Q2.(a) Why is plaster of paris written as CaSO4.1/2 H 2 O? How is it possible to have half a water molecule attached to CaS4O?

Give any two important uses of this compound.

The chemical formula of plaster of Paris is CaSO4.1/2H2O. This basically indicates that two units of calcium sulphate share one molecule of water. The uses of this compound are as follows:

1) It is used by doctors as plaster for supporting fractured bones in the right position.

2) It is also used for making toys, materials for decoration and for making surfaces smooth.

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When hydrated calcium sulphate (CaSO4.2H2O) or gypsum is heated at 373K, it loses its water molecules and forms calcium sulphate hemihydrate (). This hemihydrated form of calcium sulphate is known as Plaster of Paris. It is in the form of a white powder.

When the powder of Plaster of Paris is mixed with water, it becomes hard and solid gypsum. Plaster of Paris is generally used to support fractured bones in their correct positions.

In, only half a water molecule is shown as the water of crystallization because two molecules of CaSO4 share one molecule of water.

Plaster of Paris is used for making toys, materials for decoration, and for making smooth surfaces.

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