Colloidal particles are either positively charged or negatively charged. This charge is due to preferential adsorption of either positive or negative ions on their surface. There is adsorption of common ion present in excess.
When aqueous solution of AgNO3 is added to KI solution, positively charged sol of AgI is obtained due to the adsorption of Ag+ions on AgI molecules. excess AgNO3+KI⟶AgI+KNO3 AgI+AgNO3⟶fixed part positive sol[AgI]Ag+:diffused partNO3−