Hardy Schulze rule states that the ion that causes a soluble to coagulate is opposite in sign to the electric charge of the colloidal particle; further, the coagulating power increases with the valency of the ion.
The effective ions of the electrolyte in bringing about coagulation are those which carry charge opposite to that of the colloidal particles. These ions are called coagulating ions or flocculating ions.
Greater is the valency of the coagulating or the flocculating ion, greater is its power to bring about coagulation.
Example: For coagulating negative sol order of coagulating capacity is:
Al3+ > Mg2+ > Na+
Al3+ coagulates a negatively charged sol more effectively than Na+ ions.