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When a liquid or solid material is in thermodynamic equilibrium with its vapor, the pressure of the latter, called the vapor pressure, is a function of the temperature of the liquid/solid-vapor interface. In its simplest form the pressure is given by the Clausius-Clapeyron law

, (1)

where is the latent heat of fusion (for solids), or the latent heat of vaporization (for liquids), the Boltzmann constant, and the absolute temperature, measured in .

Equation (1) can be rewritten as

,

where and are material- (and phase-) dependent constants. The constants and used in this Demonstration are taken from an article by C. B. Alcock, V. P. Itkin, and M. K. Horrigan. They reproduce the observed pressures, reported in the 2003 CRC Handbook, to an accuracy of or better (see the Wikipedia vapor pressure data page).

The number density of the vapor is given by

.

References:

C. B. Alcock, V. P. Itkin, and M. K. Horrigan, "Vapor Pressure of the Metallic Elements," Canadian Metallurgical Quarterly, 23, 1984 pp. 309–313.

D. Lide, ed., CRC Handbook of Chemistry and Physics, 84th ed., Boca Raton, FL: CRC, 2003.