When copper sulphate (CuSO4) is added to water, it undergoes a process called hydration. The water molecules surround and interact with the copper sulphate ions, forming a complex called a hydrated copper sulphate ion. The overall reaction can be represented as follows:

CuSO4(s) + 5H2O(l) → [Cu(H2O)6]SO4(aq)

In this hydrated form, the copper sulphate dissociates into its component ions, Cu2+ and SO42-, which become surrounded by water molecules. The copper ions are coordinated with six water molecules, forming an octahedral complex, while the sulphate ions remain free in the solution.

The hydration process of copper sulphate is exothermic, meaning that it releases heat. This can be observed as a slight rise in the temperature of the water upon the addition of copper sulphate.

The hydrated copper sulphate ions are responsible for the characteristic blue colour of the solution. This colour is due to the absorption of specific wavelengths of light by the hydrated copper ions.

In summary, when copper sulphate is added to water, it undergoes hydration, forming hydrated copper sulphate ions that give the solution its blue colour.