Relationship between Boiling Point and External Pressure

Greater the pressure exerted on the liquid, greater would be the boiling point or vice versa. Boiling point of a liquid can be increased by increasing the external pressure on the liquid, whereas it can be decreased by decreasing the external pressure on the liquid.

At 1 atmospheric pressure water boils at 100°C. At high altitude such as at Murree where atmospheric pressure is 0.921 atm (i.e. 700 mm of Hg), water boils at 98°C. While at the top of Mount Everest water boils at only 69°C. Boiling point of water is 120°C at 1489 torr and 25°C at 23.7 torr.

When vapour pressure of a liquid becomes equal to the external pressure then the liquid boils. Thus when external pressure is changed, its boiling point will also be changed. So a liquid can be made to boil at any temperature by changing external pressure. With the increase in external pressure more heat is required to equalize vapour pressure with external pressure. So the boiling point is raised. Similarly, at a lower external pressure a liquid absorbs less amount of heat and it boils at lower temperature.