The coagulation of colloidal particles of the sol can be caused by

Heating or boiling of sol disturbs the adsorbed layer due to increased collisions with the molecules of dispersion medium.charge destruction occurs leading to coagulationA positive sol can coagulate a negative sol vice versa.This is called mutual coagulationAddition of excess electrolyte leads to coagulation where the ion having opposite charge to that of solneutralizes the charge