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(Also called vaporization.) The physical process by which a liquid or solid is transformed to the gaseous state; the opposite of condensation.

Evaporation is usually restricted in use to the change of water from liquid to gas, while sublimation is used for the change from solid to gas. According to the kinetic theory of gases, evaporation occurs when liquid molecules escape into the vapor phase as a result of the chance acquisition of above-average, outward-directed, translational velocities at a time when they happen to lie within about one mean free path below the effective liquid surface. It is conventionally stated that evaporation into a gas ceases when the gas reaches saturation. In reality, net evaporation does cease, but only because the numbers of molecules escaping from and returning to the liquid are equal, that is, evaporation is counteracted by condensation. Because the molecules that escape the condensed phase have above-average energies, those left behind have below-average energies, which is manifested by a decrease in temperature of the condensed phase (unless compensated for by energy transfer from the surroundings).
See also evapotranspiration.

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