Question

6. Describe a simple experiment to show that 1/5th of air is oxygen

Answer 1

Aim : To show that 1/5th of the air is oxygen.

Apparatus required: Glass trough, bell jar, stopper, water, phosphorus, evaporating dish

PROCEDURE

1. A glass trough is filled with water
2. A dry evaporating dish containing white phosphorus is made to float on the water.
3. This apparatus is covered with a well-stoppered bell jar with markings on its side.
4. A glowing iron wire is introduced into the bell jar by opening the stopper
5. The phosphorus is carefully ignited.
6. Re-stopper the apparatus .
7. The phosphorus burns with a brilliant flame farming dense white fumes of phosphorus pentoxide (P₂O₅).
8. Soon the burning ceases and the water level starts rising.
9. The water occupies approximately $\frac{1}{5}\mathrm{th}$ of the original volume, as the fumes dissolve in water.
10. Some phosphorus is left unburnt in the dish. The air left behind in the bell jar does not allow the phosphorus to burn.

In this way we came to know that the volume of air occupied by oxygen is $\frac{1}{5}\mathrm{th}$.