Solutions

A solution is a single phase consisting of a mixture on a molecular level of two or more components. A solution may be a gas, a liquid or a solid. If a component forms roughly micron sized clusters it is called a colloidal suspension. Colloids include a wide range of properties and are an entire field of study. Larger clusters are simply mechanical mixtures rather than solutions.

Under normal conditions gases can mix in any proportions as long as they do not react. A mixture of oxygen and nitrogen is stable in any proportion from pure oxygen to pure nitrogen at ordinary temperature and pressure. In a mixture of oxygen, nitrogen and argon the amount of each component can be adjusted independently. If the components react, the situation is different. 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. Pure water contains three types of particles; water molecules, hydronium ions and hydroxide ions. Water also consists of transient structures involving large numbers of molecules. Yet there is only one component.