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Vapor pressure [a] or equilibrium vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. The equilibrium vapor pressure is an indication of a liquid's thermodynamic tendency to evaporate.
The boiling point of water is the temperature at which the saturated vapor pressure equals the ambient pressure. Water supercooled below its normal freezing point has a higher vapor pressure than that of ice at the same temperature and is, thus, unstable. Calculations of the (saturation) vapor pressure of water are commonly used in meteorology.
Water vapor has lower density than that of air and is therefore buoyant in air but has lower vapor pressure than that of air. When water vapor is used as a lifting gas by a thermal airship the water vapor is heated to form steam so that its vapor pressure is greater than the surrounding air pressure in order to maintain the shape of a ...
The relation of equilibrium vapor pressure to the saturation vapor pressure can be thought of as a relative humidity measurement for the atmosphere. As P v /P sat increases, vapor will continue to condense inside a given capillary. If P v /P sat decreases, liquid will begin to evaporate into the atmosphere as vapor molecules. [2]
In a scientific notion, the relative humidity (or ) of an air-water mixture is defined as the ratio of the partial pressure of water vapor in air to the saturation vapor pressure of water at the same temperature, usually expressed as a percentage: [11] [12] [5] = % /
The definition of a w is where p is the partial water vapor pressure in equilibrium with the solution, and p* is the (partial) vapor pressure of pure water at the same temperature. An alternate definition can be a w ≡ l w x w {\displaystyle a_{w}\equiv l_{w}x_{w}} where l w is the activity coefficient of water and x w is the mole fraction of ...
A vapor can exist in equilibrium with a liquid (or solid), in which case the gas pressure equals the vapor pressure of the liquid (or solid). A supercritical fluid (SCF) is a gas whose temperature and pressure are above the critical temperature and critical pressure respectively. In this state, the distinction between liquid and gas disappears.
In a plot that compares pressure and specific volume (commonly called a p-v diagram), compressed fluid is the state to the left of the saturation curve. Conditions that cause a fluid to be compressed include: Specific volume and enthalpy inferior to that of a saturated liquid; Temperature below the saturation temperature;