JEE Main 2016 — Surface Chemistry Question with Solution

Gold number of a protective colloid is a minimum weight of it in milligrams which must be added to 10 ml of a standard red gold sol. so that no coagulation of the gold sol. (i.e. change of colour from red to blue) takes place when 1 ml of 10% sodium chloride solution is rapidly added to it. Obviously, smaller the gold number of a protective colloid, the greater is the protective actioin.

It is given that the gelatin has lowest gold number thus, it would serve as a better protective colloid than colloids in the rest of the options.