Heat Transfer

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 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 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.

(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.

The three states of matter are Solid, Liquid and Gas.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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).

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.

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).

all the above three factors

Reason — The increase in length of a solid rod on heating depends on its original length, temperature and material.

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.

A matter is something that has mass, occupies space and can be perceived by our senses. It is composed of tiny particles called molecules.

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.

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.

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.

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.

The average kinetic energy of molecules of a liquid increases during boiling of the liquid at its boiling point.

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.

The two ways to change liquid state to vapour state are:

1. Boiling
2. Evaporation

Differences between Boiling and Evaporation are listed below:

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

Iron and alcohol expand on heating.

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

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.

When a cubical metal solid block is heated, its length, breadth and thickness increases. So the volume also increases.

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

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.

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.

No, water will not expand. Water contracts on heating it from 0°C to 4°C. On heating beyond 4°C it expands.

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.

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.

(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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

(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.

(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.