How do we know the charge of a sol particle?

Read the given passage and answer the question that follow: Colloidal particles always carry an electric charge which may be either positive or negative. For example, when AgNO_3 solution is added to KI solution, a negatively charged colloidal sol is obtained. The presence of equal and similar charges on colloidal particles provide stability to the colloidal sol and if, somehow, charge is removed. coagulation of sol occurs. Lyophobic sols are readily coagulated as compared to lyophilic sols. What is the reason for the charge on sol particles ?

Read the given passage and answer the question that follow: Colloidal particles always carry an electric charge which may be either positive or negative. For example, when AgNO_3 solution is added to KI solution, a negatively charged colloidal sol is obtained. The presence of equal and similar charges on colloidal particles provide stability to the colloidal sol and if, somehow, charge is removed. coagulation of sol occurs. Lyophobic sols are readily coagulated as compared to lyophilic sols. What is the reason for the charge on sol particles ?