Why only colloids have brawnian motion?

Answer 1

Brownian motion occurs in colloids due to the random collisions between the larger particles (colloidal particles) and the smaller, fast-moving molecules of the dispersing medium (such as water or air). In a colloid, the particles are small enough to be affected by these collisions, but large enough to be seen under a microscope. In contrast, in solutions or pure substances, the particles are either too small (like solute molecules) or too large (like bulk materials) to exhibit noticeable Brownian motion. Therefore, Brownian motion is a unique characteristic of colloidal systems where the balance between particle size and medium interaction is just right.