Calcium sulfate has the chemical formula CaSO4 and contains one calcium ion bonded to one sulfate ion. The calcium ion has a charge of positive two, while the sulfate ion has a charge of negative two.
Calcium sulfate is often found in its dihydrate form, gypsum. For dihydrates, every molecule of calcium sulfate has two corresponding molecules of water. Calcium sulfate also has a hemihydrate form, known as "plaster of Paris." This means that every two molecules of calcium sulfate in plaster of Paris has one corresponding water molecule. Since dehydrated calcium sulfate easily absorbs water to return to one of its hydrate forms, it is often used as a desiccant.