When a liquid is heated in an open container, its vapour pressure keeps on increasing with the rise in temperature till it becomes equal to the atmospheric pressure. At this state, molecules of the liquid are readily converted to the vapour phase and the liquid is said to boil. The temperature at which the vapour pressure of the liquid becomes equal to the atmospheric pressure (i.e. external pressure) is called the boiling point of the liquid. When the external pressure is equal to the standard atmospheric pressure (i.e. 760 mm of mercury), the boiling point is called normal boiling point. For example, normal boiling points of water and benzene are 100°C and 80°C respectively.

Effect of change in external pressure. It should be possible to change the boiling point of a liquid by changing the external pressure. Clearly, if the external pressure is higher, more heat will be required to make the vapour pressure equal to the external pressure and hence higher will be the boiling point. Similarly, if the external pressure is decreased, the boiling point is lowered. Boiling at a lower temperature under reduced pressure is used in the purification of such liquids which decompose at or below their normal boiling points.