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.
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. |
= C = O = H |
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ethyl ether (C4H10O) Pvapor (25oC) = 520 torr The relatively weak dipole-dipole forces and London dispersion forces between molecules results in a much higher vapor pressure compared to ethyl alcohol. |
ethyl alcohol (C2H6O) Pvapor (25oC) = 75 torr Although dipole-dipole forces and London dispersion forces also exist between ethyl alcohol molecules, the strong hydrogen bonding interactions are responsible for the much lower vapor pressure compared to ethyl ether. |
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. |