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Vapor Pressure

The Macroscopic View

The vapor pressure of a liquid is the equilibrium pressure of a vapor above its liquid (or solid); that is, the pressure of the vapor resulting from evaporation of a liquid (or solid) above a sample of the liquid (or solid) in a closed container. Examples:

substance vapor pressure at 25oC
diethyl ether 0.7 atm
bromine 0.3 atm
ethyl alcohol 0.08 atm
water 0.03 atm

The vapor pressure of a liquid varies with its temperature, as the following graph shows for water. The line on the graph shows the boiling temperature for water.

Graph of vapor pressure vs temperature for water

As the temperature of a liquid or solid increases its vapor pressure also increases. Conversely, vapor pressure decreases as the temperature decreases.

The vapor pressure of a liquid can be measured in a variety of ways. A simple measurement involves injecting a little of the liquid into a closed flask connected to a manometer. Click here for an illustration.

The Microscopic View

Microscopic equilibrium between gas and liquid. Note that the rate of evaporation of the liquid is equal to the rate of condensation of the gas. Microscopic equilibrium between gas and solid. Note that the rate of evaporation of the solid is equal to the rate of condensation of the gas.

Factors That Affect Vapor Pressure

Microscopic equilibrium between gas and liquid at low temperature. Note the small number of particles in the gas. Microscopic equilibrium between gas and liquid at high temperature. Note the large number of particles in the gas.

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