Explanation:

In a colloidal suspension, particles will settle very slowly or not at all because the colloidal particles carry surface electrical charges that mutually repel each other. A coagulant (typically a metallic salt) with the opposite charge is added to the water to overcome the repulsive charge and "destabilize" the suspension. For example, the colloidal particles are negatively charged and alum is added as a coagulant to create positively charged ions. Once the repulsive charges have been neutralized ( since opposite charges attract), the van der Waals force will cause the particles to cling together (agglomerate) and form micro floc.Coagulation : In water treatment, coagulation occurs when a coagulant is added to water to "destabilize" colloidal suspensions.