Page 22 - Partition & Adsorption of Organic Contaminants in Environmental Systems

VAPOR–LIQUID AND VAPOR–SOLID EQUILIBRIA 13

pressure (P) of the supercooled liquid. The heat of fusion of a solid compound
can be obtained with the relation DH fus =DH sub -DH evap.
The observation that DH evap (or DH sub) is not very sensitive to temperature
below the normal boiling point of a substance may be understood on the basis
that the powerful molecular forces that keep the substance in a liquid or solid
state are not significantly affected by the thermal energy of the molecules
below the boiling point. As the temperature increases significantly toward the
critical point of the substance (at which the properties of the gas and the liquid
coalesce to form a single phase), DH evap will then decrease very sharply and
become zero at the critical temperature.