Plaster of Paris reverts back to gypsum when mixed with water, forming a hard solid mass. This process involves the rehydration of calcium sulfate hemihydrate, restoring its original chemical composition.

When gypsum is heated at 373 K, it loses water molecules and transforms into calcium sulfate hemihydrate, also known as “Plaster of Paris.”

Calcium sulfate hemihydrate is called “Plaster of Paris” because it was historically obtained from gypsum deposits near Paris, France. It was widely used in the city during the 18th century for making plaster molds and sculptures.

Gypsum is used in construction materials such as plasterboard and cement, agriculture as a soil conditioner, and in manufacturing processes such as making molds and casts.

The chemical formula for hydrated gypsum is CaSO4.2H2O, indicating it contains two water molecules as water of crystallization per formula unit.