Vapor

In physics, a vapor (American English) or vapour (English English and Canadian English; see spelling differences) is a substance in the gas phase at a temperature lower than its critical temperature,[1] which means that the vapor can be condensed to a liquid by increasing the pressure on it without reducing the temperature. A vapor is different from an aerosol.[2] An aerosol is a suspension of tiny particles of liquid, solid, or both within a gas.[2]

An ampule of nitrogen oxide vapor: brown nitrogen dioxide and colourless dinitrogen tetroxide, in equilibrium

For example, water has a critical temperature of 647 K (374 °C; 705 °F), which is the highest temperature at which liquid water can exist. In the atmosphere at ordinary temperatures, therefore, gaseous water (known as water vapor) will condense into a liquid if its partial pressure is increased sufficiently.

A vapor may co-exist with a liquid (or a solid). When this is true, the two phases will be in equilibrium, and the gas-partial pressure will be equal to the equilibrium vapor pressure of the liquid (or solid).[1]


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