The number of water molecules which form part of the structure of a crystal are called water of crystallisation.
for example-copper sulphate contains 5 molecules of water of crystallisation in one formula unit.

Water of crystallization refers to the water molecules that are bound within the crystal structure of a compound. These water molecules are typically present in a fixed ratio to the compound, and their presence is essential for maintaining the stability and structure of the crystal lattice. Several compounds, such as bleaching powder, baking soda, washing soda, and plaster of Paris, exhibit water of crystallization.

Bleaching Powder:
- Bleaching powder, also known as calcium hypochlorite, has the chemical formula Ca(ClO)2.
- It contains water of crystallization in the form of calcium hypochlorite hydrate, Ca(ClO)2•xH2O, where x represents the number of water molecules.
- The water of crystallization in bleaching powder plays a crucial role in stabilizing the compound and preventing it from decomposing.

Baking Soda:
- Baking soda, or sodium bicarbonate, is represented by the chemical formula NaHCO3.
- It contains water of crystallization in the form of sodium bicarbonate hydrate, NaHCO3•xH2O.
- The water molecules are held within the crystal lattice, contributing to the stability of the compound.

Washing Soda:
- Washing soda, or sodium carbonate, is chemically represented as Na2CO3.
- It contains water of crystallization in the form of sodium carbonate decahydrate, Na2CO3•10H2O.
- The presence of water molecules within the crystal structure helps maintain the integrity of the compound.

Plaster of Paris:
- Plaster of Paris, also known as calcium sulfate hemihydrate, has the chemical formula CaSO4•0.5H2O.
- It contains water of crystallization in the form of half a water molecule per calcium sulfate unit.
- The water of crystallization is necessary for the formation of the gypsum crystals that make up plaster of Paris.

Conclusion:
Water of crystallization is an essential component of various compounds, including bleaching powder, baking soda, washing soda, and plaster of Paris. These compounds rely on the presence of water molecules within their crystal structures to maintain stability and structural integrity. Understanding the role of water of crystallization helps in comprehending the properties and applications of these compounds.