Colloids are the heterogenous mixture which differ from true solution and suspension. In true solution, the solute cannot be separted from the solvent by simple physical methods, where as the suspensions are mixtures where the particles are not distributed through the mixture uniformly and can be separted by simple physcial process. Thomas Graham, in 1861, during his work on diffusion, found that while certain substances, such as sugars, salts, acids and bases diffused readily through a parchment membrane, others, such as gelatin, albumen and glue, diffused at a very slow rate. The substances belonging to the former category which generally exist in crystalline state, were called crystalloids while the substances belonging to the second category were given the name colloids. These observations led to the development of a new branch known as colloidal chemistry.
In a true solution as sugar or salt in water, some of the alloys, gases mixture, the solute particles are dispersed in the solvent as single molecules or ions. Thus the diameter or the size of the dispersed particles ranges from 1A° to 10A° that is they are very small compared to the wavelength of the light so they do not scatter the light . On the other hand, in a suspension as sand stirred into water, the dispersed particles are aggregates of millions of molecules. The diameter or size of these particles is of the order of 2000A° or more they are larger or sometime equal to the magnitude of the wavelength of the light so they tend to scatter the light. The colloidal solutions are intermediate between true solutions and suspensions. When the diameter of the particles of a substance dispersed in a solvent ranges from about 10A° to 2000A°, the system is termed a colloidal solution. The size of the particles are such that it is exactly in the wavelengh region of the visible light so that they tend to scatter the light. The path of the light in the colloidal solution can be clearly seen. When in a cinema projection hall there will be bright cone of line which starts from projector and ends in the screen this effect is due to the scattering of light by the dust particle present in the atmosphere that is smoke which is colloidal solution of solid in gas.