Question

how we can say that colloidal solution is a heterogenous mixture

Answer 1

A colloid is somewhere in between a truly homogenous mixture and a heterogenous mixture. A Lorri suggested, the individual particle in a colloid are small enough so that the thermal movement (Brownian mostion) prevents them from settling to the bottom of a container through the action of gravity, although they can be separated by centrifugation (which is not possible for a true homogeneous mixture). On the other hand, colloids are usually not transparent, they disperse light because the particles are as large or larger than the wavelength of visible light. 
Whereas some substances form colloids because molecules cluster to form colloidal particles, in polymers and, in particular, proteins, the molecules are large enough that they form colloidal particles on their own.

Answer 2

if you will pass a beam of light through colloidal solution then it scatter the light . It shows that the solute particles are not dissolved , hence colloidal solution is hetrogeneous mixture.
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