A mixture of two gases that will react to form a third gas is an example where there are three species present but the concentration of each is not independent. A mixture of hydrogen and nitrogen, in the presence of a platinum catalyst, will react to form ammonia. The three species are in equilibrium. The amount of ammonia present will depend on the initial ratio of oxygen and nitrogen. Solid solutions are often very complex in behavior.
Many metal alloys have limited possibilities of composition.
Liquid solutions are the most familiar type. A liquid solution can contain components that would by themselves be gases, liquids or solids. A water solution might contain some dissolved air, some alcohol and some dissolved salt. If the concentration of one component is much greater than the others, that component is referred to as the solvent, the others are referred to as solutes. A single substance can contain more than one type of particle.