Energy is needed when bonds are broken. However energy is released when bonds are formed.

Lets take hydrated copper sulfate for example. Its formula is given as CuSO₄.6H₂O. It shows that for each CuSO₄ unit 6 water molecules are bonded via ion-dipole bonding. To drive the water out we supply energy in the form of heat in order to break the bonds ion-dipole bonds.

Once the water is removed the copper sulfate is grey in colour.

When water is added to the crystals, energy is released as ion-dipole bonds form.

View the video on the right to see the energy released when water is added to anhydrous copper sulfate.