Some important properties of colloidal solutions are listed below.
A colloidal solution is heterogeneous though solute particles are invisible under an ordinary microscope. They can be detected with the help of an ultramicroscope.
The osmotic pressure of a colloidal solution is very small because of the number of colloidal particles are small as compared to a solution containing the same amount of solute particles in a given volume of solvent.
The colloidal particles can pass through ordinary filters. However, by the use of ultrafilters, it becomes possible, to filter colloidal particles.
“When a strong beam of light is concentrated on a colloidal solution, the path of the beam is illuminated by colloidal particles. It is called the Tyndall effect.” The colloidal particles absorb light and then scatter light of shorted waves. Because of this reason, the path of the beam is illuminated by bluish light and becomes visible when observed from the other side. “The park of light made visible throughout the colloid is called the Tyndallbeam or Faraday Tyndall-cone”.
The colloidal solution is usually coloured due to the scattering effect of particles. The colour depends upon the size and the nature of particles, e.g.
It is because of unequal bombordment by the solvent molecules or the molecules of the dispersion medium, which has comparatively smaller sizes.
Since colloidal particles carry a charge, they move towards respective electrodes when placed in an electric field. “The migration of colloidal particles under the influence of the electric field is called cataphoresis”. It is employed to determine the sign of the electric charge on the colloidal particles.
If the colloidal particles in sol are maintained in a stationary state, the dispersion medium
moves. “The movement of dispersion medium under the influence of the electric field is
known as electro-osmosis”. This phenomenon has been used dewatering peat, moist clay,
drying dyes and pastes.
Precipitation or Coagulation.
The precipitation or coagulation of a colloidal solution can be brought about by the addition of a small amount of an electrolyte, e.g., a few drops of barium chloride to arsenious sulphide precipitate it. This is because of the reason that ions of electrolyte are absorbed by the colloidal particles. These form bigger aggregates and settle down as precipitate. In our example, sol particle being negatively charged, are neutralized by Ba+2 ions and form a precipitate.
It is concerned with the power of an ion to coagulate a sol, It can be stated as. “The higher the valency of the active ion, the greater is its precipitating action”, e. g. the precipitating action of cations Na+, Mg2, and Al3+ is of the Order.
Na+ < Mg2+ < Al3+
“The process by which the sole particles are protected from precipitation by electrolytes due to the previous addition of some hydrophilic colloid is called protection” The colloid added for protection is called a protective colloid. e.g., if a little gelatin is added to gold sol, it is not coagulated by the addition of sodium chloride. In this case, gelatin is a protective colloid.
The protective colloid particles form a protective layer about sol particles and thus prevent their precipitation.
The particles at the surface of a solid or liquid carry residual valencies. In order to satisfy it, they attract other particles and retain at their surfaces. “This physical contact at the surface, between two particles, is called adsorption”, The large surface offered by colloidal particles and their free valencies are responsible for adsorption by colloids.