NCERT Solutions for Class 7th Science Chapter 7 .2 Convection — In-text Questions
Book page 937 Updated on2026-09-19
Q1.
You observed that the cup under which the candle was placed, rises up (Fig. 7.3b). Why is it so?
Redrawn sketch of Fig. 7.3(b), page 93 — the set-up after a burning candle is placed below one of the two paper cups.
Answer
Because the air inside that cup becomes hot, expands and turns lighter, and rising hot air lifts the cup.
Candle flame heats the air around it Air inside the cup warms up Warm air expands → occupies more space Same air in a bigger space → becomes lighter ⇒ it rises up and the cup goes up
Try This: place a partially inflated balloon in the Sun (Fig. 7.4). After some time the air inside gets heated, expands, and the balloon becomes visibly larger — the same expansion, made easy to see.
Q2.
How does heat transfer take place in liquids? Do liquids also rise up when heated like air?
Answer
Heat travels through liquids mainly by convection — and yes, a liquid also rises when it is heated, exactly as air does.
Water at the bottom gets heated It expands → becomes lighter → rises Cooler, heavier water at the sides comes down to take its place This water is heated in turn and also rises ⇒ a convection current is set up until all the water is hot
Why it happens: unlike a solid, the particles of a liquid can move about freely. So they do not merely pass heat to their neighbours — they carry the heat with them from one place to another. That is the whole difference between conduction and convection.
Check it yourself: Activity 7.3, with a grain of potassium permanganate at the bottom of a beaker, makes this invisible current visible as a coloured streak.