Plaster of paris is calcium sulphate and addition of water of crystallization to it leads to formation of gypsum which becomes hard material. It should be stored in water proof container.\\\\({ CaSO }_{ 4 }\\cdot \\dfrac { 1 }{ 2 } { H }_{ 2 }O+\\dfrac { 3 }{ 2 } { H }_{ 2 }O\\rightarrow { CaSO }_{ 4 }\\cdot 2{ H }_{ 2 }O\\\\)