1. In these, hydrogen bonding is very important. Often the water molecules serve to fill in the interstices and bind together structure.
2. A specific example is CuSO4 . 5H2O.
3. Although there are five water molecules for every divalent copper ion, only four are coordinated to the cation, it’s six-coordination being completed from sulphate anions. The fifth water molecule is held in place of hydrogen bonds, O – H – O, between it and two coordinated water molecules and then coordinate sulphate anion.
4. Water forms hydrated salts during crystallization.
Examples, Na2CO3 . 10H2O, FeSO4 .7H2O.
5. The water present in the hydrates is called as water of hydration.