Colloidal state





Thomas Graham (1861) classified all the soluble substances into two following classes:

1. Crystalloids: The substances, aqueous solution of which can readily pass through vegetable and animal membranes such as inorganic salts, acids, bases, urea, sugar etc. are called crystalloids.

 

2. Colloids: The substances, aqueous solution of which can not pass through vegetable and animal membranes such as gelatin, starch, proteins etc. are called colloids. The term colloid is derived from Greek word kolla meaning glue-like.

This view of Graham has now been discarded. Because it has been shown that the same substance may exhibit both crystalliod and colloid states e.g. NaCl is crystalloid in water but colloid in benzene; soap is colloid in water but crystalloid in benzene etc.

It means that crystalloids or colloids simply represent a state of matter in which every substance can be brought by suitable methods.

When crystalloids dissolve in water they exist as single molecule or ions like those of the solvent and constitute a homogeneous system called true solution. On the other hand substances in colloidal state exist either as large number of aggregates of thousands of molecules (associated colloids) or as single big molecules i.e. of starch molecular mass 52000 (molecular colloids).

If the colloidal particles grow still bigger then they are said to form suspension. Therefore “A colloidal state may be regarded as intermediate stage between molecules and particles of coarse suspension”.

The size of the particles of solute in a colloidal solution is from 10^{-4} to 10^{-7} cm (1 nm to 100 nm). Particles bigger than 10^{-4} cm form a suspension while particles smaller than 10^{-7} cm form a true solution.

Colloidal State

Colloidal State




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