### Theory:

Calcium sulphate hemihydrate is commonly known as plaster of Paris. The name is derived from the place of its origin Montmartre in the Paris district. The chemical formula is $\left({\mathit{CaSO}}_{4}\right)\cdot 1}{2}\cdot {H}_{2}O$.
Preparation of plaster of Paris:

When gypsum (${\mathit{CaSO}}_{4}\cdot {2H}_{2}O$), is heated in a rotating kiln at $$300$$$\mathrm{°}F$, it loses its water molecule and forms calcium sulphate hemihydrate$\left({\mathit{CaSO}}_{4}\right)\cdot 1}{2}\cdot {H}_{2}O$.

Properties of plaster of Paris:
• It is a white powder.
• It acts as an oxidizing agent under extreme conditions.
• It produces gypsum when mixed with water.
Uses of Plaster of Paris:

Uses of plaster of Paris
• It is used in making casts for statue, dentistry, surgical instruments, and toys etc
• It is used in construction for making smooth walls, and in the creation of walls and ceilings  designs.
• It is used in making chalks and also helps in setting fractured bones during surgery.
• In laboratories, it is used to seal air gaps in the apparatus.