* Homogeneous mixture: A mixture where the components are evenly distributed throughout and have a uniform composition. You can't visually distinguish the different parts.

* Heterogeneous mixture: A mixture where the components are not evenly distributed and have a non-uniform composition. You can usually see the different parts.

When oxygen dissolves in water, the oxygen molecules spread out evenly throughout the water molecules. You can't see the individual oxygen molecules, and the mixture appears uniform.