The
vapor pressure is the
pressure (if the vapor is mixed with other gases, the
partial pressure) of a
vapor.
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 saturated vapor pressure. A substance with a high vapor pressure at normal temperatures is often referred to as
volatile.
Liquids
When the partial pressure of any liquid
equals its vapor pressure, the liquid is partially vaporized: liquid and vapor are in
equilibrium.
Given a constant temperature, if the pressure is reduced, the equilibrium is changed in favour of the substance's gas phase: The liquid eventually gets totally vaporized. If pressure is increased, the opposite occurs: Eventually, all vapor will
condense to liquid.
With constant pressure but variable temperature, even lower temperatures will cause all vapor to condense to liquid, while a continual increase in temperature will cause the liquid to wholly
evaporate (turn to vapor).
At any given pressure, the
boiling point of a substance is the temperature at which the vapor pressure of the substance
in liquid form equals the total ambient pressure.
The processes of condensation and evaporation can be delayed, which is referred to as
supersaturation and
superheating, respectively.
Solids
When the ambient pressure equals the vapor pressure of any solid, the solid and vapor are in equilibrium. Below that temperature, vapor will condense to solid; above that temperature, solid will sublime (turn to vapor). Thus, at any given pressure, the sublimation point of a substance is the temperature at which the vapor pressure of the substance
in solid form equals the ambient pressure.
Relation
It may be noted that the vapor pressure of a substance in liquid form may be (and, in general, usually
is) 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
The boiling temperature of
water for pressures around 100
kPa can be approximated by
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
Category:Chemical properties
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