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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 saturated vapor pressure of that substance at that temperature. The term vapor pressure is often understood to mean saturated vapor pressure.

1 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, ever 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.

2 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.

3 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.

4 Water

The boiling temperature of waterDrinking water This article focuses on water as we experience it every day. The water (molecule) article describes water from a scientific and technical perspective. Water is an abundant substance on Earth. It exists in many forms, such as sea, rain, and for pressures around 100 kKilo (symbol: k is a prefix in the SI system denoting 103 or 1,000. For example: kilogram is 1000 grams kilometre is 1000 metres kilowatt is 1000 watts kilojoule is 1000 joules Adopted in 1795, it comes from the Greek , meaning thousand''. Kilo" is often Pa can be approximated by

where the temperature is in degrees CelsiusThe degree Celsius (°C) is a unit of temperature named after the Swedish astronomer Anders Celsius ( 1701 1744), who first proposed it in 1742. The Celsius temperature scale was designed so that the freezing point of water is 0 degrees, and the boiling po and the pressure p is in Pascals. One gets the vapor pressure by solving this equation for p.

See also Relative humidityRelative humidity is the ratio of the current vapor pressure of water in any gas (especially air) to the vapor pressure at which the gas would become saturated at the current temperature, normally expressed as a percentage. Equivalently, it is the ratio o, absolute humidityAbsolute humidity is the mass of water vapor in a given volume of air or gas, usually measured in grams per cubic meter. It may also be measured as the partial pressure of the water vapor. See also relative humidity External link Psychrometrics..