Q. Phosphorus is kept in water because:
Answer: its ignition temperature is very low
Notes: 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 60o. Having very low ignition temperature, it readily catches fire in air to give dense white fumes of P4O10. So it is kept in water to avoid oxidation by the oxygen present in air.