Explain what is observed:
$(i)$ When a beam of light is passed through a colloidal sol.
$(ii)$ An electrolyte,$NaCl$,is added to hydrated ferric oxide sol.
$(iii)$ Electric current is passed through a colloidal sol.

(N/A) $(i)$ When a beam of light is passed through a colloidal solution,the scattering of light is observed. This phenomenon is known as the $Tyndall$ effect. The scattering of light illuminates the path of the beam within the colloidal solution.
$(ii)$ When $NaCl$ is added to hydrated ferric oxide sol,it dissociates into $Na^+$ and $Cl^-$ ions. Ferric oxide sol particles are positively charged. Therefore,they undergo coagulation in the presence of the negatively charged $Cl^-$ ions.
$(iii)$ Colloidal particles carry a net positive or negative charge. When an electric current is passed,these particles migrate towards the oppositely charged electrode. Upon contact with the electrode,they lose their charge and coagulate. This process is known as electrophoresis.