White phosphorus is the unstable allotrope of phosphorus and it has to be stored under water. If it is kept in the open, it smoulders and ultimately catches fire. When it kept in dark, it glows or emits light. This property of white phosphorus is called phosphorescence.When freshly prepared, it is a white, wax-like solid and after some time it changes its color and turns into yellowish. That is why, it is also called yellow phosphorus. It is very poisonous.