Answers to all "Let Us Enhance Our Learning" questions of Chapter 7, Heat Transfer in Nature (NCERT Class 7 Science, Curiosity, 2026-27): conduction, convection and radiation, conductors and insulators of heat, land and sea breezes, the water cycle, infiltration and groundwater. All 10 questions are answered, with the key answer highlighted.
Conduction: heat passes from particle to particle in solids, without the particles moving from their places. Convection: heat is carried by the actual movement of particles in liquids and gases (hot fluid rises, cooler fluid sinks). Radiation: heat travels without any medium, e.g. from the Sun.
Choose the correct option in each case. (i) A saucepan is made of two materials, A (the pan) and B (the handle). (a) Both are good conductors of heat (b) Both are poor conductors (c) A is a good conductor and B is a poor conductor (d) A is a poor conductor and B is a good conductor. (ii) Pins are stuck to a metal strip with wax and a burning candle is kept below one end (near pin I). Which 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. (iii) The most suitable place for a smoke detector in a room is (a) near the floor (b) in the middle of a wall (c) on the ceiling (d) anywhere in the room.
Solution
(i) (c): the pan (A) must be a good conductor so that heat reaches the food quickly, and the handle (B) a poor conductor so that we can hold it without burning our hands.
(ii) (b): heat travels along the strip by conduction from the end above the candle; the wax near pins I and II melts first, so they fall first, then III and IV.
(iii) (c) On the ceiling: smoke from a fire is hot and rises upwards by convection, so it reaches the ceiling first.
A shopkeeper serves you cold lassi in a tumbler that has a small leak. You were given another tumbler by the shopkeeper to put the leaky tumbler in. Will this arrangement help to keep the lassi cold for a longer time? Explain.
Solution
Yes. A thin layer of air is trapped between the two tumblers. Air is a poor conductor of heat, so it slows down the flow of heat from the warm surroundings into the lassi. The lassi stays cold for longer (this is the idea behind double-walled flasks).
Yes: the air trapped between the two tumblers is a poor conductor of heat and reduces heat flowing into the lassi.
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.
Solution
(i) F. In solids, heat is transferred by conduction; the particles cannot move from their places.
(ii) T.
(iii) F. Clay particles are very fine and packed closely, so water seeps through clay slowly; sandy soil has bigger spaces and allows more seepage.
(iv) T. At night the land cools faster; cooler air from the land blows towards the sea.
(i) F, solids conduct heat (ii) T (iii) F, sandy soil allows more seepage (iv) T
Some ice cubes placed in a dish melt into water after some time. Where do the ice cubes get heat for this transformation?
Solution
From the surroundings: the warmer air around them (by convection and conduction), the dish they are kept in (by conduction), and the warm objects and sunlight in the room (by radiation).
From the surroundings: the warm air, the dish and nearby warm objects, by conduction, convection and radiation.
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.
Solution
The smoke moves upwards. The smoke and the air around the burning tip become hot, so they become lighter and rise (convection), whichever way the stick points.
Even when the incense stick points downwards, the hot smoke rises upwards (convection)
Upwards: the hot smoke and air rise by convection, even though the stick points down.
Two test tubes with water are heated by a candle flame as shown in Fig. 7.16 ((a) heated at the bottom; (b) heated near the top). Which thermometer will record a higher temperature? Explain.
Solution
The thermometer in Fig. 7.16(a). When water is heated at the bottom, the hot water rises and the cooler water sinks, setting up convection currents; soon all the water becomes hot. In (b), the water is heated near the top, so the hot water stays at the top; water is a poor conductor, so the water lower down (around the thermometer bulb) stays cooler.
Fig. 7.16(a): heating from the bottom sets up convection currents that heat all the water; in (b) the heated water stays at the top.
Why are hollow bricks used to construct the outer walls of houses in hot regions?
Solution
Hollow bricks have air trapped inside them. Air is a poor conductor of heat, so it slows down the flow of heat from the hot outside into the house. The rooms stay cooler in summer (and warmer in winter). Hollow bricks are also lighter and use less material.
The air trapped in hollow bricks is a poor conductor of heat, so less heat enters the house and it stays cooler.
Explain how large water bodies prevent extreme temperatures in areas around them.
Solution
Water heats up and cools down slowly compared with land. During the day the sea or lake absorbs a lot of heat without getting very hot, and the cool sea breeze blows towards the land, keeping it cooler. At night the water gives out its heat slowly and stays warmer than the land, and the land breeze and the warm water keep nights from getting very cold. So places near large water bodies have a moderate climate, without very hot days or very cold nights.
Water heats and cools slowly; it absorbs heat by day (sea breeze cools the land) and releases it at night, so nearby areas avoid extremes of temperature.
Explain how water seeps through the surface of the Earth and gets stored as groundwater.
Solution
When it rains, some water flows away on the surface, but some of it soaks (seeps) into the ground through the spaces between soil particles and cracks in rocks. This seeping is called infiltration. The water moves down through layers of soil, sand and gravel until it reaches rock layers that it cannot pass through. It collects and fills the pore spaces in the sediments and openings in rocks above these layers. This stored water is groundwater, and the underground layers that hold it are called aquifers. We reach it through wells, tube wells and hand pumps.
Rainwater infiltrates through gaps in soil and cracks in rocks and collects in the pore spaces of underground layers (aquifers) above rock it cannot pass, forming groundwater.
The water cycle helps in the redistribution and replenishment of water on the Earth. Justify the statement.
Solution
Redistribution: water evaporates from oceans, rivers and lakes and transpires from plants. The water vapour is carried by winds over long distances, cools and condenses into clouds, and falls as rain or snow in other places, including inland areas and mountains far from the sea. So water moves from one part of the Earth to another.
Replenishment: the rain and melted snow refill rivers, lakes, ponds and wells, and some seeps into the ground to recharge groundwater. So the water we use is continuously restored.
Without the water cycle, fresh water on land would be used up and not replaced.
Evaporation, condensation and precipitation carry water from oceans to land far away (redistribution), and rain and snowmelt refill rivers, lakes and groundwater (replenishment).
