Water

Vapor

pressure

Air

pressure

Pressure – boiling point relationship

The Boiling Point of Water

As energy is transferred from a heating device to the water, the kinetic energy of the water molecules increases and the molecules become more mobile. If the kinetic energy is large enough to break the intermolecular forces of attraction between water molecules in the liquid phase then the molecules can escape in the form of water vapor. This is indicated by bubbles forming near the bottom of the container of water, nearest to the heat source. These gaseous water molecules exert a force on the atmosphere, called the vapor pressure. The vapor pressure is opposed by another force, created by a column of air pushing down on the surface of the water. This pressure is the atmospheric pressure.

The atmospheric pressure will initially squash the pressure of the water vapor causing the bubbles to burst, but as more energy is provided the pressure exerted by the bubbles will exceed the atmospheric pressure and the bubbles of water vapor will escape the surface of the liquid. A diagram of these forces is provided. Water begins to boil. The temperature at which water boils is related to the vapor pressure required for boiling, which is equal to the atmospheric pressure. The implication of this is that as the atmospheric pressure changes, the boiling point of water changes as well. When you go up a mountain, the air pressure is lower (the column of air pushing downward is less) and water boils at a lower temperature.

Water

Vapor

pressure

Air

pressure

Pressure – boiling point relationship

Water

Vapor

pressure

Air

pressure

Pressure – boiling point relationship

Water

Vapor

pressure

Air

pressure

Pressure – boiling point relationship