NCERT Solutions for Class 7 Science Chapter 4 Heat

1. State similarities and differences between the laboratory thermometer and the clinical thermometer.

Answer:

Similarities:-
i) In both laboratory thermometer and clinical thermometer, mercury is used

ii) Both laboratory and clinical thermometer consist of a long, narrow uniform tube of glass.

Differences:-

i) The least count of both the thermometer is different.

ii) Range of the temperature in a laboratory thermometer is from $-{10}^{\circ }Cto{110}^{\circ }C$ whereas in clinical thermometer, it is from ${35}^{\circ }Cto{42}^{\circ }C$

iii) Clinical thermometer is used to measure body temperature whereas laboratory thermometer is not used for such purposes

2. Give two examples each of conductors and insulators of heat.

Answer:

Two examples of

i) Conductor :-
1) Copper
2) Aluminium

ii) insulator:-
1) Plastic
2) Wood

3. Fill in the blanks :

(a) The hotness of an object is determined by its __________.

(b) Temperature of boiling water cannot be measured by a _____________ thermometer.

(c) Temperature is measured in degree ______________.

(d) No medium is required for transfer of heat by the process of __________.

(e) A cold steel spoon is dipped in a cup of hot milk. Heat is transferred to its other end by the process of ______________.

(f ) Clothes of ______________ colours absorb more heat better than clothes of light colours.

Answer:

a) The hotness of an object is determined by its temperature.

b) Temperature of boiling water cannot be measured by a clinical thermometer.

c) Temperature is measured in degree celsius.

d) No medium is required for transfer of heat by the process of radiation.

e) A cold steel spoon is dipped in a cup of hot milk. Heat is transferred to its other end by the process of conduction.

f) Clothes of dark colours absorb more heat better than clothes of light colours.

4. Match the following :

(i) Land breeze blows during (a) summer

(ii) Sea breeze blows during (b) winter

(iii) Dark coloured clothes are preferred during (c) day

(iv) Light coloured clothes are preferred during (d) night

Answer:

These are the correct match

i) Land breeze blows durin $\to$ (d) night

ii) Sea breeze blows during $\to$ (c) day

iii) Dark coloured clothes are preferred during $\to$ (b) winter

iv) Light coloured clothes are preferred during $\to$ (a) summer

5. Why wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing.

Answer:

During winters, wearing more layers of clothing keeps us warmer more than compared to wearing just one thick piece of clothing because air gets trapped in between the layers of various clothes and as being a bad conductor of heat, air prevents heat loss from our body.

Because of that, wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing.

6. Look at Fig. 4.13. Mark where the heat is being transferred by conduction, by convection and by radiation.

Answer:

Three stages are:-

i) Transfer of heat from the burner to the pan takes place through radiation.

ii) Transfer of heat from the pan to water is taking place through conduction.

iii) Transfer of heat within water is take place through convection.

7. In places of hot climate it is advised that the outer walls of houses be painted white. Explain.

Answer:

In places of hot climate, it is advised that the outer walls of houses be painted white because white is a light colour and because of that it reflects back most of the heat that falls on it and a very less amount of heat is absorbed by the walls.
Therefore, a light colour tends to keep the house cool.

8. One litre of water at 30°C is mixed with one litre of water at 50°C. The temperature of the mixture will be

(a) 80°C

(b) more than 50°C but less than 80°C

(c) 20°C

(d) between 30°C and 50°C

Answer:

If one litre of water at ${30}^{\circ }C$ is mixed with one litre of water at ${50}^{\circ }C$ . Then, the temperature of the mixture will be

(d) between ${30}^{\circ }C$ and ${50}^{\circ }C$

Because as the volume of both the liquids with temperature ${30}^{\circ }C$ and ${50}^{\circ }C$ is equal. Therefore, when we add both the liquids there is transferred of heat take place from a liquid with high temperature to a liquid with a low temperature. That's why temperature is between ${30}^{\circ }C$ and ${50}^{\circ }C$

9. An iron ball at 40°C is dropped in a mug containing water at 40°C. The heat will

(a) flow from iron ball to water.

(b) not flow from iron ball to water or from water to iron ball.

(c) flow from water to iron ball.

(d) increase the temperature of both.

Answer:

If an iron ball at ${40}^{\circ }C$ is dropped in a mug containing water at ${40}^{\circ }C$ . Then, the heat will

(b) not flow from iron ball to water or from water to iron ball.

Because heat will flow from an object with high temperature to an object having a low temperature. But here the temperature of both liquid and iron ball is the same.
Therefore, no flow of heat takes place

10. A wooden spoon is dipped in a cup of ice cream. Its other end

(a) becomes cold by the process of conduction.

(b) becomes cold by the process of convection.

(c) becomes cold by the process of radiation.

(d) does not become cold.

Answer:

If a wooden spoon is dipped in a cup of ice cream. Then, its other end

(d) does not become cold.

Because wood is a bad conductor of heat.
Therefore, no or very very less transfer of heat takes place.

11. Stainless steel pans are usually provided with copper bottoms. The reason for this could be that

(a) copper bottom makes the pan more durable.

(b) such pans appear colourful.

(c) copper is a better conductor of heat than the stainless steel.

(d) copper is easier to clean than the stainless steel.

Answer:

Stainless steel pans are usually provided with copper bottoms. The reason for this could be that

(c) copper is a better conductor of heat than stainless steel.