Question

why can some mixtures not be classified as solutions?
not all mixtures are heterogeneous.
not all mixtures are homogeneous.
mixtures are made of only one substance.
not all mixtures have solutes and solvents.

Explanation:

A solution is a homogeneous mixture. Some mixtures are heterogeneous (not uniform throughout), so they can't be classified as solutions.

Answer:

Not all mixtures are homogeneous.