Water of crystallisation
- The fixed number of water molecules present in one formula unit of a salt is called water of crystallisation.
- For instance, there are five molecules of water in one formula unit of copper sulphate and hence the chemical formula for hydrated copper sulphate is CuSO4. 5H2
Fig. Copper sulphate crystal
- Gypsum has two molecules of water as water of crystallisation and hence the chemical formula for hydrated gypsum stands out to be CaSO4.2H2
- This gypsum on getting heated loses water molecules and becomes calcium sulphate hemihydrate (CaSO4.1/2 H2O). This is known as plaster of Paris.
- Uses of Plaster of Paris are as follows:
- Plaster for supporting fractured bones in their appropriate position.
- When mixed with water, it again changes to gypsum giving a hard solid mass. The reaction is as follows:
- It is also used for making toys, materials for decoration and for making smooth surfaces
Fig. Statue made of plaster of Paris