The enthalpy of fusion ($\Delta \bar{H}_{fus}$), is the heat required to convert one mole of solid to liquid.

Enthalpy of vaporization ($\Delta \bar{H}_{vap}$), is the heat required to change one mole of liquid to gas.

Enthalpy of sublimation ($\Delta \bar{H}_{sub}$), is the heat required to change one mole of solid to gas.

At the triple point the relation is fulfilled: $\Delta H_{sub}=\Delta H_{fus}+\Delta H_{ebu}$

The enthalpy of vaporization is a measure of the intermolecular interactions within the liquid. It is actually the difference between the intermolecular forces in the gas phase minus those that exist in the liquid phase. The enthalpy of vaporization decreases with increasing temperature and vanishes at the critical point, since liquid and gas are the same.

Trouton's rule relates the enthalpy and entropy of vaporization at the normal boiling point. \begin{equation} \Delta \bar{S}_{vap,pen}=\frac{\Delta\bar{H}_{vap,pen}}{T_{pen}}\approx 10.5\;R \end{equation} In highly polar liquids (water, alcohols) that form hydrogen bonds, the expression is used: \begin{equation} \Delta\bar{S}_{vap,pen}=\approx 4.5\ ;R+Rln(T_{pen}/K) \end{equation}