Vapor pressure is the pressure exerted by a liquid in equilibrium with its pure liquid phase at a given temperature.

1. The vapor pressure of a liquid is dependent only upon the nature of the liquid and the temperature.
2. Different liquids at any temperature have different vapor pressures.
3. The vapor pressure of every liquid increases as the temperature is raised.

The normal boiling point of any pure substance is the temperature at which the vapor pressure of that substance is equal to 1 atmosphere (760 mm Hg).

                                                                A                                B

In container A, the liquid is evaporating.  Some of the molecules have enough kinetic energy to escape (turn to a gas) by pushing against the pressure of the atmosphere.  Container B shows the flask is saturated.  When new molecules of liquid are vaporized, the gas cannot hold additional molecules, therefore some of the molecules condense back to liquid.

Vapor Pressure of Water at Selected Temperatures

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