The term Colloid was first used by Graham (1861) to include the substances like starch, gum gelatin etc; whose glue like solutions have little or no tendency to pass through parchment membrane or collodion. But the term colloid refers to a heterogeneous stable mixture which intermediate between suspension and true solution.
A colloid may consist of a mixture of solid in liquid in which solid form is called dispersed phase and the liquid component is called dispersion medium. This colloid has a fluid like consistency. The particles of dispersed phase is called Colloidal particles or Micelles, The size of particle is 10 -10 cm; which can visible under ultramicroscope but invisible under light microscope.
Colloidal solutions may be Lyophilic (solvent loving) or lyopholic (solvent hating ). Lyophilic colloids are called emulsoods and later are called suspensoids. The colloidal solution in which water is the dispersion medium called as hydrophilic colloids.
Colloidal solutions may attain two physical states one is sol state and other is semisolid gel state. There two states can be inter conversible in reversible colloids.
Agar-agar when boiled in water gives a sol like colloid. But when it is cooled attains semisolid consistency. But egg albumin is irreversible colloid.
Living cell protoplasm is a colloidal solution in which water is the dispersion medium and cytoplasmic organells and inclusions are the dispersed phase.
A colloidal solution shows following properties.
Colloidal particles (dispersed) never pass through the parchment membrane or plasma membrane.
Colloidal particles have a tendency to attract and retain at their surfaces other particles with which they come in contact.
iii) Tyndall effect:
lf a strong light is passed through a colloidal solution. The path of beam is illuminated by a bluish light come due to defraction of light by particles.
Under ultra microscope it shows bright spots on dark field.
iv) Brownian Motion:
In colloidal solutions the particles of dispersed phase exhibit characteristic zig-zag motion called Brownian motion, which due to uneoual bombardment of particle by the molecules of the dispersion medium.
v) Electrical properties:
Colloidal particles constituting the dispersed phase carry electric charge probably due to the preferential adsorption of ions to it, All colloidal particle of a particular colloidal solution carry either + ve or-ve charge.
vi) Coagulation or Flocculation:
The precipitation of colloidal particles constituting the dispersed phase of the colloidal solution by the addition of an electrolyte is called Coagulation or Flocculation.
vii) Osmotic pressure:
Colloidal solutions shows very low osmotic pressure as the concentratiois higher than normal solution. Protoplasm is a polyphase complex colloidal system exhibit all above properties of colloids.