IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

# Burning Candle in Limited Air

## Objective

To show that burning needs oxygen and hot air has low density.

## Introduction

At lower classes we teach that air contains oxygen and burning needs oxygen. This statement can be supported by this demonstration. In the second part we will show that hot air has low density.

## Apparatus

two plates, three candles, two glass tumblers of same size and one with a bigger size.

## Procedure

1. Put a candle vertically in a plate. Light the candle. The candle keeps on burning. Cover the burning candle by an inverted glass. The candle goes off. Now explain that the glass originally had air in it and when covered only that much of air was made available to the candle. This air had some amount of oxygen and when that was consumed, the candle went off.
2. Now use two plates and put one candle each in them. Light them. Take two glasses of quite different sizes (one may be a glass and the other may be a glass jug), in the two hands and cover the candles with these simultaneously. The candle in the smaller glass goes off earlier than the one in the bigger glass. Explain that the bigger glass contains more air and hence more oxygen.
3. Now put one candle in one plate and two candles in the other plate. Light all of them. Take two glasses of the same size in the two hands and simultaneously cover the burning candles in the two plates. The single candle lasts longer than the double candle. Explain that same amount of oxygen was available in the two glasses but two candles together was consuming oxygen faster than a single candle.

This is a slight modification of the famous experiment given in most of the science textbooks for lower classes to demonstrate that there is 21\% oxygen in air. We haven't put water in the plate in this experiment.