Colloidal Solution 

A heterogeneous solution in which the particle size is in between 10^–7 cm to 10^–4 cm such that the solute particles neither dissolve nor settle down in a solvent, is called colloidal solution. Blood is an example of colloid. Its components can be separated using centrifugation.

The components of a colloidal solution are the dispersed phase and the dispersion medium. 

1.0Dispersed Phase

It is the component which is present in small proportions and consists of particles of colloidal dimensions (10^–9 m to 10^–6 m).

2.0Dispersion Medium

It is the component which is present in excess and acts as a medium in which colloidal particles are dispersed.

3.0Types of Colloids

4.0Properties of Colloids

(1) A colloid is a heterogeneous mixture but it appears homogeneous.

(2) The size of particles of a colloid is too small to be individually seen by naked eyes.

(3) The colloidal particles do not settle down when left undisturbed i.e. colloid is quite stable.

(4) The colloid particles cannot be separated from the dispersion medium by the process of filtration. A special technique of separation known as centrifugation can be used to separate the colloidal particles.  

(5) Colloidal solutions are not transparent, but translucent in nature.

(6) Particles in a colloidal solution follow a zig zag path (Brownian movement). 

It is normally not possible to see the colloidal particles because of their very small size. However, their path can be seen under a microscope. 

Brownian movement may be defined as continuous zig-zag movement of colloidal particles in a colloid. This Brownian movement arises due to hitting of the colloidal particles by the particles of the dispersion medium from different directions with different forces.

(7) Colloidal solution scatters the beam of light passing through it. (Tyndall effect) 

As a result, the path of light becomes visible. This scattering of a beam of light by colloidal particles is called the Tyndall effect. The reason for this observation is that the particles of a colloidal solution are big enough to scatter light and hence the path of light becomes visible i.e. the Tyndall effect is observed.

For example,

(i) Tyndall effect can also be observed when a fine beam of light enters a room through a small hole. This happens due to the scattering of light by the particles of dust and smoke in the air.

(ii) Tyndall effect can be observed when sunlight passes through the canopy of a dense forest. In the forest, mist contains tiny droplets of water, which act as particles of colloids dispersed in air.

Difference between true solutions, suspensions and colloidal solutions