A homogeneous-looking heterogeneous mixture in which particles having size between 10 -9 – 10-7 m are dispersed in a continuous medium is called a colloid.

Examples: Milk, Office paste (gum), Blood, Toothpaste, Mist (water vapor dispersed in air), Jelly, etc., are colloids.

The particles of the dispersed phase are commonly called colloid particles.

Depending upon the nature of the dispersed phase and the dispersion medium, the colloids be classified into various types.

Properties of colloid

Some characteristics properties of colloids are described below:


i) Heterogeneous nature:

A colloid (or sol) is heterogeneous in nature. The particles in a colloid can be seen only with a powerful microscope.

ii) Particle size:


The size of particles in a colloid lies between 10-9 m – 10-7 m.

iii) Separation of colloidal particles by filtration:

Colloidal particles can easily pass through the pores of a filter paper. Therefore, colloidal particles cannot be separated by filtration.

iv) Stability:


Colloids are unstable unless stabilized by adding suitable stabilizer. The colloidal particles tend to come together and settle down.

v) Brownian movement of colloidal particles:

When seen under a microscope, the colloidal particles in a colloid are seen to be moving in a random (zigzag) fashion. This zigzag motion of particles in a colloid is called Brownian motion.

vi) Light scattering by colloidal particles:


Tyndall effect. When a strong beam of light is passed through a colloid kept in a dark room, the path of the light beam through the colloid becomes visible. This occurs due to scattering of light by the colloidal particles. The scattering of light by the particles in a colloid is called Tyndall effect.

True solutions do not scatter light. Therefore, true solutions do not show Tyndall effect.

vii) Electrophoresis:

Many colloidal particles have electrical charge only them, i.e., colloidal particles may have positive or negative charge only them. So, when electric current is passed through a colloid, the colloidal particles move towards the oppositely charged electrode. The movement of colloidal particles under influence of electric field is called electrophoresis.