What is Boiling Point 2022 |Basic Concept of Boiling Point |

The temperature at which the vapour pressure of a liquid becomes equal to the atmospheric pressure or some other external pressure is called the boiling point of a liquid.


The vapour pressure of a liquid increases with an increase in temperature. When a liquid is heated its vapour pressure goes on increasing until it becomes equal to the external pressure. At this temperature, the liquid starts boiling. This is the boiling point of the liquid.

The amount of heat required to vaporize one mole of a liquid at its boiling point is called Molar heat of vaporization.

At boiling point bubbles continuously, come out of the boiling liquid. It is because bubbles formed in the interior of liquid have higher internal P than atmospheric pressure. Thus, the bubble comes out of the liquid and bursts upon the surface.

Effect of intermolecular forces on boiling point Consider the vapour pressure curves of water, ethyl alcohol, ethylene glycol and diethyl ether.

Boiling Point
Boiling Point

• Graph shows, that all the liquids boil at a temperature, at which their vapour pressure becomes equal to 760 torrs at sea level.

• At the same temperature, the curve of diethyl ether starts 200 torrs, while that of water at 4.8 torrs. It shows that due to weak intermolecular forces ether has higher vapour pressure than water.

Thus, ether bolls at a lower temperature than water. Hence weaker the intermolecular forces, the lower will be the boiling point and vice versa.

• The curve shows that vapour pressures for all liquids increase very rapidly closer to the boiling point of liquids.

Boiling Point and External Pressure

The boiling point of a liquid increase with the increase in external pressure and vice versa.

It is because, at higher external pressure, the liquid needs to absorb more heat to equalize its vapour pressure t external pressure hence boiling point is higher. While at lower external pressure liquid needs to absorb less heat, hence boiling point is lower.

         e.g. at 23.7 tom water boils at 25 ° C, while

         at 1489 torr water boils at 120 ° C  

• Water bolls at lower temperature on Murree hills and Mount Everest

On Murree hills, external atmospheric pressure is around about 700 mm of Hg, which is lower than nom Hence, water boils at a lower temperature of 98 ° C. At further higher altitudes, on Mount Everest, external atmospheric pressure is further decreased up to 323 mm of Hg; hence water boils just at 69 ° C.

Applications of Variation of Boiling Point with Pressure

The variation of the boiling point of a liquid with pressure has many applications e.g. Pressure cooking, vacuum distillation etc.

(1)  Pressure Cooking Principle

It is based on the fact that an increase in external pressure increases the boiling point of a liquid. Importance: At higher altitudes, atmospheric pressure is lower than normal (1 atm). Thus water boils at lower temperatures e.g. it bolls at 98 °C at Murree hills (P = 0.92 atm). Thus cooking takes more time.

The boiling point of water can be raised in a pressure cooker. It is a closed container in which vapours are allowed to escape. Consequently, they develop more pressure on the water. Hence, the boiling point of wat increased. This helps in cooking the meat & vegetables quickly even at high altitudes.

(2) Vacuum Distillation

The distillation carried out under reduced pressure is called vacuum distillation.

It is based on the fact that a decrease in pressure decreases the boiling point of a liquid.


It allows the distillation of liquids without decomposition. Some high boiling liquids cannot be separated by distillation at normal pressure (i.e.1 atm), since they decompose before reaching their boiling points. e.g. glycerine boils at 290 ° C but decomposes before reaching its boiling point. However, when the external pressure is reduced to 50 torrs in a vacuum distillation apparatus, it boils at 210 ° C and therefore can be distilled without decomposition.

It decreases the time for distillation.

It takes less time so it requires less fuel and is therefore economical.

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