Living Science Physics Class 8 ICSE Solutions Chapter 6 Heat Transfer

ICSE Living Science Physics for Class 8 Solutions Chapter 6 Heat Transfer

Check Your Progress

Write true or false. Correct the false statements.
Question 1.
Boiling causes cooling.
Answer:
False.
Correct : Evaporation causes cooling.

Question 2.
Wet clothes dry because the water in them boils to its vapour state.
Answer:
False.
Correct : Wet clothes dry because the water in them evaporates to its vapour state.

Question 3.
Boiling occurs in the whole volume of a liquid whereas evaporation occurs only at the surface of a liquid.
Answer:
True

Question 4.
Water evaporates at all temperatures.
Answer:
True

Think and answer

Question 1.
Does a glass tumbler get affected when ice is placed in it ?
Answer:
A thick glass tumbler could crack when ice is placed in it. This happens because glass is a bad conductor of heat. This results in the uneven expansion, the contraction of the inner surface being more than the outer one. As a result, a strain builds up in the glass due to which it cracks.

A. Tick the most appropriate answer.

Question 1.
The process that occurs when water changes into water vapours at 30°C is called
a. cubical expansion
b. boiling
c. evaporation
d. superficial expansion
Answer:
c. evaporation

Question 2.
Evaporation causes
a. heating
b. boiling
c. melting
d. cooling
Answer:
d. cooling

Question 3.
The length of an iron rod increases from 10 m to 11m, when kept over a burner flame. This increase in length is due to
a. cubical expansion.
b. linear expansion.
c. superficial expansion.
d. solidification.
Answer:
b. linear expansion.

Question 4.
There are four iron rods R₄, R₂, R₃ and R₄ of the same dimensions. The temperature of Rt is raised to 500°C, that of R₂ to 250°C, that of R₃ to 1000°C and that of R. to 800°C. Which rod will have the maximum linear expansion?
a. R₁
b. R₂
c. R₃
d. R₄
Answer:
c. R₃

Question 5.
The liquid that expands the minimum on heating is
a. benzene.
b. water.
c. alcohol.
d. ether.
Answer:
b. water.

Question 6.
The factors on which linear expansion depend on are
a. original length of the solid.
b. increase in the temperature of the solid.
c. nature of the solid.
d. all of these
Answer:
d. all of these

Question 7.
The type of thermal expansion that takes place in gases is
a. linear expansion.
b. superficial expansion,
c. cubical expansion.
d. all of these
Answer:
c. cubical expansion.

B. Fill in the blanks.

1. Evaporation occurs only at the …………. of a liquid.
2. Expansion in ………….. of a solid on heating is called superficial expansion.
3. The process of ………….. occurs at the boiling point of a liquid.
4. Boiling is a process.
5. Brass expands ………… than steel.
6. ………….. expand more than liquids.
7. A 20 m steel rod expands …………. than a SO m steel rod, when heated to 500°C for 10 minutes.
8. The density of water is maximum at ………..

Answer:

1. Evaporation occurs only at the surface of a liquid.
2. Expansion in area of a solid on heating is called superficial expansion.
3. The process of boiling occurs at the boiling point of a liquid.
4. Boiling is a brisk and violent process.
5. Brass expands more than steel.
6. Gases expand more than liquids.
7. A 20 m steel rod expands less than a 80 m steel rod, when heated to 500°C for 10 minutes.
8. The density of water is maximum at 4 °C.

C. Match the columns

1. Expansion in length	a. evaporation
2. Water to water vapour at 100°C	b. superficial expansion
3. Expansion in volume	c. linear expansion
4. Expansion in area	d. boiling
5. Water to water vapour at 25°C	e. cubical expansion

Answer:

1. Expansion in length	c. linear expansion
2. Water to water vapour at 100°C	d. boiling
3. Expansion in volume	e. cubical expansion
4. Expansion in area	b. superficial expansion
5. Water to water vapour at 25°C	a. evaporation

D. Write true or false. Correct the false statements.

Question 1.
During the process of boiling, the temperature of the liquid remains constant.
Answer:
True

Question 2.
Evaporation is a slow and silent process.
Answer:
True

Question 3.
Water in earthen pots remains cool due to thermal expansion.
Answer:
False.
Correct : Water in earthen pots remain cool due to evaporation.

Question 4.
Most of the substances decrease in their dimensions on heating.
Answer:
False.
Correct : Most of the substances experience increase in their dimensions when heated.

Question 5.
Superficial expansion can only occur in solids and liquids.
Answer:
False.
Correct: Superficial expansion only occurs in solids.

Question 6.
Gravesande’s ring-and-ball experiment shows that liquids expand on heating.
Answer:
False.
Correct : Gravesand’s ring-and-ball experiment shows that solids expand on heating.

Question 7.
Glass is a good conductor of heat.
Answer:
False.
Correct : Glass is a poor conductor of heat.

Question 8.
Out of solids, liquids and gases, gases expand the most.
Answer:
True

E. Define the following terms.

1. Evaporation
2. Boiling
3. Thermal expansion
4. Linear expansion
5. Cubical expansion
6. Superficial expansion

Answer:

1. Evaporation : The change of a substance from its liquid state to its gaseous state at any temperature below its boiling point is called evaporation.
2. Boiling : The process by which a liquid changes into its gaseous state at some fixed temperature by absorption of heat energy is known as boiling.
3. Thermal expansion : The expansion produced in matter due to the absorption of heat energy is called thermal expansion.
4. Linear expansion : The increase in length of a solid on heating is called linear expansion.
5. Cubical expansion : Expansion in volume of solid, liquid or gas on heating is called cubical expansion.
6. Superficial expansion : Expansion in area of a solid on heating is called superficial expansion.

F. Answer the following in short.

Question 1.
Why is the process of evaporation occurs only at the surface?
Answer:
Evaporation occurs only at the surface of the liquid. This is because in a liquid, the molecules are in constant random motion. The speed of these molecules keep changing due to their collisions with other molecules. If molecules near the surface of the liquid gain sufficient energy, they escape and become vapour.

Question 2.
Name three different types of thermal expansion.
Answer:
Three different types of thermal expansion are linear expansion, superficial expansion and cubical expansion.

Question 3.
Name the type of thermal expansion that occurs in solids, liquids and gases.
Answer:
Cubical expansion occurs in all – solids, liquids and gases.

Question 4.
How does a brass-iron bimetallic strip bend on heating?
Answer:
On heating, a brass-iron bimetallic strip acquires a curved shape such that brass is on the outer side of the curve.

Question 5.
Write two applications of thermal expansion of solids in our day-to-day lives.
Answer:
Two applications of thermal expansion of solids in our day to day lives are :
a. Telephone and electric wires are always kept sagging between utility poles to prevent them from snapping in winter.
b. A cemented floor is not laid in one block. Instead, the floor is laid in small blocks leaving very small spaces in between them to allow their expansion in summer.

Question 6.
Name the factors that decide the linear expansion In an aluminum rod.
Answer:
The factors on which linear expansion in an aluminium rod depends are :
a. original length of the rod, and
b. increase in temperature of the rod.

Question 7.
Explain the phenomenon of thermal expansion in solids.
Answer:
When a solid is heated, the kinetic energy of its particles increases and the particles start moving more vigorously. As a result, the distance between the particles increases and the substance expands. But, as the particles are closely packed and are held together firmly, so, solids expand very little on heating.

G. Answer the following in detail.

Question 1.
Explain the process of evaporation.
Answer:
The change of a substance from its liquid state to its gaseous state at any temperature below its boiling point is called evaporation. In a liquid, the molecules are in constant random motion. The speed of these molecules keep changing due to their collisions with other molecules. If molecules near the surface of the liquid gain sufficient energy, they escape and become vapour.

This is the process of evaporation. When the temperature of the liquid increases, the average kinetic energy of the molecules increases. This means that there will be a larger proportion of the molecules that have sufficient energy to escape from the surface. Hence, the rate of evaporation increases with increase in temperature.

Question 2.
Show, with the help of an activity, that gases expand on heating.
Answer:
The following activity shows that gases expand on heating. Materials required : A metal can, a balloon, a Bunsen burner and a tripod stand
Procedure : Fix the balloon around the neck of the empty metal can. Heat the can by using the burner. Observation : The balloon grows bigger in size as the air inside it expands on heating.
Conclusion : Gases expand on heating.

Question 3.
How is evaporation different from boiling?
Answer:

Question 4.
Explain how an iron rim is mounted on a wooden cart wheel.
Answer:
An iron rim, whose diameter is made slightly smaller than the wooden wheel, is heated. It expands on heating. The hot iron rim is then placed around the wooden wheel to fit easily. Finally, cold water is poured on the wheel, which causes the metal rim to contract and grip the wheel tightly.

Question 5.
Explain, with the help of a diagram, the working of an automatic fire alarm.
Answer:
An automatic fire alarm uses an iron-brass bimetallic strip.

The end A of the strip is tightly fixed on a wooden board and the end B is free to move up and down. An adjustable contact screw C made of brass is also provided. This screw is held 1 mm above the end B of the strip. In case of a fire, the bimetallic strip gets heated and bends. The end B rises up as brass expands more than iron. When the end B touches the contact screw C, the electrical circuit is completed and the electric bell starts ringing.

Question 6.
Prove that different liquids expand differently when subjected to the same amount of heat.
Answer:
The following activity shows that different liquids expand differently when subjected to the same amount of heat. Materials required : Four identical round-bottomed flasks with long narrow stems, a glass trough, a stand to hold the stems of the flasks, a heater, water, ether, benzene and alcohol

Procedure : Label the flasks as A, B, C and D. Fill equal volumes of water, ether, benzene and alcohol in different flasks. Place all the flasks in the empty glass trough. Now, pour heated water in the trough and observe the increase in the level of the liquid in each flask.

Observation : You will notice that all liquids rise to different levels showing different amounts of expansion. Ether expands the maximum followed by benzene, alcohol and water.

Conclusion : This clearly shows that different liquids expand differently even when they are subjected to the same amount of heat.

Question 7.
Describe an experiment to show that solids expand on heating.
Answer:
The following activity shows that solids expand on heating.
Materials required : An iron ball, an iron ring, a metal stand with a hook and a burner

Procedure : Take the iron ball and the iron ring such that the ball should just pass through the ring. Suspend the ball by the metal chain from the hook of the stand. Heat the ball and then try passing the ball through the ring. What do you observe’? Now, cool the ball and try passing it through the ring again. What do you observe now?

Observation :
1. When the ball is heated, it does not pass through the ring, because it expands on heating.
2. When the ball is cooled, it easily passes through the ring, as it regains its original size.

Conclusion : Solids expand on heating.

H. Give reasons for the following.

Question 1.
Evaporation is accompanied by cooling.
Answer:
Evaporation is accompanied by cooling because the heat energy required for the surface molecules to escape is provided by the surface the liquid is in contact with.

Question 2.
Solids expand the least on heating.
Answer:
In solids, the particles are closely packed and are held together firmly. So, solids expand very little on heating. In liquids, particles are loosely packed and have more free space. On heating, the distance between the particles increases. So, liquids expand more than solids. In gases, particles are free to move and the force of attraction between them is almost zero. So, gases expand the most and solids expand the least.

Question 3.
Small gaps are left between successive rail tracks.
Answer:
Railway tracks are made of steel, which expands in summer when temperature rises. If the rails are fixed tightly next to each other, leaving no space for expansion, they will bend outward, and may cause derailment of a train. Hence, small gaps are left between successive rail tracks to allow them to expand in summer without any change in their shapes.

Question 4.
Electric transmission lines are kept loose between electric poles.
Answer:
Electric transmission lines are always kept sagging between the electric poles to prevent them from snapping in winter due to contraction.

Question 5.
A glass tumbler cracks on heating.
Answer:
A glass tumbler cracks on heating. This happens because glass is a poor conductor of heat. Heating a thick glass tumbler results in the uneven expansion, the expansion of the inner surface being more than the outer one. As a result, a strain builds up in the glass due to which it cracks.

Question 6.
Liquids and gases have no linear or superficial expansion.
Answer:
Liquids and gases have no linear or superficial expansion as they have no definite shape.

Question 7.
A little space is left in medicine bottles. –
Answer:
A little space is left in medicine bottles to allow space for expansion when they are sent from cold to hot conditions.

I. Equal volumes of four liquids – alcohol, benzene, ether and water – are poured in four flasks and kept in hot water. Label the liquids corresponding to A, B, C and D in the following figure.

Answer:
A – Water
B – Ether
C – Benzene
D – Alcohol

Think and answer

Question 1.
Why does a tightly fixed lid of a bottle come loose when kept under hot water ?
Answer:
A tightly fixed lid of a bottle when kept under hot water expands and gets loosened. So, it can be opened easily.

Question 2.
What would happen to aquatic life if water contracts on cooling till 0 °C and expands on heating ?
Answer:
If water contracts on cooling till 0°C and expands on heating, then the whole depth of water of sea and ocean will freeze and aquatic life will be destroyed.

Question 3.
Why does tea cool more rapidly in a saucer than in a cup ?
Answer:
In a saucer, larger area of tea is exposed. So, evaporation takes place from a larger area than in a cup. Hence, tea cools more rapidly in a saucer than in a cup.

Question 4.
Why do the tyres of a car filled with compressed air become slightly fiat in winter ?
Answer:
In winters, the air inside the tyres contracts and gets compressed. Hence, the tyres of a car filled with compressed air becomes slightly flat in winters.

Living Science Physics Class 8 ICSE Solutions