phosphate

It is white phosphorus, but not phosphate, that should be stored in water.

White phosphorus (also known as yellow phosphorus) is one of the allotropes (forms) of phosphorus, which has an ignition point (ignition temperature) as low as 35oC. It is dangerous that white phosphorus can burn in air when the air temperature is high, especially in summer. To avoid burning air, white phosphorus is stored in water. Water is used to separate white phosphorus and the air (oxygen), and to lower the temperature of white phosphorus.