NCERT Solutions Curiosity Chapter 7 –104Chapter exercises — Let Us Enhance Our Learning

Book page 102 Updated on2026-09-05

Q1.

Choose the correct option in each case. Your father bought a saucepan made of two different materials, A and B, as shown in Fig. 7.14. The materials A and B have the following properties — (a) Both A and B are good conductors of heat (b) Both A and B are poor conductors of heat (c) A is a good conductor and B is a poor conductor of heat (d) A is a poor conductor and B is a good conductor of heat

Answer

(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.

Part	What it must do	Property needed	Usual material
A — body of the pan	Carry heat from the flame quickly into the food	Good conductor of heat	Aluminium, steel, iron
B — handle	Stop heat from reaching the hand that holds it	Poor conductor of heat	Wood, plastic, bakelite

Why the other options are wrong: if both were good conductors (a), the handle would become too hot to hold. If both were poor conductors (b), the food would never cook. Option (d) is the arrangement turned upside down — a pan that refuses to heat but a handle that burns you.

Q1.

Pins are stuck to a metal strip with wax and a burning candle is kept below the rod, as shown in Fig. 7.15. Which of the following will happen? (a) All the pins will fall almost at the same time (b) Pins I and II will fall earlier than pins III and IV (c) Pins I and II will fall later than pins III and IV (d) Pins II and III will fall almost at the same time

Answer

(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.

Distance of pin II from the flame = distance of pin III from the flame
⇒ heat reaches both at the same time
⇒ the wax under both melts together
⇒ pins II and III fall almost together, then pins I and IV

Why it happens: in conduction, heat spreads out from the heated point in both directions along the strip. What decides when a pin falls is only its distance from the flame. Pins that are equally distant fall together.

Tip: option (b) and option (c) would be tempting if the candle were at one end. Always check where the flame is before answering.

Q1.

A smoke detector is a device that detects smoke and sounds an alarm. Suppose you are fitting a smoke detector in your room. The most suitable place for this device will be: (a) Near the floor (b) In the middle of a wall (c) On the ceiling (d) Anywhere in the room

Answer

(c) On the ceiling.

Why it happens: smoke is a mixture of hot gases and tiny solid particles. Being warmer than the surrounding air, it expands, becomes lighter and rises up by convection. So the very first place smoke collects in a room is right below the ceiling. A detector fixed there catches the smoke earliest and sounds the alarm while there is still time to act.

Why not the others: near the floor (a) the smoke would arrive only after the whole room had filled up; in the middle of a wall (b) it would still be late; and (d) is simply not true — position matters a great deal.

Q2.

A shopkeeper serves you cold lassi in a tumbler. By chance, the tumbler had a small leak. You were given another tumbler by the shopkeeper to put the leaky tumbler in it. Will this arrangement help to keep the lassi cold for a longer time? Explain.

Answer

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

Why it happens: when one tumbler is placed inside another, a thin layer of air gets trapped between the two. Air is a poor conductor of heat. Heat from the warm room now has to cross the outer tumbler, then the trapped layer of air, and only then the inner tumbler before it can reach the lassi. The air layer slows that flow down badly, so the lassi warms up much more slowly than it would in a single tumbler.

Warm room air → outer tumbler → trapped air (poor conductor) → inner tumbler → cold lassi
⇒ heat flows in slowly ⇒ lassi stays cold longer

Did you know? A thermos flask works on exactly this principle, only better — it has two walls with the air removed from the space between them, so there are almost no particles left to conduct heat at all.

Q3.

State with reason(s) whether the following statements are True [T] or False [F]. (i) Heat transfer takes place in solids through convection. [ ] (ii) Heat transfer through convection takes place by the actual movement of particles. [ ] (iii) Areas with clay materials allow more seepage of water than those with sandy materials. [ ] (iv) The movement of cooler air from land to sea is called land breeze. [ ]

Answer

Statement	T / F	Reason
(i) Heat transfer takes place in solids through convection.	False	In solids, heat is transferred mainly by conduction. Convection needs the particles themselves to move from one place to another, and the particles of a solid are fixed in their positions — they can only vibrate. Convection therefore takes place in liquids and gases, not in solids.
(ii) Heat transfer through convection takes place by the actual movement of particles.	True	That is the definition of convection. Heated liquid or gas expands, becomes lighter and rises, while cooler, heavier matter sinks to take its place — the particles carry the heat with them, as the coloured streak in Activity 7.3 shows.
(iii) Areas with clay materials allow more seepage of water than those with sandy materials.	False	It is the other way round. The spaces between sand particles are wider than those between the very fine, closely packed clay particles, so water seeps faster through sand and slowest through clay (Activity 7.5).
(iv) The movement of cooler air from land to sea is called land breeze.	True	At night the land cools faster than the sea. The warmer air above the sea rises, and cooler air from the land moves towards the sea to take its place. This movement of cooler air from land to sea is the land breeze (Fig. 7.7b).

Tip: remember the breezes by where the air comes from — a sea breeze blows from the sea (during the day), a land breeze blows from the land (at night).

Q4.

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

Answer

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.

Surroundings (air, dish, table) are warmer than the ice
⇒ heat flows from the warmer surroundings to the colder ice
⇒ ice absorbs this heat and melts into water

Why it happens: heat always flows from a hotter body to a colder one. Ice is far colder than a room, so it keeps absorbing heat until it has melted. The heat reaches it in all three ways at once — by conduction from the dish it touches, by convection of the air currents around it, and by radiation from the warmer objects nearby.

Check it yourself: put one ice cube in a steel bowl and another in a thick woollen cloth. The cube in the steel bowl melts much faster, because steel is a good conductor and supplies heat to it quickly.

Q5.

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

Answer

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

The incense stick points downwards, but the smoke curls around it and always travels upwards.

Why it happens: smoke is a mixture of hot gases and tiny solid particles. Being hotter than the air around it, it expands, becomes lighter, and is pushed up by the cooler, heavier air that sinks around it. This is convection, and it depends only on the smoke being hot — not on the direction in which the stick is held. So the smoke leaves the tip, curls around the stick and climbs.

Check it yourself: hold a burning agarbatti sideways in a still room. The smoke bends upward within a centimetre or two of the tip, every single time.

Q6.

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

Answer

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

Set-up	Where the candle is	Where the thermometer bulb is	What happens
Fig. 7.16(a)	Below the closed bottom of the test tube	In the water, above the heated part	Heated water rises past the bulb, cooler water sinks to be heated in turn — a convection current heats the whole tube. Higher reading.
Fig. 7.16(b)	At the side, near the upper part of the water	In the water, near the bottom of the tube	The hot water stays at the top because it is lighter. Very little heat reaches the bottom. Lower reading.

Why it happens: water is heated mainly by convection, and convection currents can only carry heat upward. In (a) the water is heated from below, so the warm water rises through the whole tube and passes the thermometer bulb — the entire tube of water gets hot. In (b) the heat is given near the top; the warm water is already at the top, so it has nowhere to rise to and simply stays there. The bulb at the bottom can only be reached by the very slow conduction through water, which is a poor conductor of heat.

Did you know? This is why a kettle, a pressure cooker and a kadhai are always heated at the base. Heating a vessel near its rim would waste almost all the fuel.

Q7.

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

Answer

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

Hot outside air → outer face of the wall
→ air trapped in the hollow brick (poor conductor)
→ inner face of the wall
⇒ very little heat gets through ⇒ the rooms stay cool

Why it happens: the particles of a gas are far apart, so heat cannot be passed on easily from one to the next. The trapped air therefore acts as a barrier and slows down the flow of heat from the hot outside into the house. A solid brick has no such air pocket and lets heat through much more readily.

Did you know? The same wall works in reverse in winter — it stops the heat inside the house from escaping. That is why hollow bricks keep houses cool in summers and warm in winters. In the Mori block of Uttarkashi, the same idea is used with walls of two wooden layers filled with cow dung and mud.

Q8.

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

Answer

Because water heats up and cools down much more slowly than land, a large lake, sea or ocean acts like a huge cushion that smooths out the temperature of the region around it.

By day and in summer: the Sun's heat falls on both land and water. The land becomes very hot quickly, but the water warms up only a little — it absorbs a great deal of heat for a small rise in temperature. The cooler air over the water then moves inland as a sea breeze and brings the temperature of the coast down.
By night and in winter: the land loses its heat quickly and becomes cold. The water, having stored a lot of heat, cools very slowly and keeps giving heat to the air above it. This warm air, and the land breeze pattern it sets up, prevents the coast from becoming bitterly cold.

The evidence: Activity 7.4 shows it directly — in the same sunlight for 20 minutes, the temperature of the soil rose much more than that of the water, and on cooling the soil again lost its heat faster.

Did you know? This is exactly what Pema and Palden's grandfather meant. Kerala has a long coastline, so its winters are warm and humid, while Gangtok, far from any large water body, becomes bitterly cold. Compare Mumbai and Nagpur on a May afternoon — same state, very different heat.

Q9.

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

Answer

Rainwater that falls on the ground partly flows into ponds, lakes, rivers and oceans, and partly seeps into the ground. The seeping happens in these steps:

Water enters the spaces between the particles of soil and the cracks and openings in the rocks below. This process of surface water seeping through soil and rocks is called infiltration.
Water infiltrates more readily where these spaces are wider, open and interconnected (Fig. 7.11) — fastest through gravel, slower through sand, slowest through clay.
Moving downward, the water is finally held in the pore spaces of sediments and the openings in rocks beneath the surface. Water stored like this is called groundwater.
The underground layers of sediments and rocks that store water in their pore spaces are called aquifers (Fig. 7.12).

Rain → infiltration through soil and rock → storage in an aquifer as groundwater.

Did you know? This groundwater may lie anywhere from a few metres to hundreds of metres below the ground, depending on the location. It is the water we bring up through wells, handpumps and bore wells — and it is not unlimited. Rainwater harvesting and recharge pits are used to put it back.

Q10.

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

Answer

The statement is fully justified, because the water cycle keeps moving water from where there is plenty of it to where it is needed, and keeps putting back what is used up.

Sun heats oceans, rivers and lakes → water evaporates as water vapour
Plants and trees add vapour by transpiration
Vapour rises, cools and condenses into clouds
Clouds travel over land and give rain, snow and hail (precipitation)
Water flows into ponds, lakes, rivers and oceans, or infiltrates into the ground
⇒ back to the start of the cycle

Redistribution — the vapour rises mostly over the oceans but the clouds carry it far inland, so rain falls on hills, plains, forests and fields thousands of kilometres from the sea. Snow that falls on the mountains melts in summer and flows down as rivers, feeding places that get very little rain of their own.

Replenishment — the rivers, lakes and wells we draw water from are refilled every year by the same cycle. Rainwater that seeps into the ground recharges the aquifers, so groundwater sources are made good again.

The bigger point: the water cycle also conserves the total amount of water on the Earth. Not a drop is created or destroyed — the same water is used, returned and used again. What we can lose is water at a particular place, which is why rainwater harvesting and recharge pits matter so much.

Tip: in an exam, name all four processes — evaporation, transpiration, condensation and precipitation — and then say clearly what is redistributed and what is replenished.

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