Name the phenomenon/process involved
i) mixing of hydrated ferric oxide (`+ve` sol) and arsenious sulphide (`-ve` sol)
ii) An impure sol is purified by removing dissolved particles using suitable membrane
iii) Movement of dispersion medium is observed in an electric field. 

Name the phenomenon/effect for the following i) Collidal particles are in zig zag motion. ii) When an electrical potential is applied across two platinum electrodes dipping in a collidal solution, particles move towards one or the other electrodes. iii) Scattering of light by collidal sol.

Explain the following observations : (i) A beam of light passing through a colloidal solution has a visible path. (ii) Passing an electric current through a colloidal solution removes colloidal particles from it. (iii) Ferric hydroxide sol coagulates on addition of potassium sulphate,