to an extent in the gas phase (as is the case with hydrogen fluoride), and so the calculated value of the bond strength will be too low. This is particularly true of metals, which often form covalently bonded molecules in the gas phase: in these cases, the enthalpy of atomization must be used to obtain a true value of the bond energy. An alternative description is to view the enthalpy of condensation as the heat which must be released to the surroundings to compensate for the drop in entropy when a gas condenses to a liquid. As the liquid and gas are in equilibrium at the boiling point (Tb), ΔvG = 0, which leads to: As neither entropy nor enthalpy vary greatly with temperature, it is normal to use the tabulated standard values without any correction for the difference in temperature from 298 K. A correction must be made if the pressure is different from 100 kPa, as the entropy of a gas is proportional to its pressure (or, more precisely, to its fugacity): the entropies of liquids vary little with pressure, as the compressibility of a liquid is small. These two definitions are equivalent: the boiling point is the temperature at which the increased entropy of the gas phase overcomes the intermolecular forces. As a given quantity of matter always has a higher entropy in the gas phase than in a condensed phase ( is always positive), and from , the Gibbs free energy change falls with increasing temperature: gases are favored at higher temperatures, as is observed in practice. Vaporization enthalpy of electrolyte solutions Estimation of the enthalpy of vaporization of electrolyte solutions can be simply carried out using equations based on the chemical thermodynamic models, such as Pitzer model or TCPC model. Selected values Elements The vaporization of metals is a key step in metal vapor synthesis, which exploits the increased reactivity of metal atoms or small particles relative to the bulk elements. Other common substances Enthalpies of vaporization of common substances, measured at their respective standard boiling points: See also Clausius–Clapeyron relation Shimansky equation, describes the temperature dependence of the heat of vaporization Enthalpy of fusion, specific heat of melting Enthalpy of sublimation Joback method, estimation of the heat of vaporization at the normal boiling point from molecular structures Latent heat References CODATA Key Values for Thermodynamics NIST Chemistry WebBook Enthalpy