Vapor pressure is a measure of the tendency of a material to change into the gaseous or vapour state, and it increases with temperature. As liquids change physically into a gas (as during a temperature rise), their molecules travel with greater velocity, and some break out of the liquid to form a vapour above the liquid. These molecules create a vapour pressure, which (at a specified temperature) is the only pressure at which a pure liquid and its vapor can exist in equilibrium.
If, in a closed liquid–vapor system, the volume is reduced at constant temperature, the pressure will increase imperceptibly until condensation of part of the vapor into liquid has lowered the pressure to the original vapour pressure corresponding to the temperature. Conversely, increasing the volume at constant temperature will reduce the pressure imperceptibly, and molecules will move from the liquid phase to the vapor phase until the original vapour pressure has been restored. Temperatures and vapour pressures for a given gas always move together.
The temperature corresponding to any given vapour pressure is obviously the boiling point of the liquid and also the dew point of the vapor. Addition of heat will cause the liquid to boil, and removal of heat will start condensation. The terms saturation temperature, boiling point, and dew point all mean the same physical temperature at a given vapor pressure. Their use depends on the context in which they appear.
The vapour pressure that a single component in a mixture contributes to the total pressure in the system is called partial pressure. For example, air at sea level, and saturated with water vapor at 20 °C, has partial pressures of about 2.3 kPa of water, 78 kPa of nitrogen, 21 kPa of oxygen and 0.9 kPa of argon, totaling 102.2 kPa, making the basis for standard atmospheric pressure. Vapour pressure is measured in the standard units of pressure. The International System of Units (SI) recognizes pressure as a derived unit with the dimension of force per area and designates the pascal (Pa) as its standard unit.
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