Aarohi mishra 8th red
Chapter 6: Heat Transfer
25-32 minutes
In evaporation:
1. all molecules of liquid begin to escape out
2. only the molecules at the surface escape out
3. the temperature of liquid rises by absorbing heat from
surroundings
4. the molecules get attracted within the liquid
Answer
only the molecules at the surface escape out
Reason — In evaporation, some molecules near the surface
of liquid acquire sufficient kinetic energy by collisions with
the other molecules, to reach the surface. These molecules
absorb heat from the surroundings to escape out in space.
The rate of evaporation of a liquid increases when:
1. temperature of liquid falls
2. liquid is poured in a vessel of less surface area
3. air is blown above the surface of liquid
4. humidity increases
Answer
air is blown above the surface of liquid
Reason — Blowing air above the surface of liquid takes
away the molecules escaping from liquid surface and to take
their place other molecules move to the surface of liquid thus
increasing evaporation rate.
During boiling or vaporization:
1. all molecules take part
2. temperature rises
3. no heat is absorbed
4. the average kinetic energy of molecules increases
Answer
all molecules take part
Reason — During boiling or vaporization, all the molecules
of the liquid absorb heat energy supplied to it from an
external source. Hence, all the molecules of the liquid
acquire sufficient kinetic energy to take part in boiling or
vaporization.
The boiling point of a liquid is increased by:
1. increasing the volume of liquid
2. increasing the pressure on liquid
3. adding ice to the liquid
4. decreasing pressure on liquid
Answer
increasing the pressure on liquid
Reason — At boiling point vapour pressure is equal to
atmospheric pressure, thus boiling point depends upon
surrounding atmospheric pressure. With increase in
atmospheric pressure, boiling point increases.
Two rods A and B of the same metal, but of length 1 m and 2
m respectively, are heated from 0°C to 100°C. Then:
1. both the rods A and B elongate the same
2. the rod A elongates more than the rod B
3. the rod B elongates more than the rod A
4. the rod A elongates, but the rod B contracts
Answer
the rod B elongates more than the rod A
Reason — The increase in length of a solid rod on heating
depends on its original length, temperature and material.
Longer the rod, greater is the increase in its length. Since
temperature and material are same so greater length rod B
(2 m) will elongates more than rod A (1 m).
Two rods A and B of the same metal, same length, but one
solid and other hollow, are heated to the same rise in
temperature Then:
1. the solid rod A expands more than the hollow rod B
2. the hollow rod B expands more than the solid rod A
3. the hollow rod B contracts, but the solid rod A expands
4. both the rods A and B expand the same
Answer
both the rods A and B expand the same
Reason — The increase in length of a rod on heating does
not depend on whether it is hollow or solid.
A given volume of alcohol and the same volume of water are
heated from the room temperature to the same temperature
then:
1. alcohol contracts, but water expands
2. water contracts, but alcohol expands
3. water expands more than alcohol
4. alcohol expands more than water
Answer
alcohol expands more than water
Reason — Equal volumes of different liquids when heated
to same temperature expand by different amount. Expansion
of a liquid depends upon its coefficient of linear expansion
(γ).
The increase in length of a metal rod depends on:
1. the initial length of the rod only
2. the rise in temperature only
3. the material of the rod only
4. all the above three factors
Answer
all the above three factors
Reason — The increase in length of a solid rod on heating
depends on its original length, temperature and material.
The correct statement is:
1. Iron rims are cooled before they are placed on the cart
wheels.
2. A glass stopper gets tightened on warming the neck of
the bottle.
3. Telephone wires sag in winter, but become tight in
summer.
4. A little space is left between two rails on a railway track.
Answer
A little space is left between two rails on a railway track
Reason — During summer season due to rise in
temperature the rails expands, so little space is left between
two rails on a railway track, otherwise the rails will bend
sideways.
Assertion (A) : A humid environment slows the rate of
evaporation.
Reason (R) : In humid environment, vapour molecules do
not find the space to escape.
1. Both A and R are true and R is the correct explanation of
A
2. Both A and R are true and R is not the correct
explanation of A
3. Assertion is true but reason is false
4. Assertion is false but reason is true
Answer
Both A and R are true and R is the correct explanation of A
Explanation
Assertion (A) is true because a humid environment already
has a high concentration of water vapour in the air thus, the
rate of evaporation becomes slow.
Reason (R) is true because since the air is already saturated
with vapour, fewer water molecules from the surface can
escape into the air so they don’t find enough space to move
into.
Here, reason clearly justifies why the rate of evaporation is
slow in humid environment.
Assertion (A) : Glass containers with narrow necks are
more likely to break when hot water is poured into them
suddenly.
Reason (R) : Glass is a good conductor of heat.
1. Both A and R are true and R is the correct explanation of
A
2. Both A and R are true and R is not the correct
explanation of A
3. Assertion is true but reason is false
4. Assertion is false but reason is true
Answer
Assertion is true but reason is false
Explanation
Assertion (A) is true because when hot liquid is poured into a
glass container, the inner surface of the glass becomes hot,
while its outer surface remains at the room temperature.
Therefore, the inner surface of the container expands, while
its outer surface does not expand so this unequal expansion
cracks and ultimately breaks the glass container.
Reason (R) is false because glass is a poor conductor of
heat.
Assertion (A) : A gas expands less than a liquid for the
same rise in temperature.
Reason (R) : Gases have negligible intermolecular forces of
attraction.
1. Both A and R are true and R is the correct explanation of
A
2. Both A and R are true and R is not the correct
explanation of A
3. Assertion is true but reason is false
4. Assertion is false but reason is true
Answer
Assertion is false but reason is true
Explanation
Assertion (A) is false because, for a given increase in
temperature, a gas expands much more than either a liquid
or a solid.
Reason (R) is true because in a gas the molecules are much
farther apart than those in liquids or solids, so the
intermolecular forces of attraction are negligible and the
molecules move freely throughout the available space.
Fill in the blanks:
(a) Boiling occurs at ............... .
(b) Evaporation takes place at ............... .
(c) The molecules of liquid ............... heat from surroundings
in evaporation.
(d) Heat is ............... during boiling.
(e) Cooling is produced in ............... .
(f) A longer rod expands ............... than a shorter rod on
being heated to the same temperature.
(g) Liquids expand ............... than the solids.
(h) Gases expand ............... than the liquids.
(i) Alcohol expands ............... than water.
(j) Iron expands ............... than copper.
Answer
(a) Boiling occurs at a fixed temperature.
(b) Evaporation takes place at all temperatures.
(c) The molecules of liquid absorb heat from surroundings in
evaporation.
(d) Heat is absorbed during boiling.
(e) Cooling is produced in evaporation.
(f) A longer rod expands more than a shorter rod on being
heated to the same temperature.
(g) Liquids expand more than the solids.
(h) Gases expand more than the liquids.
(i) Alcohol expands more than water.
(j) Iron expands less than copper.
Write true or false for each statement:
(a) Evaporation is rapid on a wet day.
(b) Evaporation takes place only from the surface of a liquid.
(c) All molecules of a liquid take part in the process of
evaporation.
(d) Temperature of a liquid rises during boiling or
vaporization.
(e) All molecules of a liquid take part in boiling.
(f) Boiling is a rapid phenomenon.
(g) All solids expand by the same amount when heated to
the same rise in temperature.
(h) Telephone wires are kept tight between the two poles in
winter.
(i) Equal volumes of different liquids expand by different
amounts when they are heated to the same rise in
temperature.
(j) Solids expand the least and gases expand the most on
being heated.
(k) A mercury thermometer makes use of the property of
expansion of liquids on heating.
(l) Kerosene contracts on heating.
Answer
(a) False
Correct Statement — Evaporation is rapid on a dry day.
(b) True
(c) False
Correct Statement — Only the molecules on the surface of
a liquid take part in the process of evaporation.
(d) False
Correct Statement — Temperature of a liquid remains
constant during boiling or vaporization.
(e) True
(f) True
(g) False
Correct Statement — All solids do not expand by the same
amount when heated to the same rise in temperature.
(h) True
(i) True
(j) True
(k) True
(l) False
Correct Statement — Kerosene expands on heating.
Match the following:
Column A Column B
(i) increase in inter-molecular
(a) Blowing air increases
separation
(b) Increase in pressure
(ii) pendulum of a clock
increases
(c) Thermal expansion (iii) cooking utensils
(d) Invar (iv) boiling point
(e) Pyrex glass (v) evaporation
Answer
Column A Column B
(a) Blowing air increases (v) evaporation
(b) Increase in pressure
(iv) boiling point
increases
(i) increase in inter-molecular
(c) Thermal expansion
separation
(d) Invar (ii) pendulum of a clock
(e) Pyrex glass (iii) cooking utensils
What is matter? What is it composed of?
Answer
A matter is something that has mass, occupies space and
can be perceived by our senses. It is composed of tiny
particles called molecules.
Do all the molecules of a liquid take part in evaporation? If
not, explain your answer.
Answer
No, all the molecules of a liquid do not take part in
evaporation. Only the molecules on the surface of the liquid
take part in evaporation.
In evaporation, some molecules near the surface of the
liquid acquire sufficient kinetic energy by collisions with the
other molecules, to reach the surface. These molecules
absorb heat from the surroundings to escape out in space
forming vapours of the liquid. Hence, only the molecules on
the surface of the liquid take part in evaporation.
No heat is supplied to a liquid during evaporation. How does
then the liquid change into its vapour?
Answer
Due to weak inter-molecular force of attraction the molecules
of a liquid move freely inside it. In the process, they collide
with each other transferring energy between them.
Molecules with higher kinetic energy reach the surface of the
liquid. These molecules absorb some more heat from the
surroundings to gain sufficient kinetic energy to overcome
the force of attraction on them due to other molecules inside
the liquid and escape out from the surface into space. These
escaping molecules form the vapour of the liquid. Thus,
evaporation of the liquid happens even though no heat is
supplied to it.
Comment on the statement 'Evaporation is a surface
phenomenon'.
Answer
Due to weak inter-molecular force of attraction the molecules
of a liquid move freely inside it. In the process, they collide
with each other transferring energy between them.
Molecules with higher kinetic energy reach the surface of the
liquid while those with lesser kinetic energy remain inside
the liquid. The higher energy molecules on the surface
absorb some more heat from the surroundings to gain
sufficient kinetic energy to do work against the force of
attraction on them due to other molecules inside the liquid
and escape out from the surface into space. These escaping
molecules form the vapour of the liquid. Thus, evaporation of
the liquid happens only from the surface of the liquid. Hence,
Evaporation is a surface phenomenon.
Why is cooling produced when a liquid evaporates.
Answer
In the process of evaporation, a liquid changes into vapour
and for this purpose some heat is needed. In evaporation,
no external heat is supplied so the liquid will draw the
necessary heat from its surroundings. This produces cooling
effect when the liquid evaporates.
What is the change in average kinetic energy of molecules
of a liquid during boiling at its boiling point?
Answer
The average kinetic energy of molecules of a liquid
increases during boiling of the liquid at its boiling point.
How is the heat energy supplied to a liquid used during
boiling at a fixed temperature?
Answer
During boiling at a fixed temperature, the heat energy
supplied to the liquid is used in increasing the average
kinetic energy of the liquid molecules so that they acquire
sufficient kinetic energy to overcome the force of attraction
of other molecules and escape out from the liquid in form of
vapour.
Name two ways of changing liquid state to the vapour state
and distinguish them.
Answer
The two ways to change liquid state to vapour state are:
1. Boiling
2. Evaporation
Differences between Boiling and Evaporation are listed
below:
Boiling Evaporation
Boiling takes place at fixed Evaporation takes place at all
temperature. temperatures.
Boiling is a rapid process. Evaporation is a slow process.
Boiling takes place
Evaporation takes place only
throughout the mass of the
from the surface of the liquid.
liquid at the same instant.
In evaporation, liquid molecules
In boiling, external heat is absorb heat from its
supplied. surroundings to convert into
vapour state.
What do you understand by thermal expansion of a
substance?
Answer
The expansion of a substance when heated is called thermal
expansion.
Give two examples of the substances which expand on
heating.
Answer
Iron and alcohol expand on heating.
State three factors on which depend the linear expansion of
a metal rod on heating.
Answer
The three factors on which the linear expansion of a metal
rod on heating depends are:
1. Original length of the rod
2. Increase in temperature
3. Material of the rod
A metal plate is heated. State three factors on which the
increase in its area will depend.
Answer
The increase in area of a metal plate depends on following
three factors:
1. The initial area of the plate.
2. The increase in temperature.
3. Material of the plate.
A cubical metal solid block is heated. How will its volume
change?
Answer
When a cubical metal solid block is heated, its length,
breadth and thickness increases. So the volume also
increases.
State one application of thermal expansion of liquids.
Answer
Thermal expansion of liquids is used in the working of
mercury thermometer.
Which of the following will expand more, when heated to the
same temperature:
(a) solid,
(b) liquid or
(c) gas?
Answer
Gas will expand more as compared to solid and liquid
because the inter-molecular force of attraction in gas
molecules is the least and they have high kinetic energy.
100 ml of each of the following liquids is heated from 10°C to
50°C. Which will expand more:
(a) water
(b) benzene
(c) alcohol?
Answer
Benzene will expand more than alcohol and water because
expansion of a liquid depends upon the nature of liquid since
volume and temperature are same. Coefficient of cubical
expansion of benzene is more than alcohol and water. So it
expands the most.
Water is heated from 0°C to 4°C. Will it expand?
Answer
No, water will not expand. Water contracts on heating it from
0°C to 4°C. On heating beyond 4°C it expands.
What do you mean by anomalous behavior of water?
Answer
Water contracts on heating it from 0°C to 4°C and then
beyond 4°C on further heating, it expands. This is called
anomalous behavior of water.
How does the density of a substance (solid, liquid and gas)
change on heating?
Answer
When a substance is heated, its volume increases while its
mass remains same. So density of substance decreases
with increase in temperature.
In case of solids, when temperature increases, increase in
volume is negligible so decrease in density is not
appreciable.
But in case of liquids and gases as temperature increases,
volume increases by an appreciable amount, so density
decreases considerably.
Name the three states of matter and distinguish them on the
basis of their (i) volume, and (ii) shape
Answer
The three states of matter are Solid, Liquid and Gas.
Property Solid Liquid Gas
Solids have a Gases do not
Liquids have a
Volume definite have a definite
definite volume.
volume. volume.
Liquids do not Gases do not
Solids have a
Shape have a definite have a definite
definite shape.
shape. shape.
Distinguish between liquid and vapour (or gas) states of
matter on the basis of the following factors:
(a) Arrangement of molecules
(b) Inter-molecular separation
(c) Inter-molecular force and
(d) Kinetic energy of molecules
Answer
Property Liquids Gases
Molecules are not rigid
Arrangement Molecules are very
and they are less
of molecules loosely packed
closely packed
The inter-molecular
The inter-molecular
Inter-molecular space is very large
spaces are more than
separation as compared to
solids
solids and liquids
The force is weak and
The force is very
molecules are free to
Inter-molecular weak so molecules
move within the
force are free to move
boundary of the
anywhere
container
Kinetic energy The kinetic energy is Kinetic energy is the
of molecules more than solids most in gases.
What is evaporation? Explain it on the basis of molecular
motion.
Answer
The change of liquid into its vapour at all temperatures from
its surface is called evaporation.
The inter-molecular force of attraction is lesser in liquids as
compared to solids. Hence, liquid molecules can move
throughout the liquid and in the process they collide with
each other transferring energy between them. Molecules
with higher kinetic energy reach the surface of the liquid
while those with lesser kinetic energy remain inside the
liquid. The higher energy molecules on the surface absorb
some more heat from the surroundings to gain sufficient
kinetic energy to do work against the force of attraction on
them due to other molecules inside the liquid and escape out
from the surface into space. These escaping molecules form
the vapour of the liquid. The process continues till all the
liquid evaporates.
What is boiling? Explain it on the basis of molecular motion.
Answer
The change from the liquid state to gaseous state on heating
at a constant temperature is called boiling.
When a liquid is heated the average kinetic energy of the
liquid molecules increases. When the molecules acquire
sufficient kinetic energy to overcome the force of attraction
of other molecules, they escape out in form of vapour
throughout the mass of liquid and not just it's surface.
Describe an experiment to demonstrate the thermal
expansion in solids.
Answer
The thermal expansion in solids can be demonstrated by the
following experiment:
1. Take a ball and ring set-up as shown in figure. It consists
of a metal ball and a ring. The metal ball just slips
through the ring when both are at room temperature
2. Now heat the ball on a burner and place it over the ring.
We will observe that the ball does not pass through the
ring.
3. Now allow the ball to cool by itself and after some time
again place it over the ring. We will observe that the ball
passes through the ring.
Conclusion — On heating the ball expands and becomes
bigger than the diameter of the ring so it does not pass
through the ring. But on cooling the ball contracts and
passes through the ring.
Describe an experiment to show that liquids expand on
heating.
Answer
The following experiment demonstrates that liquids expand
on heating:
1. Take an empty bottle with a tight fitting cork with a hole
drilled in its middle, a drinking straw, two bricks, a wire
gauze and a burner.
2. Fill the bottle completely with water. Add few drops of ink
to make it coloured for easy contrast.
3. Fix the cork in the mouth of the bottle. Pass the drinking
straw through the cork. Put some molten wax around the
hole to avoid leakage of water.
4. Pour some more water in the drinking straw so that water
level in the straw can be seen. Mark the water level in
the straw as shown in figure.
5. Place the bottle on the wire gauze kept over the two
bricks. Heat the bottle using a burner as shown in figure.
6. Look at the level of water in the straw.
When water is heated more and more, the level of water in
the drinking straw rises. This shows that water expands on
heating.
Describe an experiment to show that air expands on heating.
Answer
The following experiment demonstrates that air expands on
heating:
1. Take an empty bottle. The empty bottle contains air.
2. Attach a rubber balloon to its neck as shown in figure.
3. The balloon is deflated initially.
4. Place the bottle in a water bath containing boiling water.
It heats the air present in bottle.
5. After sometime it is noticed that the balloon gets inflated
as shown in figure.
Conclusion — This shows that on heating, air present inside
bottle expands and fills the balloon. So the balloon gets
inflated. This shows that air expands on heating.
Describe an experiment to show that same volume of
different liquids heated to same rise in temperature expand
by different amounts.
Answer
The following experiment demonstrates that same volume of
different liquids heated to same rise in temperature expand
by different amounts:
1. Take four identical glass bottles each fitted with a narrow
glass tube through a cork at its mouth.
2. Fill them up to the same level with different liquids like
water, alcohol, kerosene, benzene.
3. Place them in a common water bath containing boiling
water as shown in figure.
Observations — After sometime it is noticed that different
liquids rise to different levels. Benzene expands the most
followed by alcohol and kerosene. Water expands the least.
Conclusion — From the above experiment it is concluded
that, same volume of different liquids heated to same rise in
temperature expand by different amounts.
Give reason for the increase in rate of evaporation of a liquid
when:
(a) air is blown above the liquid.
(b) surface area of liquid is increased.
(c) temperature of liquid is increased.
Answer
(a) Blowing air above the surface of liquid takes away the
molecules escaping from liquid surface and to take their
place other molecules move to the surface of liquid thus
increasing evaporation rate.
(b) On increasing the surface area of the liquid, number of
molecules escaping out of the surface increases. This
increases the rate of evaporation.
(c) Energy of molecules increases with increase in
temperature. So more and more molecules come to the
surface of the liquid and escape out. Hence, rate of
evaporation increases with increase in temperature.
Why does bubbles appear when a liquid is heated?
Answer
When a liquid is heated the average kinetic energy of the
liquid molecules increases. When the molecules acquire
sufficient kinetic energy to overcome the force of attraction
of other molecules, they escape out in form of vapour
throughout the mass of liquid. As a result, bubbles are
formed. These bubbles move to the surface and boiling of
liquid takes place.
Two iron rods — one 10 m long and the other 5 m long, are
heated to the same rise in temperature. Which will expand
more?
Answer
The Iron rod that is 10 m long will expand more than the rod
of 5 m length because if two rods of different lengths but of
same metal are heated to same temperature, longer rod will
expand more than the shorter one.
Two identical rods of copper are heated to different
temperatures — one by 5°C and the other by 10°C. Which
rod will expand more?
Answer
The copper rod heated by 10°C will expand more than the
rod heated by 5°C because if two identical rods of same
metal and same length are heated to different temperatures,
the rod heated to higher temperature expands more than the
rod heated to lower temperature. More the increase in
temperature of the rod greater will be the increase in its
length.
One rod of copper and another identical rod of iron are
heated to the same rise in temperature. Which rod will
expand more? Give reason.
Answer
Copper rod will expand more than iron rod. The increase in
length of the rod depends on the material of the rod. The
coefficient of linear expansion of copper is more than that of
iron, so copper rod expands more than iron rod for the same
rise in temperature.
Two identical rods — one hollow and the other solid, are
heated to the same rise in temperature. Which will expand
more?
Answer
Both the rods will expand to the same extent as the increase
in length of the rod on heating does not depend upon
whether it is hollow or solid.
In the ball and ring experiment, if the ball after heating is left
to cool on the ring for some time, the ball again passes
through the ring. Explain the reason.
Answer
On heating the ball, it expands and becomes bigger than the
diameter of the ring so it does not pass through the ring. But
on cooling, the ball contracts to its original size so it again
passes through the ring.
Explain the following:
The telephone wires break in winter.
Answer
In winters, the telephone wires contract due to drop in
temperature. When they become too tight between the poles
due to this contraction, they break.
Explain the following:
Iron rims are heated before they are fixed on the wooden
wheels.
Answer
The iron rims are made slightly smaller in diameter than the
wooden wheel. On heating, the rims expand and can easily
slip over the wooden wheel. On cooling, it contracts and
make a tight fit over the wooden wheel.
Explain the following:
Gaps are left between the successive rails on a railway
track.
Answer
In summer season due to rise in temperature, each rail
expands in length. By leaving a gap between the two rails,
the rails get a chance to expand in this space. This prevents
the rails from bending sideways and derailing the train.
Explain the following:
A glass stopper stuck in the neck of a bottle can be removed
by pouring hot water on the neck of the bottle.
Answer
When hot water is poured on the neck of the bottle, the neck
expands and the glass stopper stuck in it gets space to
loosen and then can be easily removed.
Explain the following:
A cement floor is laid in small pieces with gaps in between.
Answer
In summer when temperature rises cement expand. So a
cement floor is laid in small pieces with gaps in between to
allow expansion, otherwise the cement floor will crack.
Why is one end of a steel girder in a bridge kept on rollers
instead of fixing it in a pillar?
Answer
One end of a steel girder in a bridge is kept on rollers
instead of fixing it in a pillar because in summer or in winter
when there is rise or fall in temperature, the girder may
expand or contract without affecting the pillar or bridge.
An empty glass bottle is fitted with a narrow tube at its
mouth. The open end of the tube is kept in a beaker
containing water. When the bottle is heated, bubbles of air
are seen escaping into water. Explain the reason.
Answer
The empty glass bottle is filled with air. When the bottle is
heated, the air inside the bottle expands and it escapes out
through the tube into water in the form of bubbles.
An iron washer is heated. State the effect on its:
(a) mass
(b) internal diameter
(c) external diameter and
(d) density.
Answer
(a) Mass does not change on heating.
(b) When iron washer is heated, it expands so internal
diameter increases.
(c) External diameter increases as iron washer expands on
heating.
(d) Density decreases as volume of the iron washer
increases on heating.
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