CLASS 7 · SCIENCE · CHAPTER 7

Heat Transfer in Nature

Simple answers to the chapter activities, in-text questions, all 10 end questions, and three exploratory projects.

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Activities and In-text Questions

Textbook pages 89–101

Question Opening discussion

Why can Kerala be warmer and more humid than Gangtok in winter?

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Why can Kerala be warmer and more humid than Gangtok in winter?

Answer

Kerala is closer to the equator and has a long coastline. These factors help make its winter weather warmer and more humid than Gangtok.

Question Activity 7.1

What happens to the pins on a heated metal strip, and why? What would happen if the strip were wood or glass?

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What happens to the pins on a heated metal strip, and why? What would happen if the strip were wood or glass?

Answer

The pin nearest the flame falls first, followed by the others towards the cooler end. Heat travels along the metal by conduction and melts each pin’s wax. Wood and glass are poor conductors, so the pins would not fall in the same way.

Question Section 7.1

Give examples of good and poor conductors of heat. Where does air belong?

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Give examples of good and poor conductors of heat. Where does air belong?

Answer

Good conductors: iron, steel, aluminium, and copper.

Question Section 7.1

Why do woollen clothes and two thin blankets keep us warm?

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Why do woollen clothes and two thin blankets keep us warm?

Answer

Wool and the gap between blankets trap air. Air is a poor conductor of heat, so it slows the loss of body heat.

Question Activity 7.2

What happens to the paper cup above a candle? Why?

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What happens to the paper cup above a candle? Why?

Answer

The cup above the candle rises. The candle warms the air; warm air expands, becomes lighter, and rises.

Question Activity 7.3

How does the coloured water move when water is heated from below?

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How does the coloured water move when water is heated from below?

Answer

The coloured water rises above the heated centre and sinks along the cooler sides. Warm water becomes lighter and rises; cooler water moves down to take its place. This circulating movement transfers heat by convection.

Question Activity 7.4

Which heats faster in sunlight, soil or water? How does the temperature change?

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Which heats faster in sunlight, soil or water? How does the temperature change?

Answer

Soil usually heats faster than water. Record both thermometer readings at 0, 5, 10, 15, and 20 minutes; the exact temperature rise depends on the day and the setup.

Question Section 7.2.1

What causes sea breeze during the day and land breeze at night?

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What causes sea breeze during the day and land breeze at night?

Answer

During the day, land heats faster than the sea. Warm air above land rises, and cooler air from the sea moves towards land: this is a sea breeze. At night, land cools faster, so cooler air moves from land towards the warmer sea: this is a land breeze.

Question Section 7.3

How does heat from a fire or the Sun reach us without touching us?

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How does heat from a fire or the Sun reach us without touching us?

Answer

It reaches us by radiation. Radiation transfers heat without needing air or another material in between.

Question Section 7.3

Why are light-coloured clothes comfortable in summer and dark clothes in winter?

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Why are light-coloured clothes comfortable in summer and dark clothes in winter?

Answer

Light colours reflect more heat, so they absorb less in summer. Dark colours absorb more heat, which can help keep us warmer in winter.

Question Section 7.3

Which types of heat transfer occur when water is heated in a pan?

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Which types of heat transfer occur when water is heated in a pan?

Answer

The pan receives heat from the flame mainly by radiation and conducts heat through its metal. The water circulates and warms by convection. We feel warmth from the flame and hot pan by radiation.

Question Activity 7.5

In which material does water seep fastest: clay, sand, or gravel? Why?

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In which material does water seep fastest: clay, sand, or gravel? Why?

Answer

Water usually seeps fastest through gravel, then sand, and slowest through clay. Gravel has larger spaces between its particles, so water can pass through more easily.

Question Section 7.4.1

What are infiltration and an aquifer?

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What are infiltration and an aquifer?

Answer

Infiltration is rainwater seeping through soil and rocks. The water can collect underground in layers of rock or sediment called aquifers.

Question Section 7.4.1

Why should we conserve groundwater, and how can it be replenished?

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Why should we conserve groundwater, and how can it be replenished?

Answer

Groundwater can run low when people pump it faster than rainwater can refill it. Rainwater harvesting, recharge pits, and protecting open soil help more rain soak into the ground.

Let Us Enhance Our Learning

Textbook pages 102–104

Question 1

Choose the correct option: (i) What are materials A and B in the saucepan made of? (ii) Which pins fall first as heat travels along a metal strip? (iii) Where should a smoke detector be placed?

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Pan: conducting body and insulating handle; heated-end pins fall first; smoke alarm goes high.

Answer

(i) c
(ii) b
(iii) c

Question 2

Will placing the leaky tumbler inside another tumbler help keep cold lassi cold longer? Explain.

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Trapped air is an insulator and slows heat from entering.

Answer

Yes, it helps keep the lassi cold longer.

Question 3

State True or False and give reasons: (i) Heat transfer in solids takes place through convection. (ii) Convection transfers heat by actual movement of particles. (iii) Clay allows more seepage than sand. (iv) Cooler air moving from land to sea is called land breeze.

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Conduction in solids; convection moves particles; sand seeps more than clay; land-to-sea wind is land breeze.

Answer

(i) F
(ii) T
(iii) F
(iv) T

Question 4

Where do ice cubes in a dish get the heat that melts them?

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Heat flows from the warmer surroundings into the colder ice.

Answer

The surroundings provide the heat for melting.

Question 5

An incense stick points downwards. In which direction does its smoke move? Show the movement with a diagram.

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Warm air rises and carries the smoke upward.

Answer

The smoke moves up.

Question 6

Which thermometer in the two test-tube diagrams records a higher temperature, and why?

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Heated water rises, so the thermometer near the top reads higher.

Answer

Fig. 7.16(a).

Question 7

Why are hollow bricks used for the outer walls of houses in hot regions?

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Air pockets slow heat from passing through the wall.

Answer

Hollow bricks help keep the house cooler.

Question 8

How do large water bodies prevent extreme temperatures in nearby areas?

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Water changes temperature slowly and moderates the nearby air.

Answer

Large water bodies make nearby temperatures less extreme.

Question 9

Explain how water seeps through Earth’s surface and gets stored as groundwater.

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Rain infiltrates through soil and rock spaces and collects in aquifers.

Answer

Groundwater is water stored underground in aquifers.

Question 10

How does the water cycle redistribute and replenish water on Earth?

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Evaporation and transpiration → clouds → precipitation → water bodies and groundwater.

Answer

The water cycle moves and replenishes water around Earth.

Exploratory Projects

Textbook pages 104

Question Project 1

Will paper tightly wrapped around a rotating metal rod burn when held near a candle? Explain the expected observation.

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The metal carries heat away, so the paper may stay below its ignition temperature.

Answer

The rod conducts heat away; an adult must supervise the fire activity.

Question Project 2

What happens to a paper spiral suspended above a candle, and why?

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Rising warm air pushes the spiral and makes it turn.

Answer

The spiral rotates because of convection.