Lyophilic colloid or sol are unstable, so they are stabilised by adding some protective which protect them from precipitation. Thus, lyophilic colloids are protecting colloids. Their protecting power is denoted in terns of gold number. It defined as the minimum amount of lyophilic colloid in mg which present the flocculation of 10‌mL gold sol by the addition of 1‌mL of 10% NaCl solution. Lesser the gold number, higher the protecting power.