Mechanical Property (Brownian Movement)

Robert Brown, a Botanist discovered in 1827 that pollen grains placed in water do not remain at rest but move about continuously and randomly. Later on, this phenomenon was observed in case of colloidal particles when they were seen under the ultramicroscope. The particles were seen to be in constant motion in zig – zag path in all possible directions.
This zig – zag motion of colloidal particles is called Brownian Movement.

Cause of Brownian Movement

Brownian Movement arises because of impact of the molecules of the dispersion medium with the colloidal particles. It has been postulated that the impacts of the particles of dispersion medium on colloidal particles are unequal which ultimately causes its zig – zag motion. However, as the size of particle increases, the probability of unequal impacts decreases and the Brownian movement becomes slow. Ultimately, when the dispersed particles become bigger enough to acquire the dimensions of suspension, no Brownian movement is observed.
Significance of Brownian Movement
(i).     Brownian Movement provides direct demonstration of ceaseless motion of molecules as postulated by Kinetic Theory.
(ii).    It neutralizes the force of gravity acting on colloidal particles and hence helps in providing stability to colloidal sols.