The chemical formula of gypsum is:

Gypsum is the common name of calcium sulphate dihydrate, a soft white or grey mineral. Each unit of calcium sulphate is combined with two molecules of water, called water of crystallisation. Gypsum is very soft; it has a hardness of 2 on the Mohs scale and can be scratched with a fingernail.

Uses of gypsum:

Gypsum is added to cement in small amounts (about 3-5%) to slow down its setting. Without it, cement would harden too quickly to be worked.

It is used as a fertiliser and soil conditioner, especially to reclaim alkaline (sodic) soils and to supply calcium and sulphur to crops such as groundnut.

It is used to make plaster of Paris, blackboard chalk and wallboards.

When gypsum is heated to about 373 K (100°C), it loses three-fourths of its water and becomes calcium sulphate hemihydrate, CaSO4·½H2O. This is plaster of Paris, a white powder. When plaster of Paris is mixed with water, it takes up water again, changes back into gypsum and sets into a hard, solid mass. This is why plaster of Paris is used by doctors to support fractured bones, and for making toys, statues, decorative designs on ceilings and casts. It is called "plaster of Paris" because it was made from large gypsum deposits found near Paris.

If gypsum is heated above about 200°C, it loses all its water and forms anhydrous calcium sulphate, called "dead burnt plaster", which does not set with water.

In India, Rajasthan is by far the largest producer of gypsum.

Why not the other options?

• ✗CaSO4·½H2O is plaster of Paris.
• ✗CaCO3 is calcium carbonate (limestone, marble and chalk).
• ✗Ca(OH)2 is calcium hydroxide (slaked lime).