NCERT Solutions for Class 7 Science Chapter 3 – Heat

Chapter 4 – Heat

Hot and Cold

While performing our daily tasks, we often need to touch a number of objects, among which some are cold and hot. Obviously, an iron rod exposed to the sun for a longer period becomes hot and an ice cream stored in the refrigerator stays cool. Without noticing, we keep playing between hot and cold. Now, the degree of temperature varies depending on the situation and objects. So, how is it possible to determine which objects are hotter than others? Naturally, by touching the object it is possible to figure out, but finding accuracy in temperature can become a problem.

Measuring Temperature

Have you ever seen a thermometer? It is quite natural to get your hands on one of them when someone in your family suffers from a fever. A thermometer is used for measuring the temperature of our body. There are different types of thermometers available. What we see commonly in our house is a clinical thermometer. It is used for measuring the body’s temperature only. A clinical thermometer looks like a cylindrical pipe made of glass. A bulb containing mercury stays attached at the bottom. Outside the bulb, you can notice a small shining thread of mercury. If it’s hard to locate, consider rotating the thermometer slightly to see the meter. It is usually seen through the two meeting ends of the thermometer’s radius. The scale used for the clinical thermometer is measured in °C.

When reading a thermometer, make sure to note the difference in temperature indicated by two bigger marks. Also, you can note the number of divisions. If a bigger mark reads one degree and is divided into five divisions, it indicates 1/5 = 0.2 °C. Before using a thermometer, give it some nice jerks so that the mercury accumulates down at the bulb and prevents wrong readings. Now, place the thermometer under the tongue or underarm. After one minute precisely, take out the thermometer and note down the temperature. It is our natural body temperature. Our normal body temperature should be around 37°C.

Laboratory Thermometer

Now, let’s understand how is it possible to take readings from other objects. For this, we need a laboratory thermometer. The teacher can probably show you a laboratory thermometer from the chemistry lab. Here, the range of measuring scales differs from that of a clinical thermometer. A lab thermometer’s measuring scale typically ranges from -10°C to 110°C. Each reading should be noted to discern the correct information about an object.

Transfer of Heat

Have you ever thought why a frying pan becomes hot when placed on a source of flame? This happens due to the transfer of heat. The heat from the source passes through the frying pan and cooks the food inside it. Gradually, it starts cooling down when removed from the source of heat. Again, why does it cool down? The heat escapes from the pan by transferring through the floor or anything that touches it. Therefore, it is clear that heat flows from hotter objects to cooler ones. The same happens even in the case of nature which builds the cycle of heat and rain.

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

Solution:

Similarities

• Both are made of glass and consist of a long narrow glass tube
• At one end both of them have a bulb
• The bulbs of both thermometers consist of mercury
• Celsius scale is present in both the thermometers

Differences

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

Solution:

Conductors: Iron and copper

Insulators: Plastic and 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 the 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 heat better than clothes of light colours.

Solution:

(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 degrees Celcius.

(d) No medium is required for the 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 heat better than clothes of light colours.

4. Match the following: 

Solution:

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

Solution:

More layers of clothing during winter keep us warmer than wearing just one thick piece of clothing because air gets trapped in between the layers. As air is a bad conductor of heat it does not allow the heat to escape from the body.

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

Solution:

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

Solution:

In places of hot climate, it is advised that the outer walls of houses be painted white because white colour reflects back most of the heat that is incident on it, as white is a poor absorber of heat. This helps in keeping 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

Solution:

The temperature of the mixture will be between 30°C and 50°C because hot water loses heat and simultaneously cold water gains heat. This keeps the temperature in between 30°C and 50°C.

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

(a) flow from the iron ball to water.

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

(c) flow from water to the iron ball.

(d) increase the temperature of both.

Solution:

The answer is (b) not to flow from the iron ball to water or from water to the iron ball because both of them have the same temperature of 40°C.

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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.

Solution:

The answer is (d) does not become cold because wood is a bad conductor of heat.

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 stainless steel.

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

Solution:

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