The melting and boiling points of water are much higher than predicted based on criteria such as molar mass (number of electrons available for temporary dipole interactions) or extrapolation from available data on related substances (H2S, H2Se).
The entropy of vaporization can be an indicator of intermolecular forces in liquids, particularly hydrogen bonding. Trouton's rule may be used to indicate the absence of strong intermolecular bonds in the liquid phase. Substances with little intermolecular interactions in the liquid have an entropy of vaporization of 88 J/mol-K. A more ordered state in the liquid, which results from hydrogen bonding or large dipole-dipole interactions, results in larger values for the entropy of vaporization because the order in the liquid must be randomized in addition to forming a mole of gas from a liquid.
When a new compound is prepared, vapor pressure measurements over a range of temperatures are used to determine heat of vaporization using the Clausius-Clapeyron equation. The entropy of vaporization is determined from the heat of vaporization and the boiling point.
If a compound follows Trouton's rule, the compound has little intermolecular interactions in the liquid state.
Deviations from Trouton's rule are not as conclusive. Compounds with very low boiling points such as He and H2 exhibit very low entropies of vaporization which must be corrected for the difference in volume of the gas at these temperatures. These calculations become more complex. Other factors may also prove important is special cases.
Water exhibits an entropy of vaporization of 109 J/mol-K. This suggests an ordered liquid state which is expected in those liquids where hydrogen bonding can occur.
