Vapour Pressure
The pressure exerted by a vapour in dynamic equilibrium with its liquid or solid at a given temperature.
Formula / equation
ln p = −ΔH_vap / RT + C
Unit
Pa
In depth
Vapour pressure measures a liquid's tendency to evaporate and depends only on temperature and the substance's intermolecular forces, never on the quantity present or the container size. It rises exponentially with temperature, as described by the Clausius–Clapeyron equation.
Examples in the real world
Water is 3.17 kPa at 25 °C but 101.3 kPa at 100 °C; diethyl ether reaches 58 kPa at 20 °C and evaporates rapidly.