How are vapor pressure and boiling point related?

1 Answer

The boiling point decreases as the vapour pressure increases.


Vapour Pressure

Some of the molecules at the surface of a liquid have enough kinetic energy to escape into the atmosphere.

These molecules exert a pressure on the walls of a closed container.

The vapour pressure is the pressure exerted when the molecules leave the surface at the same rate as they return.

If the intermolecular forces in a liquid are small, the molecules can easily escape from the surface of the liquid.

The liquid will have a high vapour pressure.

Boiling Point

The normal boiling point is the temperature in which the vapour pressure of a liquid becomes equal to atmospheric pressure.

If the intermolecular forces are small, the liquid has a high vapour pressure.

Little heat energy will have to be added to separate the molecules, so the boiling point will be low.

Conversely, if there are strong intermolecular forces, the molecules will be strongly attracted to each other.

Few molecules will enter the gas phase, and the vapour pressure will be low.

More heat will be required to separate the molecules, so the boiling point will be higher.