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Evaporation

Evaporation is one of the two forms of vaporization.

It is the process whereby atoms or molecules in a liquid state (or solid state if the substance sublimes) gain sufficient energy to enter the gaseous state.

It is the opposite process of condensation.

The thermal motion of a molecule of liquid must be sufficient to overcome the surface tension and evaporate, that is, its kinetic energy must exceed the work function of cohesion at the surface.

Evaporation therefore, proceeds more quickly at higher temperature, at higher flow rates between the gaseous and liquid phase and in liquids with lower surface tension (i.e. higher vapor pressure).

Since only a small proportion of the molecules are located near the surface and are moving in the proper direction to escape at any given instant, the rate of evaporation is limited.

Also, as the faster-moving molecules escape, the remaining molecules have lower average kinetic energy, and the temperature of the liquid thus decreases.

This phenomenon is also called evaporative cooling.

This is the reason that evaporating sweat cools the human body.

Note:   The above text is excerpted from the Wikipedia article "Evaporation", which has been released under the GNU Free Documentation License.
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September 5, 2015

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