Water of Crystallization

Water molecules are a part of the crystal structure of many compounds.

Crystals have gaps in them, that allows the water molecules to enter and change the structure.

The change in structure is visible in the form of different colours of the crystal structure.

The water molecules present inside a crystal structure are called water of crystallisation.

Salts which contain water of crystallisation in their crystal structure are called hydrated salts.

Hydrated copper sulphate is represented as . It means 5 water molecules are associated with each molecule of .

It is also called copper sulphate pentahydrate. It is blue in colour.

Similarly, washing soda exists as a hydrated salt with the formula .

Another hydrated salt is gypsum with the formula .

These molecules of water of crystallisation are loosely bound to salt molecules.

They can be removed easily by heating. The salt obtained is called anhydrous salt.

On heating copper sulphate pentahydrate, we obtain anhydrous copper sulphate. It is white in colour.

On adding water, it changes back to blue colour copper sulphate pentahydrate.

Similarly, water of crystallisation can be removed from gypsum by heat.

On heating gypsum to we get a white substance called plaster of paris.

It has the formula .

1/2 means for every 2 molecules of one molecule of water is present.

On adding water, plaster of Paris changes back to gypsum.

This property of plaster of Paris is useful in making sculptures and to make plaster casts to hold fractured bones in place.

Revision

Water molecules are a part of the crystal structure of many compounds.

The water molecules present inside a crystal structure are called water of crystallisation.

Salts which contain water of crystallisation in their crystal structure are called hydrated salts.

Molecules of water of crystallisation are very loosely bound to salt molecules.

The End