above their surfaces when placed in a closed container with space above them, even at temperatures well below the boiling point. Although at a constant temperature all of the liquid molecules have the same average kinetic energy, averages are made of low and high values. Molecules on the surface sometimes have enough KE to escape the pull of intermolecular forces and leave the liquid phase. Their collisions with the container walls generates the force we call vapor pressure.

Vapor pressure is related to the speed of evaporation at a given temperature and clearly both must be related to the strength of intermolecular forces. Vapor pressure decreases with increasing intermolecular forces. This means that substances with low vapor pressures will have high boiling points. And, in fact, the two are related mathematically by an unpleasant-looking formula. Suffice it to say that the normal boiling point of a liquid is defined as the temperature at which the vapor pressure equals one atmosphere.