The cause of Brownian-movement is

Options

A. the attractive force between the colloidal particales and the molecules of the dispersion medium.
B. heat change in liquid state
C. convectional current
D. the collision of the molecules of the dispersion medium with the calloidal particles

Answer

D

Explanation

Robert brown (B botanist)discovered `(1827)` that pollen grains placed in water do not remain at rest but phenomenon was observed, when colloidal solutions were seen under an ultramicrosopic, i.e., the colloidal particles were seen to be in a state of continuous zig zag motion all over the field of view. This zig-zag motion, called Brownian movement, is independent of the nature of the colloid but depends on the size of the particles and visosity of the solution. Smaller the size and lesser the viscosity, faster is the motion. Brownian movement arises because of the impact of the molecules of the dispersion medium with the colloidal particles. Since the impacts of the molecules of the dispersion medium on the colloidal particles are unequal (i.e. unbalanced bombardment), the result is zig-zag motion.
The Brownian movement has a stirring effect which does not permit the particles to settle and hence is reponsible for the stablity of sols. However, as the size of the particles increases, the effect of the impacts average out and the Brownian movement becomes slow. Ultimately, when the dispersed particles become bigh enough to acquire the dimensions of suspensions, no Brownian movement is observed, small size of particles) to be observed, whilst in suspension,the Brownian movement is too slow to be observed.