The Clausius-Clapeyron equation describes how the vapour pressure of a pure substance changes with temperature during a phase change. It has the form
where
is the vapour pressure,
is the enthalpy of vaporisation or enthalpy of sublimation,
is the universal gas constant,
is the temperature in Kelvin,
We can derive the Clausius-Clapeyron equation from the van’t Hoff equation , where for phase transitions such as
or
, the activity of the pure liquid or solid phase in the equilibrium constant
is taken as 1. This simplifies the expression and leads to eq 26.