Particles of a colloidal solution are smaller and lighter than that of a suspension. The range of diameter of particle of colloids are: 1 to 1000nm. They are always in a state of random motion. This random movement is called the Brownian movement, which counters the force of gravity acting on colloidal particles and hence, it does not allow the particles to settle down. Colloidal particles are charged and repel each other. That is another reason contributing to the fact that the particles of colloidal solution do not settle down.

On the contrary, particles of suspension are larger, heavy and have less movement. Thus, they settle down due to gravity.