In crystalline copper sulphate, sulphate ion is attracted towards Cu2+ by ionic interaction and it bonded to it by covalent coordinate bond. Also 4 molecules of water are bonded to Cu2+ by coordinate covalent bond. Although the fifth water molecule is held by hydrogen bonds between a sulphate ion and coordinated water molecule. And hence this is not easily removed by heating. Along with these bonds there is covalent bond present in sulphate ions and water molecule.