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.

The chemical formula for hydrated copper sulfate is CuSO4.5H2O, indicating it contains five water molecules per formula unit.

Heating copper sulfate crystals removes water of crystallization, causing them to turn white. Re-moistening the crystals restores their blue color as water molecules are reabsorbed.