Question

Plaster of Paris is prepared by heating one of the following to a temperature of 100°C. This is:

Options

• CaSO₃.2H₂O

• CaCl₂.2H₂O

• CaCO₃.2H₂O

• CaSO₄.2H₂O

Answer 1

CaSO₄.2H₂O

Explanation:

Plaster of Paris (POP) is prepared by carefully heating gypsum, which is chemically known as calcium sulphate dihydrate (CaSO₄.2H₂O), to exactly 100°C (373 K). At this temperature, gypsum loses three-fourths of its water of crystallisation, forming calcium sulphate hemihydrate, which is POP. If heated above 100°C, it loses all its water, becoming “dead burnt plaster” (anhydrous calcium sulphate) and losing its setting properties.