**2.2 Steam distillation method**

Steam distillation is suitable for the extraction of volatile components which can be distilled with steam without being destroyed and are insoluble in water. These compounds' boiling points of are mostly higher than 100°C, and they possess certain vapor pressures at about 100°C. The principle of steam distillation is that the vapor pressure of each component is equal to that of their pure state, while the existence of another liquid does not affect their vapor pressure. The total vapor pressure of the mixing system is equal to the sum of the vapor pressures of the two components. Because the total vapor pressure of the system is higher than that of any single component, so the boiling point of the mixture is lower than that of any component. It is mainly used to extract volatile oils, some alkaloids, and phenolic substances of small molecules from plants.
