NCERT Solutions for Class 7 Science Chapter 7 Heat Transfer in Nature (PDF) – 2027

Because metals are good conductors of heat. A material that lets heat pass through it easily is called a good conductor, and all metals belong to this group — a fact you met in the chapter ‘The World of Metals and Non-metals’.

In a solid metal, heat is transferred by conduction — from the hotter part of the object to its colder part.

The pins will not remain attached — all four pins will fall off, one after the other. Heat travels along the metal strip from the heated end by conduction; when it reaches a pin, the wax holding that pin melts and the pin drops.

The candle is kept at the end of the strip that is away from the stand, and the pins are labelled I, II, III and IV starting from that heated end (Fig. 7.1). So the correct prediction is:

This is what the completed Table 7.1 looks like after the activity is performed.

Pin I falls first because it is the nearest to the candle flame, and the pins do not fall together because heat takes time to travel along the strip.

In conduction, heat passes from one particle to the next, so the strip becomes hot gradually — first near the flame, later farther away. The wax under pin I melts first, then under pins II, III and IV.

Yes. The observations show clearly that heat is being transferred along the metal strip, from the heated end towards the colder end.

Inference: heat travels along the strip from the hotter part to the colder part, step by step. This is conduction. The particles of the metal do not move from their positions; each heated particle passes heat on to its neighbour. Metals allow this easily, so they are good conductors of heat.

Because wood and glass are non-metals, and non-metals such as these do not allow heat to pass through them easily. They are poor conductors (insulators) of heat.

Here is Table 7.2 filled with materials found in any home or classroom.

Yes — air is placed with the poor conductors (insulators) of heat. That is why woollen clothes, two thin blankets and hollow bricks, which all trap air, keep us warm.

Yes, it is. Houses in places with a very hot or very cold climate keep a poor conductor of heat between the inside and the outside:

• Hollow bricks in the outer walls — the trapped air is a poor conductor, so the house stays warm in winter and cool in summer.
• Walls of two wooden layers filled with cow dung and mud (Mori block, Uttarkashi) — wood and mud are poor conductors and prevent heat loss.

Because the smoke is hotter and lighter than the air around it.

The paper cup that has the burning candle below it rises up, and the wooden stick tilts — the other cup goes down [Fig. 7.3(b)].

The air under that cup gets heated, expands and becomes lighter, so it rises and lifts the cup.

Table 7.3, completed:

Because the air inside that cup becomes hot, expands and turns lighter, and rising hot air lifts the cup.

Heat travels through liquids mainly by convection — and yes, a liquid also rises when it is heated, exactly as air does.

As soon as the candle is lit, a streak of purple colour rises straight up from the centre of the beaker’s base, spreads out near the top, and then comes down along the sides of the beaker [Fig. 7.5(b)]. The streak keeps going round and round until the whole beaker of water is coloured.

Because a convection current is set up in the beaker:

Sample readings (bowls of soil and water side by side in bright sunlight). Your own numbers will be different, but the soil will always heat up faster than the water.

No. In the same interval of time, the temperature of the soil rises much more than the temperature of the water.

With the readings above, after 20 minutes in the same sunlight:

The soil got heated faster. In the same 20 minutes its temperature rose by about 14 °C, while that of the water rose by only about 4 °C.

Using the sample readings of Table 7.4 (both started at 30 °C):

So in the same 20 minutes and the same sunshine, the soil became about 10 °C hotter than the water did — the soil heated up roughly three to four times as much.

Yes. If the heated set-up is brought indoors and left for 20 minutes, the thermometer readings show that the soil cools faster than water — just as it got heated faster.

The heat of the fire reaches us by radiation — it travels directly from the hot object (the fire) to us, and no medium is needed for this transfer.

The hot utensil cools down by radiating heat to its surroundings. All objects radiate heat; a utensil hotter than its surroundings gives out more heat than it receives, so its temperature falls.

Because light and dark surfaces behave differently towards the heat that falls on them.

• Light-coloured clothes reflect most of the heat that falls on them. Less heat enters the body, so we feel cooler and more comfortable in summer.
• Dark surfaces absorb more heat. More of the Sun’s heat is taken in and kept close to the body, so we feel warmer and more comfortable in winter.

• Flame → pan: conduction. Heat is transferred from the flame to the utensil by conduction.
• Water in the pan: convection. Water at the bottom gets hot, becomes lighter and rises; cooler water comes down to take its place.
• Warmth felt around the flame and the hot pan: radiation. It reaches us directly, without any medium.

Varahamihira was an astronomer and mathematician of the sixth century CE who lived in Ujjaini — modern-day Ujjain, in Madhya Pradesh.

In his work Brihatsamhita he gave methods for predicting seasonal rainfall. His predictions were based on natural factors that he observed carefully, such as:

• cloud formation — the shape, colour and behaviour of clouds;
• wind patterns — the direction and nature of the winds;
• the position of the stars and the moon;
• and other natural phenomena.

Rainwater that falls on the ground does not all flow away. Part of it sinks into the tiny spaces between the particles of soil and the cracks in rocks and moves slowly downwards. This process of surface water seeping through soil and rocks is called infiltration.

Water seeps in more easily where the spaces between soil and rock particles are wider, open and interconnected (Fig. 7.11). The water that seeps in is stored below as groundwater.

200 mL of water is poured into each inverted bottle — one holding clay, one sand and one gravel — and the water is collected for 10 minutes (Fig. 7.10). A sensible prediction is:

On performing the activity, the water comes through in this order: gravel > sand > clay. Here is Table 7.5, completed.

Yes — for most students the findings match the prediction exactly: water seeps fastest through gravel, slower through sand and slowest through clay.

Because of the size of the spaces between the particles.

An ice stupa is a tall, cone-shaped structure of ice made in Ladakh (Fig. 7.13).

How it is made: in winter, water from mountain streams is brought down through underground pipes and sprayed into the cold air. It freezes as it falls, and the ice builds up layer by layer into a cone.

How it helps: in spring, when the streams dry up, the ice stupa melts slowly and gives water for farming and other needs through the summer.

(c) A is a good conductor and B is a poor conductor of heat.

In Fig. 7.14, A is the body of the saucepan — the part that sits on the flame and holds the food — and B is the handle.

(d) Pins II and III will fall almost at the same time.

Look carefully at Fig. 7.15. The pins are in the order IV, III, II, I from the stand outwards, and the candle is placed midway between pins II and III — not at the end of the strip as in Activity 7.1.

(c) On the ceiling.

Smoke is hot, so it is lighter than the surrounding air and rises by convection. It collects first just below the ceiling, where the detector catches it earliest.

Yes, it will. The double tumbler keeps the lassi cold for a longer time.

A thin layer of air gets trapped between the two tumblers. Air is a poor conductor of heat, so heat from the warm surroundings reaches the lassi much more slowly.

The ice cubes get this heat from their surroundings — from the dish they are kept in, from the air around them, and from the table or surface below.

The smoke will still move upwards — no matter which way the incense stick points.

Smoke is hot, so it is lighter than the air around it and rises by convection, whichever way the stick points.

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

Because a hollow brick has air trapped inside it, and air is a poor conductor of heat.

Water heats up and cools down much more slowly than land, so a large water body keeps the temperature of nearby areas moderate:

• By day / in summer: land heats up quickly, but water heats up slowly, so the air over the water stays cooler. This cool air moves inland as a sea breeze and keeps nearby areas from becoming too hot.
• By night / in winter: water cools slowly and keeps giving heat to the air, so nearby areas do not become too cold.

1. Rainwater enters the spaces between soil particles and the cracks in rocks and moves downward. This seeping of surface water through soil and rocks is called infiltration.
2. It seeps in faster where the spaces are wide, open and connected, as in gravel.
3. The water collects in the pore spaces of sediments and the openings in rocks beneath the surface. This is groundwater.
4. The underground layers of sediments and rocks that store this water are called aquifers.

The statement is correct. Heat from the Sun keeps water moving in a cycle:

So water evaporated mostly from the oceans falls as rain and snow far inland (redistribution), and rivers, lakes and groundwater are filled again every year (replenishment).

Sample answer (short report): We visited the recharge pit in our school. Rainwater from the roof comes down a pipe, passes through a chamber that traps leaves, and enters a pit about 3 m deep filled with boulders at the bottom, then gravel, then coarse sand. The water seeps through these layers by infiltration and reaches the aquifer. (Add a labelled sketch: roof → pipe → filter chamber → pit layers → aquifer.)

No — the paper does not burn (it may darken a little, but it does not catch fire), as long as it is wrapped tightly around the metal rod and the rod is rotated continuously.

The metal rod is a good conductor of heat. It quickly conducts the heat away from the thin paper, so the paper never gets hot enough to catch fire.

Observation: the paper spiral begins to rotate steadily above the candle, and it keeps turning as long as the candle burns below it. Take the candle away and it slows down and stops.