A homogeneous mixture has a consistent composition and occurs in a uniform state (gas, liquid, and solid). It differs from a heterogeneous mixture, which is comprised of different visible elements and/ or different states.
Pure air is an example of a homogeneous mixture because it occurs uniformly in the gaseous state and its different elements are not readily visible. When air is polluted with visibly distinct pollutants, such as dust and smoke, air is considered heterogeneous. The additional contents eliminate the air's consistency. Additionally, this type of air causes dust to occur in its solid state.
A mixture of salt and water is also considered a homogeneous mixture because salt readily dissolves in water. The mixture should be considered with the saturation point in mind because salt particles will remain visibly suspended in water beyond its saturation point. In this case, the mixture is heterogeneous.