Calcium Sulphate

Calcium Sulphate

Calcium sulphate occurs in nature in the anhydrous state as calcium sulphate, CaSO₄ and in the hydrated state as calcium sulphate dihydrate, CaSO₄.2H₂O called gypsum. In India, large quantities of gypsum are located in Rajasthan (Bikaner and Jodhpur) and Punjab.

Preparation

In the laboratory, it is prepared by the action of dilute sulphuric acid on lime or calcium carbonate.

Ca(OH)₂ + H₂SO₄ CaSO₄ + 2H₂O

CaCO₃ + H₂SO₄ CaSO₄ + H₂O + CO₂

It can also be prepared by adding sodium sulphate to a soluble calcium salt solution.

CaCl₂ + Na₂SO₄ CaSO₄ + 2NaCl

Properties

(i) Calcium sulphate dihydrate is sparingly soluble in water. On heating, its solubility decreases.

(ii) When gypsum is heated, interesting changes occur. Initial heating brings about changes only in the crystal structure. However, when heated to about 390 K, it loses water partially and forms calcium sulphate hemihydrate, CaSO₄.½H₂O or (CaSO₄)2.H₂O which is commonly known as Plaster of Paris.

2(CaSO₄.2H₂O) 2CaSO₄.H₂O + 3H₂O

Gypsum Plaster of Paris

When heated above 473 K, it loses water of crystallization completely to form anhydrous CaSO₄. This is called dead burnt plaster because it has no tendency to absorb water.

2CaSO4.H₂O 2CaSO₄  + H₂O

(Dead burnt plaster)

Uses

(i) Calcium sulphate dihydrate (gypsum) is used in the manufacture of ‘Plaster of Paris’ and cement.

(ii) It is used in the preparation of black board chalks.

(iii) Anhydrous calcium sulphate (anhydrite) is used as a drying agent.