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Colloids: Properties Of Colloidal Solutions - I

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Colloids: Properties Of Colloidal Solutions - I - Lesson Summary

The properties exhibited by colloidal solutions can be classified as colligative properties, optical properties, mechanical properties and electrical properties.

Colligative properties:

Colloidal solutions exhibit the colligative properties osmotic pressure, elevation in boiling point, depression in freezing point and relative lowering of vapour pressure, in the same way as true solutions do.

Colloids exhibit measurable osmotic pressure, which can be determined with a reasonable degree of accuracy. This property of colloidal solutions is, therefore, used to determine the average molecular masses of certain colloidal particles, like proteins and other polymers.

Optical properties:

Colloids exhibits the optical properties Tyndall effect & colour.

Tyndall effect.

When a beam of light passes through a true solution, there is no scattering, and the path of light cannot be traced.

When a beam of light is allowed to pass through a colloid, it gets scattered by the colloidal particles, and the path of the light can be traced.

The path of light gets illuminated with a bluish light. This phenomenon of scattering of light by colloidal particles is called the Tyndall effect. The illuminated, bright cone of light is called the Tyndall cone.

The Tyndall effect is observed only when two conditions are satisfied.

1) The diameter of the particles of the dispersed phase should not be too small than the wavelength of the light used.

2) The refractive indices of the dispersed phase and the dispersion medium should differ greatly in magnitude.

Another important optical property of colloids is their colour.

Colloidal solutions are generally coloured. The size, shape and nature of the particles determine the colour of a colloid. Larger particles absorb light of longer wavelength, and, therefore, transmit light of shorter wavelength.

EX: In a gold sol, if the particles are very fine, the sol is red in colour, but when the size of the particles grows, the colour changes to purple, then blue, and finally golden.

Mechanical properties of colloidal solutions:

The continuous zigzag movement of colloidal particles in a dispersion medium is called Brownian movement.

Brownian movement is independent of the nature of the colloidal particles, but depends upon their size and on the viscosity of the solution. The movement of colloidal particles increases with a decrease in their size and in the viscosity of the medium.

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