When arsenious sulphide sol is added to ferrie hydroxide sol, both of them are precipitated-why?

Explanation

`Fe(OH)_(3)` sol contains positively charged colloidal particles, while `As_(2)S_(3)` sol contains negatively charged particles. When they are mixed together in equivalent amount, the charge of one is neutralised by the opposite charge of the other. Hence, both of them are precipitated due to mutual coagulation.