Question

What are the characteristics of the following colloids ? Give one example of each.
(i) Multimolecular colloids
(ii) Lyophobic sols
(iii) Emulsions

Answer 1

(i) Multimolecular colloids : When on dispersion of a substance in the dispersion medium, a large number of atoms or smaller molecules of the substance (with diameter less than 1 nm) aggregate together to form species having size in the colloidal range, the species thus formed are called multimolecular colloids. Their molecular masses are not very high. Their atoms or molecules are held by weak van der Waals forces. e.g., gold sol, sulphur sol.

(ii) Lyophobic sols : Substances like metals, their sulphides etc. when simply mixed with the dispersion medium do not form the colloidal sol. Their colloidal sols can be prepared only by special methods. Such substances are called lyophobic and the sols formed by them are called lyophobic sols. They are also called extrinsic sols or irreversible sols. e.g., gold sol.

(iii) Emulsions : A colloidal dispersion in which both the dispersed phase and the dispersion medium are liquids is called an emulsion. Emulsions are stabilised by emulsifiers. Emulsions exhibit properties like Tyndall effect, electrophoresis etc. Their constituent liquids can be separated by boiling, freezing etc.
e.g., vanishing cream, butter etc