NCERT Solutions for Class 7th Science Chapter 5 .3.2 Combustion — Activity 5.5: Let us investigate
Book page 625 Updated on2026-09-19
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?
Fig. 5.6, page 62 — candle (a) burning in the open air, (b) covered with a glass tumbler.
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.