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Chapter: 11th Chemistry : Alkali and Alkaline Earth Metals

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Calcium Sulphate (Plaster of Paris): Preparation, Properties, Uses

It is a hemihydrate of calcium sulphate.

Plaster of paris

Calcium Sulphate (Plaster of Paris), CaSO4·½ H2O

It is a hemihydrate of calcium sulphate. It is obtained when gypsum, CaSO4·2H2O, is heated to 393 K.

 

2CaSO4 .2H2O(s)  →2CaSO4 .H2O+ 3H2O

 

Above 393 K, no water of crystallisation is left and anhydrous calcium sulphate, CaSO4 is formed. This is known as ‘dead burnt plaster’.

It has a remarkable property of setting with water. On mixing with an adequate quantity of water it forms a plastic mass that gets into a hard solid in 5 to 15 minutes.

 

Uses:

 

The largest use of Plaster of Paris is in the building industry as well as plasters. It is used for immobilising the affected part of organ where there is a bone fracture or sprain. It is also employed in dentistry, in ornamental work and for making casts of statues and busts.


 

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