Vapor pressure
Categories: Chemical properties
The vapor pressure is the pressure (if the vapor is mixed with other gases, the partial pressure) of a vapor(this vapour being formed from molecules/atoms escaping from a liquid/solid). At any given temperature, for a particular substance, there is a pressure at which the vapor of that substance is in equilibrium with its liquid or solid forms. This is the equilibrium vapor pressure or saturation vapor pressure of that substance at that temperature. The term vapor pressure is often understood to mean the saturation vapor pressure. A substance with a high vapor pressure at normal temperatures is often referred to as volatile.
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Equilibrium vapor pressure of solids
Equilibrium vapor pressure can be defined as the pressure reached when a condensed phase is in equilibrium with its own vapor. In the case of an equilibrium (i.e. crystalline) solid this can be defined as the pressure when the rate of sublimation of a solid matches the rate of deposition of its vapor phase. For most solids this pressure is very low, but some notable exceptions are naphthalene and ice. (Witness the phenomenon of ice disappearing even though the ambient temperature is below the freezing point of water.) All solid materials have a vapor pressure. However, due to their often extremely low values, measurement can be rather difficult. Typical techniques include the use of thermogravimetry and gas transpiration.
Relation between solid and liquid vapor pressures
It may be noted that the vapor pressure of a substance in liquid form is usually different from the vapor pressure of the same substance in solid form. If the temperature is such that the vapor pressure of the liquid is higher than that of the solid, liquid will vaporize but vapor will condense to a solid, i.e. the liquid is freezing. If the temperature is such that the vapor pressure of the liquid is lower than that of the solid, solid will vaporize but vapor will condense to a liquid, i.e. the solid is melting. At the temperature that equalizes the two vapor pressures, an equilibrium exists between solid and liquid phases. This temperature is referred to as the melting point.
Water vapor pressure
The boiling temperature of water for pressures around 100 kPa can be approximated by
<math>T_v = 100 + 0.0002772 \cdot (p - 101000) - 1.24 \cdot 10^{-9} \cdot (p - 101000)^2</math>
where the temperature is in degrees Celsius and the pressure p is in pascals. One gets the vapor pressure by solving this equation for p.
Raoult's law approximately governs the vapor pressure of mixtures of liquids.
See also
External links
ca:Pressió de vapor de:Dampfdruck es:Presión de vapor fr:Pression de vapeur ko:증기압 is:Gufuþrýstingur it:Pressione di vapore he:לחץ אדים nl:Dampdruk ja:蒸気圧 pt:Pressão de vapor fi:Höyrynpaine sv:Ångtryck