Class 7 Science · Chapter 7

7. Heat Transfer in Nature

Complete solutions to Let Us Enhance Our Learning. CBSE / NCERT Class 7 Science (Curiosity).

Let Us Enhance Our Learning

1Choose the correct option in each case.

(i) Saucepan made of materials A (pan) and B (handle): (c) A is a good conductor and B is a poor conductor of heat

The pan must conduct heat to cook food. The handle must not get hot, so that we can hold it.

(ii) Pins stuck with wax on a metal strip heated by a candle: (b) Pins I and II will fall earlier than pins III and IV

Pins I and II are nearer to the candle. Heat reaches them first by conduction, so their wax melts first.

(iii) Most suitable place for a smoke detector: (c) On the ceiling

Smoke is hot and light. It rises upwards by convection and collects near the ceiling first.

2A shopkeeper served cold lassi in a tumbler with a small leak. You put the leaky tumbler inside another tumbler. Will this arrangement help to keep the lassi cold for a longer time? Explain.

Yes, it will help.

A layer of air gets trapped between the two tumblers. Air is a poor conductor of heat. So less heat from the surroundings reaches the lassi, and it stays cold for a longer time.

3State with reason(s) whether the following statements are True [T] or False [F].

(i) Heat transfer takes place in solids through convection. — F

In solids, heat is transferred mainly by conduction. Particles of solids cannot move from their places.

(ii) Heat transfer through convection takes place by the actual movement of particles. — T

In convection, the hot particles of a liquid or gas move up and the cold ones move down.

(iii) Areas with clay materials allow more seepage of water than those with sandy materials. — F

Sandy soil has bigger spaces between particles. So it allows more seepage than clay.

(iv) The movement of cooler air from land to sea is called land breeze. — T

At night, land cools faster than the sea. The cooler air from the land moves towards the sea.

4Some ice cubes placed in a dish melt into water after some time. Where do the ice cubes get heat for this transformation?

The ice cubes get heat from their surroundings:

from the warm air around them (by convection),
from the dish and the table they touch (by conduction), and
from the surroundings by radiation.

5A burning incense stick is fixed, pointing downwards. In which direction would the smoke move? Show the movement of smoke with a diagram.

The smoke will move upwards.

The burning tip heats the air around it. Hot air with smoke becomes light and rises. Cooler air comes in its place. This is convection.

Smoke rises upwards even when the incense stick points downwards.

6Two test tubes with water are heated by a candle flame as shown in Fig. 7.16. Which thermometer (Fig. 7.16a or 7.16b) will record a higher temperature? Explain.

The thermometer in Fig. 7.16(a) will record a higher temperature.

In (a), the test tube is heated at the bottom. Hot water rises and cold water comes down. By convection, the whole water gets heated, including near the thermometer.
In (b), the test tube is heated near the top. Hot water stays at the top. Water is a poor conductor of heat, so the water at the bottom near the thermometer remains cool.

7Why are hollow bricks used to construct the outer walls of houses in hot regions?

Hollow bricks have air trapped inside them. Air is a poor conductor of heat. So less heat from outside enters the house, and the house stays cool.

8Explain how large water bodies prevent extreme temperature in areas around them.

Daytime: Land heats up faster than water. Hot air over land rises. Cool air from the sea moves towards the land (sea breeze). This keeps the area cool.
Night: Land cools faster than water. Air over the sea is warmer and rises. Cool air from the land moves towards the sea (land breeze).
Water heats up and cools down slowly. So areas near large water bodies do not become very hot or very cold.

9Explain how water seeps through the surface of the Earth and gets stored as groundwater.

Rainwater falls on the ground.
It seeps down through the soil and rocks. This process is called infiltration.
The water fills the pore spaces between sediments and the openings in rocks below the surface.
This stored water is called groundwater. The layers of sediments and rocks that store it are called aquifers.

10The water cycle helps in the redistribution and replenishment of water on the Earth. Justify the statement.

The Sun heats water in oceans, rivers and lakes. It evaporates and rises as water vapour.
Winds carry the water vapour to far-off places. It condenses into clouds.
The water falls as rain or snow on different places, even far from the sea. This is redistribution.
Rainwater fills rivers, lakes and ponds. Some of it seeps into the ground and refills groundwater. This is replenishment.
Water finally returns to the oceans and the cycle continues.

Exploratory Projects

Exploratory Projects are hands-on tasks. Do them with your classmates under the guidance of your teacher, and write your own observations.

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