(1) The foundation of colloidal chemistry was laid down by an English scientist, Thomas Graham, in 1861. The credit for the various advances in this field goes to eminent scientists like Tyndall, Hardy, Zsigmondy, N.R. Dhar, S.S. Bhatnagar and others.
(2) Thomas Graham classified the soluble substances into two categories depending upon the rate of diffusion through animal and vegetable membranes or parchment paper.
(i) Crystalloids : They have higher rate of diffusion and diffused from parchment paper.
Examples : All organic acids, bases and salts and organic compounds such as sugar, urea etc.
(ii) Colloids (Greek word, kolla, meaning glue-like) : They have slower rate of diffusion and can not diffused from parchment paper.
Examples : Starch, gelatin, gums, silicic acid and hdemoglobin etc.
(3) The above classification was discarded i.e., the terms colloid does not apply to a particular class of substances but is a state of matter like solid, liquid and gas. Any substance can be brought into colloidal state.
(4) The colloidal state depends on the particle size. If is regarded as intermediate state between true solution and suspension.
Table: Features of the three types of solutions
> 100 nm
1 nm – 100 nm
< 1 nm
(i) Ordinary filtration
(ii) Ultra- filtration
Settling of particles
Settle under gravity
Settle only on centrifugation
Do not settle
Does not show
Diffusion of particles
Does not diffuse