Definition Of Properties Of Colloids

A colloid is one of the three main types of mixtures, with the other two being a solution or suspension

A colloid is a solution that has particles ranging between 1 and 1000 nanometers in diameter, yet is still able to remain evenly distributed throughout the solution. These are also known as colloidal dispersions because the substances remain dispersed and don't settle to the bottom
In order to be classified as a colloid, the substance in the dispersed phase must be larger than the size of a molecule but smaller than what can be seen with the naked eye
Brownian movement: This continuous and rapid zig-zag motion of the colloidal particles is called Brownian movement. This motion is independent of the nature of the colloidal particles. It is more rapid when the size of the particles is small and the solution is less viscous
The Brownian movement is due to the bombardment of colloidal particles by molecules of dispersion medium
Tyndall Effect: Tyndall (1869) observed that when a strong beam of light is focused on a colloidal solution the path of the beam becomes visible and when viewed through microscope placed at right angle to the path of light; the colloidal particles appear as pin points of light moving against a dark background in a colloidal solution. This phenomenon is known as Tyndall effect and the illuminated path is called Tyndall cone

Electrophoresis: Since the colloidal particles are electrically charged (+ or -)with respect to the dispersion medium, hence on passing electric current through colloidal solution the charged particles move towards oppositely charged electrodes and get discharged to give precipitate. So, this migration of colloidal particles under the influence of electric field is called electrophoresis
Electro-osmosis: When electrophoresis of dispersed particles in a colloidal system is prevented by some suitable means, the dispersion medium itself begins to move in an electric field. This phenomenon is known as electro-osmosis
Coagulation: We know that the stability of the colloidal solution is due to mutual repulsion between similarly charged colloidal particles. When the charge on the colloidal particles is neutralized by the addition of an electrolyte or oppositely charged sol, the precipitation takes place. Thus “the process of precipitating a colloidal solution is known as coagulation or flocculation”
For example, the negatively charged As₂S₃ sol is readily coagulated on addition of a solution of BaCl₂ (due to Ba^{2 +} ions). The positively charged Fe(OH)₃ sol is readily coagulated on addition of a solution of NaOH (due to OH^- ions)