One remarkable property of P.O.P is it settles into _______ mass on wetting with water. 

Plaster of Paris is a white powder. When water is added to Plaster of Paris, it sets into a hard mass in about half an hour. The setting of the plaster of Paris is due to its hydration to form crystals of gypsum which set to form a hard, solid mass: 

${\mathrm{CaSO}}_4.1/2{\mathrm{H}}_2\mathrm{O} + 1 1/2 {\mathrm{H}}_2\mathrm{O}\to {\mathrm{CaSO}}_4.2{\mathrm{H}}_2\mathrm{O}$

This setting is accompanied by the slight expansion of volume due to which it is used in making statues, toys etc.