Explain what is observed when: An electrolyte ( AlCl 3 ) is added to a colloidal solution of arsenious sulphide ( As 2 S 3 ).

An electrolyte ( AlCl 3 ) is added to a colloidal solution of arsenious sulphide ( As 2 S 3 ). The As 2 S 3 Sol is a negatively charged Sol hence it can be coagulated by a positively charged electrolyte like AlCl 3 .

Explain, what is observed when a beam of light is passed through a colloidal solution.

Scattering of light by the colloidal particles takes place and the path of light becomes illuminated. This is called Tyndall effect. When a beam of light passes through a colloid, the colloidal particles present in the solution do not allow the beam to completely pass through. The light collides with the colloidal particles and is scattered (it deviates from its normal trajectory, which is a straight line). This scattering makes the path of the light beam visible, as illustrated below,