Clausius-Clapeyron equation

Vapour pressure vs. temperature, when one of the two phases is a gas:

$$\frac{d\ln P}{dT}=\frac{\Delta H_{\text{vap}}}{R{T}^2}$$

where:

• $P$: saturation vapour pressure,
• $T$: temperature (K),
• $\Delta H_{\text{vap}}$: molar enthalpy of vaporisation (of sublimation, for solid to gas),
• $R$: universal gas constant.

General form for any two phases (Clapeyron equation), with $\Delta V$ the molar volume change:

$$\frac{dP}{dT}=\frac{\Delta H}{T\Delta V}$$