Question

Why milk is a mixture not solution?

Answer 1

Milk is a heterogeneous mixture. It is a colloid solution. Colloids are unique because they act somewhat like homogeneous mixtures and somewhat like heterogeneous mixtures. The particles are light enough that they do not settle on the bottom (due to Brownian motion) but heavy enough that they can be separated by centrifugation (you cannot do this to a homogeneous mixture). They are small enough to not be distinguished without visual aid, but large enough to scatter light (that is why milk is white).