What is the difference between a heterogeneous and homogeneous mixture?
The essential difference is the level or scale on which the mixing occurs. This gives rise to important differences in properties of these mixtures.
For a homogeneous mixture, the mixing occurs on the molecular level, with molecules (or ions etc.) intermingling.
In a heterogeneous mixture, the mixing occurs on a larger scale such that aggregates (clumps) of the materials combine.
Furthermore, homogeneous mixtures (also known as solutions) tend to be transparent, as light travels through the mixture without scattering.
So, salt dissolves in water to make a solution. The air is a homogeneous mixture. On the other hand, smoke in the air is a heterogeneous mixture, as it involves tiny (but not molecular) particles of soot suspended in the air.
The particles of soot scatter light, and cause a loss of transparency.
Heterogeneous mixtures sometimes come apart on standing. Homogeneous mixtures do not.