Question

Why phosphorus is preserved in water?

Answer 1

The reason Phosphorus is stored in water is because it will ignite on contact with air.
The allotropic form of Phosphorus (white phosphorus) is less stable and therefore highly reactive because of angular strain in P4 molecule where the angles are only 60* It readily catches fire in air to give dense white fumes of P4O10.
P4 + 5O2 ------------------> P4O10
Therefore it is kept in water to avoid oxidation by the oxygen present in air. It is kept in water because it is insoluble in water.

phosphorus is a very reactive metal , when exposed to air it starts reacting vigrously , therefore it is kept in water.