NCERT · Curiosity · Grade 7 Science
Chapter 7 — Heat Transfer in Nature
Complete step-by-step solutions for every in-text activity question and every exercise question in this chapter — with diagrams, tables, and clear reasoning.
Part 1In-Text & Activity Questions
These are the questions embedded inside the chapter — in activity boxes, tables, and the speech bubbles of Pema, Palden and their grandfather.
1Why is the smoke going up? (Pema's question, before Section 7.2)
Answer: The smoke and hot gases released by burning firewood are lighter (less dense) than the cooler air around them. Because they are lighter, they rise upward — this upward movement of heated, less dense matter is called convection.
2Activity 7.1 — Heat transfer in a metal strip: Predict and record the order in which the pins fall (Table 7.1)
Fig. 7.1: Heat transfer in a metal strip
Prediction & Observation:
| Pin falling first | Order |
|---|---|
| Prediction | Pin I first |
| Observation | Pin I → Pin II → Pin III → Pin IV (in that order) |
Reason: Heat travels along the strip from the heated end (hot end) towards the stand (cold end) by conduction. Pin I, being closest to the flame, receives heat first, so the wax holding it melts first and it falls first. Pins II, III and IV are progressively farther away, so they receive heat later — that is why all the pins do not fall together, but fall one after another in order of distance from the flame.
3Why does pin I fall before pin II? Why did all the pins not fall together?
Answer: In conduction, heat passes from one particle to the next particle in contact with it — the particles themselves do not change position, only the heat energy moves along, one particle handing it to its neighbour. Since pin I sits nearest to the candle flame, the heat reaches it in the shortest time, melting its wax first. Pin II is a little farther, so it takes longer for the heat to reach it, and so on for pins III and IV. Because conduction takes time to travel through each successive section of the strip, the pins fall one by one rather than all at once.
4Table 7.2 — Classify materials around you as good or poor conductors of heat
| S.No. | Material | Good or Poor Conductor |
|---|---|---|
| 1 | Steel | Good conductor |
| 2 | Wood | Poor conductor (insulator) |
| 3 | Aluminium | Good conductor |
| 4 | Iron / Copper | Good conductor |
| 5 | Glass | Poor conductor |
| 6 | Plastic | Poor conductor |
| 7 | Rubber | Poor conductor |
| 8 | Cotton / Woollen cloth | Poor conductor |
| 9 | Air | Poor conductor (insulator) |
Does the list include air? Where is it placed? Yes — air is included, and it is placed among the poor conductors (insulators). Air trapped in small pockets (in woollen fibres, hollow bricks, or between two thin blankets) does not allow heat to pass through it easily, which is why trapped air keeps us warm in winter.
5Activity 7.2 — Two paper cups on a wooden stick, one held above a burning candle: Record your observation and probable reason (Table 7.3)
Observation: The cup that is held above the burning candle rises up, while the other cup (with no candle beneath it) stays lower — the stick tilts up on the heated side.
Probable reason: The air just above the candle flame gets heated. As this air warms up, it expands and takes up more space, which makes it lighter (less dense) than the surrounding cooler air. This lighter, heated air rises upward inside the cup and pushes/lifts it — this is convection in a gas.
6"I wonder how heat from the fire reaches us?" (Grandfather's explanation, near the fireplace)
Answer: Heat reaches us directly from the fire without needing any medium (air, water, or solid) in between — this process is called radiation. The same process brings us heat and light from the Sun across empty space.
7"How does heat transfer take place in liquids? Do liquids also rise up when heated like air?" (Palden's question)
Answer: Yes. Just like gases, liquids are also heated by convection. When the bottom layer of a liquid is heated, it expands, becomes lighter, and rises; the cooler, heavier liquid from the sides sinks down to take its place. This continuous circulating movement heats the entire liquid — Activity 7.3 demonstrates this using coloured (potassium permanganate) water.
8Activity 7.3 — Convection in heated water (using potassium permanganate)
Fig. 7.5(a): Set-up for demonstrating convection in water
Observation: As heat is supplied from below, a pink/purple streak of coloured water rises straight up from the centre of the beaker's base, spreads out near the surface, and then comes back down along the sides of the beaker — forming a continuous circulating loop.
Reason: Water at the bottom of the beaker gets heated first, expands, becomes lighter, and rises. The cooler water near the sides (denser/heavier) sinks down to take its place, gets heated in turn, and also rises. This repeating cycle — known as a convection current — continues until the entire volume of water is heated, proving that heat transfer in liquids happens through the actual movement of particles.
Reason: Water at the bottom of the beaker gets heated first, expands, becomes lighter, and rises. The cooler water near the sides (denser/heavier) sinks down to take its place, gets heated in turn, and also rises. This repeating cycle — known as a convection current — continues until the entire volume of water is heated, proving that heat transfer in liquids happens through the actual movement of particles.
9Activity 7.4 — Heating of soil and water; recording temperature every 5 minutes (Table 7.4)
| Time (min) | Temperature of soil | Temperature of water |
|---|---|---|
| 0 | Same starting (room) temperature | Same starting (room) temperature |
| 5 – 20 | Rises quickly, in bigger steps | Rises slowly, in smaller steps |
- Did the temperature rise by the same amount for both? No — the rise is not equal.
- Which one got heated faster? Soil gets heated faster than water.
- Rise in 20 minutes: The soil's temperature rises noticeably more than the water's temperature in the same 20 minutes (soil shows a clearly higher final reading), because soil has a lower heat capacity than water — it takes less heat to raise its temperature by the same amount.
Does soil also cool faster? Yes — just as it heats up quickly, soil also cools down faster than water once both are moved indoors. Water heats up slowly and cools down slowly, which is the basis of land and sea breezes.
10Land and Sea Breeze — how do they form?
Fig. 7.7(b): Land breeze (night-time)
Daytime (Sea breeze): Land heats up faster than the sea. Warm air above the land rises, and cooler air from the sea rushes in to take its place — this inflow of cool air from sea to land is the sea breeze.
Night-time (Land breeze): Land cools down faster than the sea after sunset. The sea, still relatively warm, causes air above it to rise; cooler air from the land then moves out towards the sea — this is the land breeze.
Because land and water heat and cool at different rates, the direction of the coastal wind reverses between day and night.
Night-time (Land breeze): Land cools down faster than the sea after sunset. The sea, still relatively warm, causes air above it to rise; cooler air from the land then moves out towards the sea — this is the land breeze.
Because land and water heat and cool at different rates, the direction of the coastal wind reverses between day and night.
11Activity 7.5 — Seepage of water through clay, sand and gravel (Table 7.5)
Fig. 7.10: Comparing the flow of water through clay, sand and gravel
| Bottle filled with | Prediction | Observation |
|---|---|---|
| Bottle 1 (Clay) | Very slow | Very slow |
| Bottle 2 (Sand) | Slow | Slow |
| Bottle 3 (Gravel) | Fast | Fast |
Reason: Water seeps through gravel the fastest because the gaps (pore spaces) between gravel particles are large and well connected. Sand particles are smaller and pack more closely, so water moves through more slowly. Clay particles are the finest and pack very tightly, leaving very narrow spaces, so seepage through clay is the slowest of all. This process of water seeping through soil and rock is called infiltration.
Part 2Let Us Enhance Our Learning — Exercise Solutions
All ten questions from the end-of-chapter exercise, solved in full with reasoning.
Q1Choose the correct option in each case.
(i) Your father bought a saucepan made of two different materials, A and B (Fig. 7.14). What are the properties of A and B?
Fig. 7.14: Saucepan
Explanation: A points to the metal body/base of the pan, which must conduct heat well from the flame to the food — so it is a good conductor. B points to the handle, which is usually made of plastic or wood so that it stays cool and safe to hold — a poor conductor.
(ii) Pins are stuck to a metal strip with wax, candle kept below the rod (Fig. 7.15). What happens?
Fig. 7.15: Heat transfer set-up
Explanation: Pins I and II are closer to the candle flame, so heat (travelling by conduction) reaches them sooner than pins III and IV, which are farther away.
(iii) Where is the most suitable place to fit a smoke detector in a room?
Explanation: Smoke is made of hot gases that are lighter than the surrounding air, so it always rises by convection and collects near the ceiling first — that is the fastest place to detect it.
Q2A shopkeeper serves cold lassi in a leaky tumbler, and gives you another tumbler to keep the leaky one inside. Will this keep the lassi cold for longer? Explain.
Answer: Yes, this arrangement does help keep the lassi cold for longer.
- Because the inner tumbler leaks, a small amount of lassi (liquid) continuously seeps out and collects on the outer surface of the inner tumbler / inside the outer tumbler.
- This leaked liquid keeps evaporating from the surface.
- Evaporation is a cooling process — it needs heat energy, and this heat is drawn from the lassi remaining inside the tumbler (and from the surrounding tumbler walls).
- As heat is continuously taken away for evaporation, the temperature of the lassi inside stays low for a longer time — exactly the principle used in traditional clay pots (matkas) to keep water cool.
Q3State whether the following statements are True [T] or False [F], with reason.
False (i) Heat transfer takes place in solids through convection.
In solids, particles are tightly packed and fixed in position — they cannot move from place to place. So heat transfer in solids happens mainly through conduction (particle-to-particle, without particles moving), not convection.
True (ii) Heat transfer through convection takes place by the actual movement of particles.
This is exactly the definition of convection — heated particles of a liquid or gas physically move from the hotter region to the cooler region, carrying heat with them.
False (iii) Areas with clay materials allow more seepage of water than those with sandy materials.
It is the other way round. Clay particles are very fine and packed tightly together, leaving very narrow pore spaces, so water seeps through clay slowly. Sandy soil has larger, more open spaces, so it allows faster seepage than clay.
True (iv) The movement of cooler air from land to sea is called land breeze.
At night, land cools faster than the sea. The relatively warmer air above the sea rises, and cooler air from the land moves in to replace it — this flow of cool air from land towards the sea is indeed called the land breeze.
Q4Some ice cubes placed in a dish melt into water after some time. Where do the ice cubes get heat for this transformation?
Answer: The ice cubes absorb heat from their warmer surroundings — the dish, the surrounding air, and any nearby warm objects. Since the ice is colder than the room, heat naturally flows from the warmer surroundings into the ice (by conduction from the dish, and by convection and radiation from the surrounding air), until the ice absorbs enough heat energy to melt into water.
Q5A burning incense stick is fixed pointing downwards. In which direction would the smoke move? Show with a diagram.
Answer: Even though the incense stick points downward, the smoke will still move upward, not along the direction of the stick. This is because the hot smoke released from the burning tip is lighter (less dense) than the surrounding cooler air. By the process of convection, the lighter hot smoke always rises above the denser, cooler air around it — the orientation of the stick has no effect on this.
Q6Two test tubes with water are heated by a candle flame as shown in Fig. 7.16. Which thermometer — (a) heated from the bottom, or (b) heated near the top/side — will record a higher temperature? Explain.
Answer: The test tube heated from the bottom, Fig. 7.16(a), will record the higher temperature.
This shows why, in practice, we always heat liquids from the bottom (as in cooking) — it lets convection distribute the heat efficiently through the whole liquid.
- When water is heated at the bottom, the heated water there expands, becomes lighter, and rises; cooler, denser water from above sinks to take its place. This sets up a proper convection current that circulates and heats the entire column of water fairly evenly, so the thermometer records a good, rising temperature.
- When water is heated near the top/side, the heated water is already the lightest layer, so it simply stays near the surface — it has no reason to sink and mix with the colder water below. Convection currents cannot form properly, so the heat stays localised near the top and the bulk of the water (and hence the overall/average temperature recorded) stays cooler.
Q7Why are hollow bricks used to construct the outer walls of houses in hot regions?
Answer: Hollow bricks trap air inside their hollow spaces, and air is a poor conductor of heat. This trapped layer of air acts as an insulating barrier between the hot outdoor environment and the cooler indoors, slowing down the flow of heat into the house. As a result, hollow-brick walls keep homes cooler in summer (by blocking outside heat from entering) and, for the same reason, help keep them warmer in winter (by reducing heat loss from inside).
Q8Explain how large water bodies prevent extreme temperature in areas around them.
Answer:
- Water heats up and cools down much more slowly than land, because it needs more heat energy to raise its temperature (and releases heat more slowly on cooling).
- During the day, the land near a large water body heats up faster than the water. Warm air over land rises, and cool air from the water body flows in as a sea/lake breeze, moderating the daytime heat.
- At night, land cools faster than the water body, which stays comparatively warm. Warm air rises over the water and cooler air from the land flows out as a land breeze — while the still-warm water keeps the coastal air from getting too cold.
- This continuous exchange of air (through convection) between land and water keeps coastal or lakeside regions from experiencing the extreme daytime heat or night-time cold seen further inland — giving them a milder, more moderate climate.
Q9Explain how water seeps through the surface of the Earth and gets stored as groundwater.
Water seeps fastest through gravel, then sand, then clay
Answer:
- When rain falls, water moves down through the tiny, connected spaces (pores) present between soil and rock particles. This process of surface water moving down through soil and rocks is called infiltration.
- Infiltration happens faster when these pore spaces are wide, open, and well-connected (as in gravel), and slower where the particles are packed tightly together (as in clay).
- As it moves down, this water gradually fills up the pore spaces of sediments and the openings in rocks beneath the surface — this stored water is called groundwater.
- The underground layers of rock and sediment that store this groundwater in their pore spaces are called aquifers. This is the water that people access by digging wells or drilling borewells, sometimes from just a few metres to hundreds of metres below the ground.
Q10"The water cycle helps in the redistribution and replenishment of water on the Earth." Justify this statement.
Answer:
Because this cycle continually moves water — upward as vapour and downward as precipitation — it carries water from oceans to distant land regions (redistribution), and it constantly refills rivers, lakes and underground water sources that are used up by living things and human activity (replenishment). This is why the water cycle is essential for maintaining a steady supply of fresh water everywhere on Earth.
- Heat from the Sun causes water in oceans, rivers and lakes to evaporate into water vapour; plants also release water vapour through transpiration.
- This water vapour rises, cools, and condenses to form clouds.
- Clouds release the water back to Earth's surface as rain, snow or hail — this is precipitation, which can fall over oceans as well as far-away land areas, including places nowhere near a sea.
- Precipitated water flows into rivers, lakes and oceans, or seeps into the ground through infiltration to recharge groundwater and aquifers.
BonusExploratory Projects — Quick Pointers
P1Wrap a thin paper strip tightly around a metal rod and try to burn it with a candle while rotating the rod. Does the paper burn?
Answer: No, the paper does not catch fire (or chars only very slightly). The metal rod is a good conductor of heat and quickly carries the heat away from the paper into the rest of the rod; continuous rotation also keeps moving the paper away from the flame, so no single spot ever gets hot enough for long enough to ignite.
P2Cut a paper spiral and suspend it above a burning candle. What happens?
Spiral paper spinning above a candle flame
Answer: The paper spiral begins to spin/rotate. The candle heats the air around it, which expands, becomes lighter, and rises as a column of hot air (convection). This rising air pushes against the slanted surface of the spiral, making it rotate continuously.
P3Visit a water harvesting or recharge pit site and find out how it works.
Pointer: A recharge pit is designed to collect rainwater/runoff and let it infiltrate quickly into the ground (often filled with gravel and sand to speed up seepage), so that it reaches and refills the aquifer instead of running off and being lost — directly applying the concepts of infiltration and groundwater storage learnt in this chapter.