Question

Why is homogeneous mixture not considered a pure substance

Answer 1

A homogeneous mixture is not considered a pure substance because it can be separated through mechanical and thermal processes. A pure substance, however, cannot be separated.

Answer 2

solution contains 2 or more substances dissolved in a liquid. 
Each ELEMENT involved, including those that make up the Solvent, are the PURE substances but, the solution, as it consists of a number of different elements is not pure.