When a solution containing two volatile substances is heated up, one of them will be present in the vapour above the solution in greater quantities than the other. For example in a solution containing water and alcohol, the alcohol is normally present in greater quantities in the vapour. In most cases the component that is in the vapour in the greatest quantities is the component with the lowest boiling point, although there are some cases when this is not true. The fact that one component 'prefers' to be in the vapour is used to design distillation columns which separate two or more substances based on differences in their volatility.
Distillation is the most widely used separation process in the chemical industry.
In particular it is widely used in the petroleum industry. Thermodynamics finds some of its most important applications in this area. Given a mixture of substances in a liquid solution, it can be used to predict which will prefer to be in the vapour phase and which will prefer to be in the liquid phase. This knowledge is vital in the design and operation of distillation columns.