Plaster of Paris(P.O.P) ( CaSO4 . 1/2 H2O ).

Plaster of Paris ( CaSO₄ . 1/2 H₂O )
On heating gypsum at 373 K, it loses water molecules and becomes calcium sulphate hemihydrate ( CaSO₄ . 1/2 H₂O ). This is called Plaster of Paris ( P.O.P),

the substance which doctors use as plaster for supporting fractured bones in the right position. Plaster of Paris is a white powder and on mixing with water, it changes to gypsum once again giving a hard solid mass.

Note that only half a water molecule is shown to be attached as water of crystallisation. It is written in this form because two formula units of CaSO₄ share one molecule of water.

Plaster of Paris is used for making toys, materials for decoration and for making surfaces smooth. Calcium sulphate hemihydrate is called ‘Plaster of Paris’