Define electro-osmosis.

Explanation

Coagulation: The process of aggregation of colloidal particles into an insoluble precipitate by the addition of some suitable electrolyte is called coagulation.
The ion of the electrolyte which neutralises the charge on the colloidal particles is called effective ion.
(a) For negatively charged sols like `As_2 S_3` sol, the effective ion is cation of the electrolyte.
(b) For positively charged sols like Fe `(OH)_3` sol, the effective ion is anion of the electrolyte.
Electrophoresis: The movement of colloidal particles in an electric field is known as electrophoresis. The sign of the charge on the colloidal particles is found from the direction in which the particles move.

For electrophoresis, the colloidal solution is placed in a U-tube fitted with platinum electrodes. On passing an electric current, the charged colloidal particles move towards the oppositely charged electrode. For example, if arsenious sulphide sol is taken in the U-tube, its particles move towards the anode, where they get coagulated. It is due to neutralisation of the negative charge present on `As_2 S_2` colloidal particles
Cause : It is due to electrostatic forces of attraction between the colloidal particles and the oppositely charged electrode which lead to the migration of colloidal particles in an electric field.
Electro-Osmosis : The movement of dispersion medium under the influence of the applied electric field, when the dispersed particles are prevented from moving by a suitable membrane.

For example, when electro-osmosis of a negatively charged sol (say `As_2 S_3` sol), the dispersion medium, which is positively charged moves towards the -ve electrode (cathode), thus raising the level of the water in the glass tube of cathodic compartment.