Colloidal state
(1) The foundation of colloidal chemistry was laid down by an English scientist, Thomas Graham, in the year1861. The credit for the several advances in this field goes to eminent scientists such as N.R. Dhar, Zsigmondy, Tyndall, Hardy, S.S. Bhatnagar and others.
(2) The scientist named 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 (such as Greek word, kolla, Greek word, meaning glue-like) : They have slower rate of diffusion and can not diffused from parchment paper.
Examples of it are: Starch, gums, silicic acid gelatin, and hdemoglobin etc.
(3) The above classification was discarded that is the terms colloid does not apply to a particular class of substances but is a state of matter like solid, liquid and gas. Any of the substance can be brought into colloidal state.
(4) The colloidal state depends on the particle size. If can be regarded as intermediate state between true solution and suspension.
Table : 14.2 Features of the three types of solutions
|
Property
|
Suspension
|
Colloid solution
|
True solution
|
|
Nature
|
Heterogeneous
|
Heterogeneous
|
Homogeneous
|
|
Particle size
|
> 100 nm
|
1 nm - 100 nm
|
< 1 nm
|
|
Separation by
(i) Ordinary filtration
(ii) Ultra- filtration
|
Possible
Possible
|
Not possible
Possible
|
Not possible
Not possible
|
|
Settling of particles
|
Settle under gravity
|
Settle only on centrifugation
|
Do not settle
|
|
Appearance
|
Opaque
|
Generally transparent
|
Transparent
|
|
Tyndall effect
|
Shows
|
Shows
|
Does not show
|
|
Diffusion of particles
|
Does not diffuse
|
Diffuses slowly
|
Diffuses rapidly
|
|
Brownian movement
|
May show
|
Shows
|
Negligible
|
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