In the chemical formula CaSO₄·1/2H₂O, only half a water molecule is shown because two formula units of CaSO₄ share one molecule of water during the formation of calcium sulfate hemihydrate.

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.