Question

In the given experiment, the water rises because

• A. Air pressure increases inside the tube due to phosphorus.
• B. Air is absorbed by the cotton and the water rises to take up its place.
• C. Water rises in any test tube if it is inverted in a water tub.
• D. Oxygen is used up in the tube and the water rises to take up its place.

Answer 1

The correct option is D

Oxygen is used up in the tube and the water rises to take up its place.

In the given experiment, phosphorus reacts with oxygen present in the test tube to form phosphorus pentoxide ($P_2{O}_5$). Again this phosphorus pentoxide reacts with water vapour present to form the phosphoric acid ($H_{3}P{O}_4$). Since all the oxygen is used up inside the test tube in the process, as a result, water rises up in the test tube.

The chemical reactions involved are as follows:

$4P+5O_2⟶2P_2{O}_5$

$P_2{O}_5+3H_{2}O⟶2H_{3}P{O}_4$