Question

which non metal is stored in water and why

Answer 1

Phosphorus (P)

is stored under water because it is so reactive, it may react with the oxygen present in the air and combust.
White phosphorus is highly flammable in the air because it will ignite when in contact with the oxygen in the air. For this reason, it is even used as a weapon. It forms diphosphorus pentoxide, P4O10P4O10.

However, white phosphorus is only slightly soluble in water, and does not react with it, even though water contains oxygen as one of its two elements.

So white phosphorus is almost always stored in water.

Answer 2

White phosphorus must always be kept under water. This is because it can be dangerously reactive when exposed to air and result in spontaneous ignition. It should also not be handled using bare hands as it can cause burns on the skin.