Ans. a. The empty paper cup catches fire easily and starts burning because the ignition temperature of paper reaches quickly.
b. When we heat the paper cup containing water, then the heat supplied to the paper cup is transferred to water inside it by conduction. So, in the presence of water, the ignition temperature of paper cup is not reached, and hence the paper cup does not catch fire.
c. Yes, the water in this paper cup becomes hot gradually.
Ans. Fix a lighted candle on a table. Put a glass chimney over the candle and rest it on a few wooden blocks in such a way that air can enter the chimney. We will see that candle burns freely in case (a) when air can enter the chimney from below. Now remove the blocks and let the chimney rest on the table. Now, we will observe that in case (b), when air does not enter the chimney from below, the flame flickers and produces smoke. Finally, put a glass plate over the chimney. Here, in case (c), the flame finally goes off because the air is not available.
This observation shows that air is necessary for combustion (or burning) to take place.
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Q71. Explain the structure of candle flame with diagram.
Ans. A flame consists of three zones.
i. The innermost zone of a flame is dark (or black) – It consists of hot, unburnt vapours of the combustible materials. It is the least hot part of the flame.
ii. The middle zone of a flame is yellow – It is bright and luminous. The fuel vapours burn partially in the middle zone because there is not enough air for burning in this zone. This zone produces a moderate temperature. This zone is the major part of a candle flame.
iii. The outer zone of a flame is blue – It is non-luminous zone. In this zone complete combustion of the fuel takes place because there is plenty of air around it. This zone has the highest temperature in the flame.
Image from NCERT
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