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Colloidal State

(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

PropertySuspensionColloid solutionTrue solution
NatureHeterogeneousHeterogeneousHomogeneous
Particle size> 100 nm1 nm – 100 nm< 1 nm

Separation by

(i) Ordinary filtration

(ii) Ultra- filtration

 

Possible

Possible

 

Not possible

Possible

 

Not possible

Not possible

Settling of particlesSettle under gravitySettle only on centrifugationDo not settle
AppearanceOpaqueGenerally transparentTransparent
Tyndall effectShowsShowsDoes not show
Diffusion of particlesDoes not diffuseDiffuses slowlyDiffuses rapidly
Brownian movementMay showShowsNegligible

 

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