Colloids:
A colloid is an intermediate state of true solution and suspension. A colloid is a heterogeneous system in which one substance is dispersed (disperse phase) as very fine particles in another substance called dispersion medium.
Colloidal particles have sizes between 1 and 1000 nm (10~9 to 10″6 m). For example, in milk, water is dispersion medium while fats, proteins etc., constitute disperse phase.
Image Source: umich.edu
Properties of colloidal solutions:
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1. They are heterogeneous mixtures.
2. The size of the colloidal particles ranges between 1 nm to 1000 nm.
3. The path of a beam of light becomes visible while passing through a colloidal solution (Tyndall effect).
4. Colloidal particles cannot be separated by simple sedimentation or filtration.