There are three main mechanical properties of Colloidal Solutions:
Brownian Motion: If we examine the colloidal solutions with an ultramicroscope, we find that suspended particles are in a constant rapid zig-zag motion. This zig-zag motion is called Brownian movement. Robert Brown first noticed this movement while examining pollen grains suspended in water under a microscope.
The kinetic theory gives a nice explanation of the phenomenon. The motion is due to unequal bombardment by the molecules of dispersion medium. with increase in size of particles, the probability of unequal bombardment diminishes and so Brownian movement decreases with increase of particle size.
This motion counteracts the force of gravity acting on the colloidal particles and is thus responsible to a certain extent for the stability of colloids.
Diffusion: As per the Graham’s Law the rates of diffusion decrease very rapidly with increase in the mass of the particles, colloidal solutions would diffuse much more slowly than ordinary molecular solutions. This property is used in determining molecular weights of colloids. Since the equation to be used for the purpose also has a term for the Avogadros number, the latter can also be determined from it, using solutes of known molecular masses.
Sedimentation: Although colloidal solutions are found to be fairly stable over considerable periods of time, they do very slowly settle down under the influence of gravity, even though some of them may take years to do so. The rates of settling or sedimentation can be readily determined because the boundary line between the clear medium and the sol can be easily made out.
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