Read the following passage and answer the questions.
When colloidal solutions are viewed under a powerful ultramicroscope, the colloidal particles appear to be in a state of continuous zig-zag motion all over the field of view. This motion was first observed by the British botanist, Robert Brown, and is known as Brownian movement.
This motion is independent of the nature of the colloid but depends on the size of the particles and viscosity of the solution. Smaller the size and lesser the viscosity, faster is the motion.
The Brownian movement has been explained to be clue to the unbalanced bombardment of the particles by the molecules of the dispersion medium. The Brownian movement has a stirring effect which does not permit the particles to settle and thus, is responsible for the stability of sots.
What is the cause of Brownian movement ?

Explanation

Collision between the particles of dispersed phase and dispersion medium is the cause of Brownian movement. Impact of collisions are not equal, so the motion is zig-zag.