Colloids show some unique properties which are discussed in this section.
Tyndall observed this phenomenon in 1869. He observed that when a beam of light is allowed to pass through a colloidal solution, the path of light gets illuminated. This phenomenon is known as Tyndall Effect. It occurs because light is scattered by particles present in colloidal solution. The intensity of the scattered light depends on the difference between the refractive indices of the dispersed phase and the dispersion medium. Greater is the difference, greater the intensity of light.
Figure shows Tyndall Effect in which light is scattered by a colloidal solution in first beaker. Second beaker contains only water (pure solution) and does not show Tyndall Effect. Tyndall Effect can be used to distinguish between Colloidal Solution and pure solution
In case of lyophilic solution, the colloidal particles are highly solvated, which results into lower difference in refractive index of two phases. In lyophilic solution, Tyndall effect is not pronounced. In case of lyophobic solution, the difference is quite large and it shows Tyndall effect.
There are continuous collisions between the colloidal particles and molecules of dispersion medium which are in constant motion and passes kinetic energy to colloidal particles by striking it from all sides. This results into zigzag movement of colloidal particles. This zigzag movement of colloidal particles is known as Brownian movement.
Brownian movement of Colloidal Particles
Colligative properties depend upon the number of moles of solute present in given mass of solvent. It does not depend on nature of solute particles. Colloidal particles are not simple molecules, they are aggregations of molecules. Hence, all colloidal dispersions give low value of osmotic pressure, depression in freezing point and elevation in boiling point.
The movement of colloidal particles under the influence of electric field is called Electrophoresis. When an electric field is applied across the colloidal solution, the colloidal particles migrate to oppositely charged electrode where they get neutralized. This phenomenon is known as electrophoresis.
When electrophoresis of dispersed particle in a colloidal system is prevented by some suitable means, it is observed that dispersion medium itself begins to move in an electric field. This phenomenon is known as electro osmosis.