Question

explain why particles of a colloidal solution do not settle down when left undisturbed while suspension do

Answer 1

Hey Friend,
They Do So Because Collide particles keep moving, while suspension, if we shake and keep it undisturbed, Particles will settle down.
Hope It helps.
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Answer 2

In a colloidal solution the colloidal particles are small and they are being continuously hit upon by the molecules of depression medium.

It is very unlikely that the dispersed face particles will be hit upon by an equal force from all the sides.

Therefore there is a resultant force acting upon the colloidal particles in a particular direction.

And colloidal particles execute zigzag motion called Brownian movement.

Because of this the colloidal particles do not settle down under the influence of gravity.