Dear Hanson,

The most common are sols (solid in liquids), gels (liquids in solids) and emulsions (liquids in liquids).

True solutions have particles less than 10^-9 m or 1nm.

Colloidal solutions have particle, which size between 10^-9 to 10^-7m or 1 nm to 100 nm.

Suspensions have particles, which size are more than and 10^-7 m or 100 nm.

Colloidal state of matter. Thomas Graham (1861) studied the rate of diffusion of liquids and solutions across the animal membranes and as ? result of his experiments, he divided the soluble substances into two classes crystalloids and colloids. The substances like common salt, sugar, urea, etc. which can be obtained in the crystalline form and in the dissolved state diffuse through the vegetable or animal membrane were termed as crystalloids, and the substances like starch, gum, glue etc. which are non-crystalline in nature and in the dissolved state do not diffuse or have ? little tendency to pass through the animal or vegetable membrane were given the name colloids.

Graham''s classification of substances into crystalloids and colloids was soon proved to be wrong. ? crystalloid was found to behave as ? colloid under different set of conditions and vice versa. For example, sodium chloride behaves as ? crystalloid when dissolved in water but behaves as ? colloid when dissolved in benzene.

It has since then been realised that ? clear cut difference does not exist between crystalloids and colloids.

The non-diffusibility of colloids and diffusibility of crystalloids through the animal or vegetable membrane was shown to be due to the difference in the size of particles of crystalloids and colloids. The colloids form bigger particles which cannot pass through the membrane. It is, therefore, preferable to speak of ? substance in ''colloidal state rather than calling it a crystalloid or a colloid. The particle size limits are given:

True solutions have particles less than 10^-9 m or 1nm.

Colloidal solutions have particle, which size between 10^-9 to 10^-7m or 1 nm to 100 nm.

Suspensions have particles, which size are more than and 10^-7 m or 100 nm.

Thus the size of the colloidal particles is intermediate between that of particles of true solution and those of suspension.

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