Heat Transfer

Objective Questions

Question 1

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.

Question 2

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) a fixed temperature

(b) all temperatures

(c) absorb

(d) absorbed

(e) evaporation

(f) more

(g) more

(h) more

(i) more

(j) less

Question 3

Match the following:

Answer

Question 4a

Select the correct alternative:

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.

Question 4b

Select the correct alternative:

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.

Question 4c

Select the correct alternative:

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.

Question 4d

Select the correct alternative:

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.

Question 4e

Select the correct alternative:

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).

Question 4f

Select the correct alternative:

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.

Question 4g

Select the correct alternative:

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).

Question 4h

Select the correct alternative:

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.

Question 4i

Select the correct alternative:

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.

Short/Long Answer Questions

Question 1

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.

Question 2

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.

Question 3

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

Question 4

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.

Question 5

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.

Question 6

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.

Question 7

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.

Question 8

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.

Question 9

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.

Question 10

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.

Question 11

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.

Question 12

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.

Question 13

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.

Question 14

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:

Question 15

What do you understand by thermal expansion of a substance?

Answer

The expansion of a substance when heated is called thermal expansion.

Question 16

Give two examples of the substances which expand on heating.

Answer

Iron and alcohol expand on heating.

Question 17

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.

Question 18

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

Question 19

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.

Question 20

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.

Question 21

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 (17) is more than that of iron (12), so copper rod expands more than iron rod for the same rise in temperature.

Question 22

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.

Question 23

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.

Question 24(a)

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.

Question 24(b)

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.

Question 24(c)

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.

Question 24(d)

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.

Question 24(e)

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.

Question 25

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.

Question 26

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.

Question 27

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.

Question 28

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.

Question 29

State one application of thermal expansion of liquids.

Answer

Thermal expansion of liquids is used in the working of mercury thermometer.

Question 30

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.

Question 31

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.

Question 32

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.

Question 33

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.

Question 34

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.

Question 35

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.

Question 36

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.

Question 37

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.

Question 38

An iron washer is heated. State the effect on its:

(i) mass

(ii) internal diameter

(iii) external diameter and

(iv) density.

Answer

(i) Mass does not change on heating.

(ii) When iron washer is heated, it expands so internal diameter increases.

(iii) External diameter increases as iron washer expands on heating.

(iv) Density decreases as volume of the iron washer increases on heating.