and transmitted over a long distances.
The study of electricity involves the study of charges. In MEMORY PLUS
this unit, before we discuss the electric current, we should
learn about the charges. Charges can be collected on the An atom is the smallest unit of
surface of a body. They can flow through wires too. The matter that has the properties of
a chemical element.
Modern Concept Science and Environment - 7 123
charges collected on a body produce static electricity and the flowing charges produce
current electricity. Current electricity is very useful for us in our daily life. In this unit,
we will learn about the static electricity as well as the current electricity.
Structure of an Atom
Matter is made up of atoms. Electron, proton and neutron + Electron
are the subatomic particles. The nucleus is the center of +
an atom. Positively charged protons and charge-less Nucleus
neutrons are present in the nucleus of an atom. Negatively ++ Neutron
charged electrons revolve around the nucleus of the atom. ++ Proton
Shell
The protons and electrons of an atom attract each other. An atom
In a neutral atom, there are equal number of protons and electrons.
10.2 Charge
The electric property of the tiny subatomic particles, MEMORY PLUS
namely electrons and protons is called a charge. It is
the property of the body due to which the body feels Electron is negatively charged
the force of attraction and repulsion. There are two particle whereas the proton is
types of charges. They are positive and negative. positively charged particle.
Properties of Electric Charges Like charge
repel
i) Like charges repel each other.
ii) Unlike charges attract each other. Opposite
charge attract
FACT WITH REASON
Why do two glass rods rubbed with fur repel each other?
Two glass rods rubbed with fur repel each other because both of them have same kind of
charge that repels each other.
10.3 Electricity
Electricity is the result of the movement of electrons. Thus, the energy produced on
a body by accumulation of charges on it or by the flow of charges through it is called
electricity. Electricity can be either static or a current electricity.
Electricity
Static Electricity Current Electricity
124 Electricity Comb
Paper
Static Electricity
ACTIVITY 1
1. Take a plastic comb and rub it against your dry hair or some woolen
material for a while.
2. Make small pieces of a paper and bring the comb near them. Do you
see that the pieces of paper fly and stick to the plastic comb as shown
in the figure alongside.
This happens because of the charges developed on the plastic comb by
rubbing it against the dry hair or wool.
Sometimes, electric charges accumulate on an object. A comb rubbed with hair is an
example. Electrons move from hair and collect on the comb to develop a negative
charge. The electric charge is developed on the insulators after rubbing them. Such
charges do not move through the metallic wires. These charges are called static electric
charges. The electrical energy developed on insulators due to the change in number of
electrons after rubbing is called static electricity.
FACT WITH REASON
Why does a rubbed plastic ruler attract paper pieces?
A rubbed plastic ruler attracts paper pieces because it has negative charge on it.
Cause of Static Electricity
Atoms of objects have equal number of protons and MEMORY PLUS
electrons in them. An atom is, therefore, electrically
neutral. Objects can acquire static charge through friction, Unit of charge is coulomb.
conduction, and induction.
Unit of current is ampere.
Friction
When two bodies are rubbed together, electrons move from one body to the other.
This transfer of electrons causes an imbalance in the positive and negative charges of
the matter. Thus, this transfer of electrons is the main cause of static electricity.
a) Conditions in which Negative Charge Develops on an Insulator : When two
different insulators are rubbed together, a transfer of electrons occurs. The body
that receives electrons now has a greater number of electrons than protons and it
becomes negatively charged. For example, when a glass rod is rubbed with silk,
then some electrons from the glass rod are transferred to the silk. As a result,
glass rod has deficiency of electrons. So, it becomes positively charged. The silk,
after rubbing, has an excess of electrons and becomes negatively charged.
Modern Concept Science and Environment - 7 125
b) Conditions in which Positive Charge Develops on an Insulator : When
two different insulators are rubbed together, a transfer of electrons occurs. The
body that loses electrons becomes positively charged. For example, when an
ebonite rod is rubbed with a cat’s skin, some of the electrons from the cat’s skin
are transferred to the rod. As the cat’s skin has a deficiency of electrons it gets
positively charged. The ebonite rod has an excess of electrons and becomes
negatively charged.
If one body gets charged positively due to rubbing, then the body which is used
for rubbing gets charged negatively. Hence, the rubbing produces equal and
opposite charges at the same time.
FACT WITH REASON
A body is charge-less in a normal condition. Why?
In a normal condition, atoms of a body consist of an equal number of opposite charged
electrons and protons. It neutralizes the net effect on each other. So a body in a normal
condition is charge-less.
Conduction
When a charged body comes in contact with other objects, a transfer of charge takes
place. This can happen when a person slides across a car seat and then touches a metal
door handle.
Transfer of charge
Induction
When a charged body is brought close to another object without
contact, a charge may be induced in the other object. In this case,
no electrons are transferred between the two objects. For example,
when you rub an inflated balloon on a woollen sweater and then
bring the charged balloon near someone’s hair, the hair may
respond by standing up straight. In this case, the electrons of the
negatively charged balloon repel the electrons on hair and resulting
positive charge toward the ends of the hair. This causes, hair to be
attracted towards the negatively charged balloon.
126 Electricity
Some Effects of Static Electricity
a) A comb rubbed with hair attracts paper pieces : When a negatively charged
comb is brought closer to a paper piece, there is an induction of positive charge
at one end of the paper piece and accumulation of negative charge at the other
end. Due to the attraction between the two opposite charges the negatively
charged comb attracts paper pieces.
FACT WITH REASON
A balloon rubbed against our hair sticks to the wall
A balloon rubbed against our hair acquires a negative charge. The
balloon then induces positive charge on the wall and sticks to the
wall.
b) A cracking sound comes from clothes of nylon, polyester or wool when
we remove them from our body : On being rubbed with our body, clothes
of nylon, polyester or wool develop charges. On removing the charged clothes
from our body, a transfer of charges causes a cracking sound. In the dark, even
the sparks become visible.
Lightning MEMORY PLUS
Lightning results due to the electric discharge Benjamin Franklin in 1752 A.D.
between clouds or between clouds and the ground. discovered that lightning is due to
The electric spark which occurs due to the sudden transfer of electric charge.
flow of huge number of charge in charged clouds is
Static electricity is the cause of
lightening.
called lightning. There are three types of lightning. They are cloud to itself, from one
cloud to another and between a cloud and the ground.
Cause of Lightning
Water droplets formed by condensation of water vapours in clouds slowly fall down.
Hot air and water vapour move up through air. These two motions cause friction
and charge develops on clouds. Clouds have a huge amount of static electric charges,
which may be positive or negative. Generally the clouds of upper region are positively
charged and the clouds on the lower region are negatively charged.
A negatively charged cloud formed above buildings and tall tress induces a positive
charge over them. If positively charged cloud forms above buildings and tall trees,
then there is an induction of negative charge over them. As the positive charges attract
negative charges, the free electrons from the cloud start pushing their way through
the moist air. When these electrons reach the building, suddenly all the charges in the
cloud flow into the building or tree. Thus, a lightning strikes with a devastating effect
due to the attraction between a sudden and huge flow of opposite charges.
Modern Concept Science and Environment - 7 127
Cause of Thunder
Thunder is the loud noise followed by a flash of lightning. Thundering
Enormous heat gets produced by transfer of electrons from
negatively charged cloud to the positively charged cloud.
This causes expansion and contraction of air in the form of
wave. It causes thunder to occur.
Lightning Conductor (Safety Measures to Prevent Lightning on Buildings)
The best method to protect buildings from lightning is storm cloud
offence. This is done by installing a lightning conductor
on buildings. A lightning conductor is made of a copper stream of positive ions
rod with sharp points at one end. A copper conductor, Spikes
like a TRISHUL with copper wire is connected to the Copper strip
ground. This TRISHUL is placed over the roof of the
building. The lower end of the wire is connected to a Electrons repelled to Earth
flat copper plate and buried deep inside the earth. IfTall building Metal plate in ground
lightning occurs on a building then it gets discharged
into the ground through the copper wire connected to
the TRISHUL avoiding any damage to the building.
FACT WITH REASON
Lightning conductors are installed on a building. Why?
Lightning conductors are installed on a building to transfer electric discharge of thunder
into the earth.
10.4 Current Electricity
Elements are made up of atoms. Each atom has electrons revolving around its
nucleus. The electrons from the outer shell move from one atom to another. In the case
of metals like silver, copper, aluminium etc., these electrons can flow continuously
when connected to a source of electricity. Thus, the energy produced due to the
continuous flow of electrons through a conductor is called a current electricity. It can
be converted to different forms of energy easily. It can be stored and transmitted over
a long distances.
Differences between Static Electricity and Current Electricity
Static Electricity Current Electricity
1. It is due to accumulation of electrons. 1. It is due to flow of electrons through wires.
2. It is produced in insulators. 2. It occurs in good conductors.
3. It does not need a circuit. 3. It needs a closed circuit to flow.
4. It cannot be transported from one place 4. Its transmission is possible through a
to another through a conducting wire. conducting wire.
128 Electricity
10.5 Electric Circuit
ACTIVITY 2
1. Take out a torch bulb, holder, dry cell, a switch and a conducting wire.
2. Connect the two terminals of the cell with the bulb.
3. Connect a switch on a wire. Does the bulb glow when you switch it on?
The circuit becomes closed when the switch is on. The current flows in a closed circuit
and the bulb glows.
An electrical circuit is a complete path through which Copper wire
electrons flow. Electric source, electric load and a
conducting wire are the three main components Switch
of an electric circuit. Electric load is a device which
consumes electricity. For example, electric fan, bulb, Dry cell
TV, heater, etc. Thus, a continuous path made by
connecting an electric source, electric load, switch, Bulb
etc. with the help of a conducting wire is called an
electric circuit. Electric circuit
Components of an Electric Circuit
A complete electric circuit normally consists of the following components.
a) Electric Source
The device which produces electricity is called a source of electricity. For
example, cell, photo cell, dynamo, generator etc.
b) Conductors or the Connecting Wires
The substance through which electrons can flow is called a conductor. Metals
are conductors of electricity. For example, silver, gold, copper, etc. But graphite,
a non-metal, also conducts electricity.
c) Electric Device/Load
The device which runs from electrical energy is called an electric device or
load. For example, electric bulb, fan, television etc. It uses electrical energy and
converts it into other forms of energy like heat, light, etc.
d) Switch
The current flows in a circuit when a load is connected to the source by some connecting
wire. To break the flow of the current, there is a device in a circuit. A device which is
used to open or close an electric circuit is called a switch. It is connected in a series
with the electric source, electric load, etc. So, it can interrupt a circuit. With its switch
in an ‘on’ position, the circuit is closed and the circuit becomes open when the switch
is in the ‘off’ position.
Modern Concept Science and Environment - 7 129
Symbols Used for the Components in Electric Circuit
It is inconvenient to indicate different electric loads by drawing their figures in an
electric circuit. In order to draw a circuit diagram, various electric components are
represented by suitable symbols. Some of the most common circuit symbols are given
below.
Electrical Components Symbol Function
Connecting wire
Jointed wire To connect components in an electric
circuit
To conduct electricity
Wire crossing Overlap of wires without joint
One cell Source of electricity
Two cells Source of electricity
Many cells Source of electricity
Bulb
Resistor To produce light from electricity
Open switch To convert electrical energy into other
forms of energy
To make a circuit open
Closed switch To make a circuit close
Ammeter To measure electric current in a circuit
Voltmeter To measure voltage in a circuit
Fuse To protect a circuit from excessive
heating
Types of Electric Circuit
a) Open Circuit Copper wire
An open circuit does not have a complete path for Dry cell Switch
current to flow. The electric circuit through which off
current cannot flow due to an interrupted path is
called an open circuit. A circuit becomes open when Bulb
the switch is turned off. The electric load does not
work in an open circuit. Open circuit
130 Electricity
b) Closed Circuit Copper wire Switch
on
A closed circuit has a complete path for current to Dry cell
flow. The electric circuit through which current Bulb glowing
can flow in an uninterrupted path is called a closed
circuit. A circuit becomes closed when the switch is Close circuit
turned on. The electric load works in a closed circuit.
Direction of Current in a Closed Circuit
In an electric circuit, there is a flow of electrons. MEMORY PLUS
However, electrons were not known at the time
when the phenomenon of electricity was first 1. Emf is more than potential
observed. So, electric current was considered to be difference in a open circuit.
the flow of positive charges and the direction of flow
of positive charges was taken to be the direction of 2. In closed circuit, electric appliance
like bulb, heater, television etc. work.
electric current. Conventionally, in an electric circuit, the direction of electric current
is taken as from the positive terminal to the negative terminal, which is opposite to the
direction of the flow of electrons.
10.6 Combination of Cells
Sometimes a higher voltage or a current for a longer duration is required. In such
situations, the grouping of the source of electricity in a particular way can solve the
problem. Grouping of two or more cells is called the combination of cells. For example,
there is a combination of cells in a torch light, radio etc. A group of cells is called a battery.
FACT WITH REASON
Why do we need to combine cells?
We need to combine cells to draw large voltage or to keep current flow for longer time.
Types of Combinations of Cells
There are mainly two types of combination of cells. They are:
a) Series combination
b) Parallel combination
a) Series Combination
When a higher voltage than a single cell is required, many cells are connected in a
series. In such a combination, the cells are connected one after another. The grouping
of cells in which the negative terminal of the first cell is connected to the positive
terminal of the second cell, the negative terminal of the second cell is connected to the
positive terminal of the third cell and so on is called a series combination.
Modern Concept Science and Environment - 7 131
1.5 V 1.5 V 1.5 V 1.5 V 1.5 V 1.5 V
+
–
+
–
+
–
+
–
+
–
+
–
(a) (b) (c)
Characteristics of a Series Combination MEMORY PLUS
i) Voltage of all cells is added in a series
The combination of cells in which
combination. opposite terminals of the cells are
connected together is called series
Total voltage, V= V1 + V2 + V3 + …… combination of the cells.
ii) The current in the electric circuit increases with
the increase in the number of cells.
FACT WITH REASON
The brightness of a bulb connected in a series combination of cells increases on increasing the
number of cells. Why?
In a series combination, all the cells are connected one after another. The total e.m.f of the
combination is equal to the sum of e.m.f. of the individual cells. i.e., V= V1 + V2 + V3+ … . Thus, e.m.f.
increases in a series combination of cells.
Demerits of a Series Combination
i) In this combination, it is not possible to obtain the current continuously for a
longer duration.
ii) If any one of the cells in the combination gets damaged then the whole
combination stops working.
b) Parallel Combination
If current is required for a longer duration than the current given by one cell, several
cells are joined in parallel to one another. In such a combination, all the positive
terminals of the cells are connected at one junction. Similarly, the negative terminals
are connected at the other junction. Thus, the grouping of cells in which positive
terminals of all cells are connected to one point and negative terminals are connected to
another point is called a parallel combination. The parallel combination is equivalent
to a single cell.
+ ++ + ++
1.5V 1.5V 1.5V 1.5V 1.5V 1.5V
– –– – ––
132 Electricity
Characteristics of a Parallel Combination
i) The total voltage of the parallel combination is equal to the voltage of a cell.
Total voltage, V= V1= V2 = V3 = ……
ii) The current in the electric circuit does not MEMORY PLUS
change with the change in the number of cells.
Cells in parallel combination last
Demerits of a Parallel Combination of Cells longer.
Electric current does not increase by adding cells in the parallel combination. So,
brightness of bulbs in electric circuit doesn’t change on increasing the number of cell.
Differences between Series Combination and Parallel Combination of Cells
Series Combination Parallel Combination
1. The negative terminals of each cell is 1. The negative terminal of all the cells
connected to the positive terminal of are connected at one point and the
the next cell. positive terminals at another point.
2. The total voltage is equal to the sum 2. The total voltage is equal to the
of the voltage of the individual cells. voltage of a single cell.
3. The current is increased with the 3. The current does not increase with
increase in the number of cells. the increase in the number of cells.
FACT WITH REASON
The cells connected in a parallel combination should have the same voltage. Why?
If the cells connected in a parallel combination have different voltages then the cells with a
higher voltage will supply current to the cells with a lower voltage.
STEPS EXERCISE
STEP 1
1. Tick () the correct statement and cross () the incorrect one.
a) There is attraction between unlike charges.
b) A glass rod rubbed with silk acquires a positive charge.
a) Metallic wires are conductors of electricity.
b) An electric bulb is a source of electricity.
c) The number of electrons is more in a positively charged cloud.
d) Parallel combination of cells is present in a torch light.
e) The e.m.f. increases in a parallel combination of cells.
Modern Concept Science and Environment - 7 133
2. Fill in the blanks with appropriate words
a) …... charge develops on comb by rubbing it with hairs.
b) There is attraction between ….... charges.
c) The number of …… is more in a negatively charged cloud.
d) ……. is caused due to the discharge of oppositely charged clouds.
e) In 1752 A.D. ……. discovered the cause of lightning.
f) In …… combination of cells, electric current increases.
g) The …….. combination of cells is equivalent to a single cell.
STEP 2
3. Answer in one word.
a) What is the simple device used to ‘open’ or ‘close’ a circuit?
b) What is the term used for the electrical property of a proton?
c) Which current develops in an insulator after rubbing?
d) How many types of electricity are there?
e) What is the term for the material which allows electric current to pass
through it?
f) In which type of combination of cells is the electric current obtained for a
longer time period?
g) In which type of circuit, the electric load does not work?
4. Differentiate between
a) electric source and electric load
b) closed circuit and open circuit
c) series combination and parallel combination
5. Give reason.
a) A body is neutral in normal conditions.
b) Negative charges develop on an ebonite rod when it is rubbed with fur.
c) A negatively charged comb attracts paper pieces.
d) Tick-tick sound is produced while removing clothes of nylon, polyster,
wool etc. from our body.
e) Thunder occurs during lightning.
6. Answer the questions with the help of the given figure. 1.5 V 1.5 V 1.5 V
a)
i. Identify the combination of cells in the given
figure.
+
–
+
–
+
–
134 Electricity
ii. What is the formula for total e.m.f. in this combination?
iii. If one cell is added to this combinations then what will happen to the
brightness of the bulb?
b) Name the types of circuit shown in the figures.
Copper wire Copper wire
Dry cell Switch Dry cell Switch
off Bulb glowing on
Bulb
i) ii)
STEP 3
7. Answer the following questions.
a) What is charge? How many types of charges are there?
b) What is electricity? Write its types.
c) How does a negative charge develop on an insulator?
d) How does a positive charge develop on an insulator?
e) How does a charge develop on a comb rubbed with hair?
f) What is static electricity?
g) Explain two effects of static electricity.
h) How does electricity get generated in the sky?
i) What is lightning? Write its causes.
j) How can we protect houses and buildings from lightning?
k) What are the sources of electricity? Write three examples.
l) What is an electric circuit? Show with diagram.
m) What is a combination of cells? Write its types.
n) What is a series combination of cells? Write its two characteristics.
o) What is a parallel combination of cells? Write its two characteristics.
p) You are given four cells of 1.5V. How would you combine them to obtain
a voltage of 6V?
8. Draw the diagram
a) to show series and parallel combinations of three cells each of 1.5V.
b) to show a circuit diagram having a bulb, cell and switch.
UNIT Estimated teaching periods TheorMyoderPnraCcotniccaelpt Science and Environment - 7 135
12 1
11
Matter
Syllabus issued by CDC Balloon
States of matter (solid, liquid and gas)
Element – Introduction, names and symbols of the first 20 elements
Compound–Introduction and examples
Differences between element and compound
Atom, molecule and molecular formula
Physical change and chemical change
LEARNING OBJECTIVES
At the end of this unit, students will be able to:
describe general states of matter (solid, liquid and gas) and explain their properties.
tell the names and symbols of first twenty elements, i.e. from atomic number 1 to 20.
define compounds and write the molecular formulae of some common compounds.
introduce atom and molecules and explain the method of writing molecular formula.
define physical and chemical change and differentiate between them.
Key terms and terminologies of the unit
1. Matter: Anything that has mass and occupies space is called matter.
2. Solid: The matter which has fixed volume and shape is called a solid.
3. Liquid: Liquid is a matter that has a fixed volume but takes the shape
of its container.
4. Gas: The matter which does not have definite shape, size and
volume is called a gas.
5. Element: An element is the purest form of a matter that contains only
one type of atoms.
6. Atom: An atom is the smallest particle of an element that takes part in
a chemical reaction without division.
7. Compound: A compound is a pure substance which is formed by the
combination of two or more atoms in a definite proportion by
their weight.
136 Matter
8. Sub- atomic particles: The protons, neutrons and electrons which make up an atom
are called sub-atomic particles.
9. Symbol: A symbol of an element is the shorthand representation the
element in a unique way. A symbol also represents the atom of
that element.
10. Molecule: The smallest part of a compound that can exist freely in nature
is called a molecule.
11. Molecular formula: The symbolic representation of a molecule is called molecular
formula.
12. Physical change: The temporary reversible change in which new substance is
not formed is called chemical change.
13. Chemical change: The permanent irreversible change in which new substance is
formed is called chemical change.
11.1 Introduction
There are different types of things around us. Some of these things are books, benches,
houses, stones, bricks, tables, etc. Anything that has mass and occupies space is called
matter. Mass is the amount of matter contained in a body and volume is the space
occupied by that body.
A matter has certain properties. Different matters have different physical and chemical
properties. For example, matters have different physical properties like taste, colour,
odour, shape and size. Similarly, some matters are water soluble while some do not.
Some matters can conduct heat and electricity while others cannot. Some matters are
opaque, some others are transparent and the remaining are translucent.
States of Matter MEMORY PLUS
Matter can be traditionally categorized into three
major physical states. They are solid, liquid and
gas. However, there are two other unique states
of matter as plasma and Bose-Einstein condensate rigid not rigid not rigid
(BEC) that occur in extreme conditions. Plasma fixed shape no fixed shape no fixed shape
is a hot ionized state of matter which requires fixed volume fixed volume no fixed volume
extremely high temperature. Stars are made of MEMORY PLUS
plasma, so plasma is the most abundant form of The flame produced in an electric arc
matter in the universe. On the other hand, Bose- welding is an example of plasma.
Einstein condensate can only form at extremely low
temperature. It was first produced in a lab in 1995. In this unit, we shall learn about
the physical characteristics of three major states of matter.
Modern Concept Science and Environment - 7 137
a) Solids
The matter which has definite shape, size and volume is called a solid. Ice, rock,
wood, glass, plate, ruler, etc. are the examples of solid matters.
Ice cubes Stone Plate Wood
Common characteristics of solids MEMORY PLUS
i) They are usually hard.
Solid molecules have strong
ii) They have definite shape and size. intermolecular force of attraction than
in liquid and gas.
iii) They have definite volume.
iv) Solids cannot be compressed because their molecules are densely packed.
v) Intermolecular force of attraction is very high in solids. Therefore, they do
not flow.
b) Liquids
Liquid is a matter that has a fixed volume but takes the shape of the container.
Oil, water, milk, alcohol, juice, etc. are some examples of liquids.
Common characteristics of liquids
i) Liquids do not have definite
shape and size and take the
shape of the vessel in which
they are kept.
ii) They have definite volume.
iii) They cannot be compressed.
iv) Molecules of liquids are loosely packed MEMORY PLUS
in comparison to solids. liquid molecules flow up to down
v) They flow from higher level to the due to less intermolecular force of
lower level because they have less attraction.
intermolecular force of attraction.
Milk Juice Water Alcohol Oil
138 Matter
c) Gas
Gases are those matter which do not have definite shape, size and volume.
They spread in any direction. Oxygen, carbon dioxide, water vapour, ammonia,
perfume sprays, smoke, etc. are some examples of gases.
Balloon Gas Sprey Water vapour
Common characteristics of gases MEMORY PLUS
a) They do not have a definite shape, size Gas molecules move here and there
and volume. Their shape and volume due to very less intermolecular force
change according to the vessel in which of attraction.
they are kept.
b) They are highly compressible because they have the highest intermolecular
distance as compared to solids and liquids.
c) They easily flow or spread in air because they have the lowest intermolecular
force of attraction among the three basic states of matter.
ACTIVITY 1
Take a brick, a glass of water and a bottle of body spray. Observe the properties of these
matters and compare how they differ from each other.
Element
The substances on the earth are either in a pure form or
in an impure form. The pure substance that cannot be
divided into other simpler substances is called an Gold
element. Each element is composed of same type of the
smallest particles known as atoms. The atoms of an
element are always identical while the atoms of different
elements differ from each other. For example, gold is Carbon
made up of only gold atoms, carbon is made up of only carbon atoms, etc. So, when
we try to divide them into smaller pieces, we get only their atoms, not the atoms of
other elements. Due to this reason, elements are the purest form of matters.
Most of the elements occur naturally. However, MEMORY PLUS
scientists have also synthesized some elements
in laboratory. There are 118 elements discovered Promethium, technetium, Curium,
till now. Out of 118 elements, 92 elements occur Plutonium, etc. are examples of
synthesized elements.
Modern Concept Science and Environment - 7 139
in natural while remaining are synthesized in the laboratory. Hydrogen, helium,
oxygen, carbon, iron, gold, lead, copper, sodium, magnesium, etc. are some examples
of natural elements.
At standard temperature and pressure, different elements are found in different
chemical and physical states. Elements like carbon, copper, gold, iron, magnesium,
silver, etc. occur in solid state. Elements like bromine, mercury, caesium, gallium, etc.
occur in liquid state. Similarly, elements like hydrogen, oxygen, nitrogen, etc. occur
as gases.
FACT WITH REASON
Why is sodium called an element?
Sodium is made up of only one type of atoms called sodium atoms. So, sodium is called an element.
Atom
Elements are made up of atoms. An atom is the MEMORY PLUS
smallest particle of an element that takes part in The smallest particle of an element
chemical reaction without division. Atoms cannot be is called an atom and the smallest
seen with our naked eyes. They can neither be particle of a compound is called
created nor be destroyed. An element contains only molecule.
the same type of atoms. Different elements have different types of atoms. An atom
shows all the properties of the element it belongs to.
There are 118 different atoms of 118 different elements.
All 118 types of atoms do not take part is chemical
reaction. The atoms that do not take part in chemical
reaction are called inert atoms. The elements that have
inert atoms are called inert elements. Atoms of gases like
helium, neon, argon, etc. do not take part in chemical
reaction and thus exist freely. So, they are also known as A neutral atom
inert gases. Atoms of other elements like hydrogen, oxygen, nitrogen, etc. cannot exist
freely.
Structure of an atom
An atom is made up of three major sub-atomic particles called protons, electrons
and neutrons. Protons are the positively charged particles that are present in the
nucleus (centre) of an atom. Neutrons have no charge. They are present along with
the protons in the nucleus. Electrons are negatively charged particles that revolve
around the nucleus in different shells or orbits. Usually, the number of electrons
in the outermost orbit of an atom describes its characteristics. Shells or orbits are
imaginary spheres where electrons revolve around.
140 Matter
FACT WITH REASON
An atom is electrically neutral, why?
An atom is electrically neutral as the number of positively charged protons is equal to the
negatively charged electrons in an atom.
Symbol MEMORY PLUS
It is tedious and time consuming to write the exact Symbol of silver (Ag), copper (Cu), gold
names of the elements or atoms in chemistry. (Au), iron (Fe), etc. are derived from
Therefore, symbols are used as the short, quick and their Latin and German roots. Their
convenient way to write the name of the elements. A Latin names are Argentum, Cuprum,
symbol of an element is the shorthand representation Aurum, and Ferrum respectively.
of that element in a unique way. It also represents the atom of that element.
The symbol of all elements should be different from each other for their identification.
Therefore, unique one or two letters from English alphabet has been assigned to each
element. The symbols of some elements have been derived from English roots while some
from Latin and German roots. The symbols of elements are derived from either first,
first and second or first and third letter or first and any other significant letter of their
English or Latin names. A symbol always has the first letter capital and second letter
small. Therefore, the symbol of an element is the abbreviation of an element which is
represented by one or two English alphabets.
The symbol of Hydrogen is its first letter ‘H’. Similarly, symbol of Carbon is ‘C’.
Chlorine and Chromium both have ‘Ch’ as their first and second letter. Since, any two
elements cannot have same symbols; therefore, we write ‘Cl’ for chlorine and ‘Cr’ for
Chromium as their first and third letters. The symbol of Sodium should be “So” or
“S” but it is ‘Na’ after its Latin name Natrium. Similarly, the symbol of potassium is
‘K’ after its Latin name Kalium.
The symbol of first twenty elements are provided in the table below:
Atomic Number Name of Elements Symbol
1. Hydrogen H
2. Helium He
3. Lithium Li
4. Beryllium Be
5. Boron B
6. Carbon C
7. Nitrogen N
8. Oxygen O
Modern Concept Science and Environment - 7 141
9. Fluorine F
10. Neon Ne
11. Sodium ( Latin: Natrium) Na
12. Magnesium Mg
13. Aluminium Al
14. Silicon Si
15. Phosphorous P
16. Sulphur S
17. Chlorine Cl
18. Argon Ar
19. Potassium (Latin: Kalium) K
20. Calcium Ca
The names of some elements with their Latin or German root names are presented in
the table below:
S.N. Name of Element Root name and origin Symbol
1 Sodium Natrium (Latin) Na
2 Potassium Kalium (Latin) K
3 Copper Cuprum (Latin) Cu
4 Iron Ferrum (Latin) Fe
5 Mercury Hydrargyrum (Latin) Hg
6 Gold Aurum (Latin) Au
7 Silver Argentum (Latin) Ag
8 Tin Stannum (Latin) Sn
9 Antimony Stibium (Latin) Sb
10 Lead Plumbum (Latin) Pb
11 Tungsten Wolfram (German) W
Compound
A compound is a pure substance formed by the combination of two or more elements
in a definite proportion by their weight. Compounds are pure substances because
they are composed of elements which are pure. A compound is a completely new
substance that has totally different properties than their combining elements. For
example, hydrogen gas and oxygen gas are combined to form a compound called
water. Here, hydrogen and oxygen are gases while the product formed is liquid in
nature. Similarly, hydrogen burns itself; oxygen helps in burning; but the compound
water, neither burns nor supports burning.
142 Matter
FACT WITH REASON
Compound is a pure substance although it has two or more elements. Why?
Compound is a pure substance although it has two or more elements because it has two or more
atoms or elements combined in fixed ratio by their wight.
Atoms of an element have a capacity
to combine with atoms of some other
elements. Such a combination makes a
large number of compounds. Substances CO2 H2O
like glucose (C6H12O6), carbon dioxide
(CO2), water (H2O), etc. are the examples C6H12O6
of some common compounds. Observe
the compounds glucose, carbon dioxide and water in the given figures. The compound
glucose is formed by the combination of 6 atoms of oxygen (O), 12 atoms of hydrogen
and 6 atoms of carbon. In carbon dioxide, 2 atoms of oxygen has combined with 1
atom of carbon. Likewise, in water, there is a combination between 1 atom of oxygen
and 2 atoms of hydrogen. In the same way, can you say about the combination of
atoms in the compounds ammonia (NH3), methane (CH4) and citric acid (C6H8O7)?
FACT WITH REASON
Sodium chloride is a compound. Give reason.
Sodium chloride is a compound because it is formed by the combination of sodium and chlorine
in a definite ratio.
Molecule
The smallest particle of a compound that can exist MEMORY PLUS
freely in nature is called a molecule. To exist freely
means to remain in nature without the need to The symbol of molecular formula of a
compound and molecule is the same.
combine with other atoms. Molecules of same compounds are alike while molecules
of different compounds are different in their properties. Molecules are formed by the
combination of similar or dissimilar atoms.
A molecule of hydrogen gas (H2) is formed by
the combination of two similar atoms. Other
such examples are Nitrogen gas (N2), Oxygen
gas (O2), etc.
Hydrogen Nitrogen Oxygen
Methane Salt
Similarly, a molecule of methane (CH4) is formed by the
combination of four atoms of hydrogen (H) and one atom
of carbon (C). In sodium chloride, one atom of sodium (Na)
and one atom of chlorine (Cl) combine to form a molecule
of sodium chloride (NaCl).
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