NCERT Solutions Curiosity Chapter 7 –937.2 Convection — Activity 7.2: Let us investigate

Book page 92 Updated on2026-09-05

Q1.

Observe what happens to the cup.

Answer

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)].

Why it happens: the candle heats the air around its flame. This hot air enters the inverted cup and warms the air inside it. Warm air expands, occupies more space and becomes lighter than the surrounding air, so it pushes up against the inside of the cup and the cup is lifted. The cup on the other side has only ordinary, cooler air under it, so it stays down and the stick tips.

Check it yourself: the stick must be balanced horizontally before the candle is lit. Only then can you be sure that the tilt is caused by the heated air and not by unequal weights.

Q2.

Record your observations in Table 7.3 and think of probable reasons.

Answer

Table 7.3, completed:

Table 7.3: Recording observations and probable reasons
Observation about the cups	Probable reasons for the observation
The cup kept above the burning candle rises up; the other cup comes down and the wooden stick no longer stays horizontal.	The air near the candle flame gets heated. As the air inside that cup warms up, it expands and occupies more space, so it becomes lighter and rises up, carrying the cup with it. The air under the second cup is not heated, so that side stays down.

Why it matters: this is the first direct evidence in the chapter that a gas transfers heat by actually moving — the beginning of convection. Compare it with Activity 7.1, where nothing moved at all.

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