Q1.
Place two identical candles on two separate petri dishes and light them. Cover one of these with a glass tumbler, as shown in Fig. 5.6. What happens to the candle flames in the two cases?
Answer
The open candle keeps burning. The covered candle grows dim, flickers and goes out after a short while.
| Candle | Air supply | What the flame does |
|---|---|---|
| (a) Not covered | Fresh air reaches it all the time | Burns steadily and goes on burning |
| (b) Covered with a glass tumbler | Only the air trapped inside the tumbler | Becomes smaller and is extinguished after some time |
Why it happens: Burning uses up the oxygen of the air. The open candle is supplied with fresh oxygen continuously, so it keeps burning. Under the tumbler the amount of oxygen is fixed. As the candle burns, the oxygen is used up and carbon dioxide takes its place, so the flame is starved and dies out. This shows that oxygen is essential for combustion.
Try This: Repeat with a big tumbler and a small tumbler over identical candles. The candle under the bigger tumbler burns longer, because a bigger tumbler traps more air and therefore more oxygen.