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[[Image:watervapor cup.jpg|thumb|right|पानीको बाफ; हावा जस्तै भएर नदेखिने भए पनि घामका किरणका ठक्करका कारण देखिन्छ ।]]
==सिद्धान्त ==
== Theory ==
{{See also|Kinetic theory}}
For [[molecule]]s of a liquid to evaporate, they must be located near the surface, be moving in the proper direction, and have sufficient [[kinetic energy]] to overcome liquid-phase intermolecular forces.<ref name="Silberberg">{{cite book |first=Martin A. |last=Silberberg |title=Chemistry |edition=4th edition |pages=431–434 |publisher=McGraw-Hill |location=New York |year=2006 |isbn=0-07-296439-1}}</ref> Only a small proportion of the molecules meet these criteria, so the rate of evaporation is limited. Since the kinetic energy of a molecule is proportional to its temperature, evaporation proceeds more quickly at higher temperatures. 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 why evaporating [[sweat]] cools the human body.
Evaporation also tends to proceed more quickly with higher flow rates between the gaseous and liquid phase and in liquids with higher [[vapor pressure]]. For example, laundry on a clothes line will dry (by evaporation) more rapidly on a windy day than on a still day. Three key parts to evaporation are heat, humidity, and air movement.
On a molecular level, there is no strict boundary between the liquid state and the vapor state. Instead, there is a [[Knudsen layer]], where the phase is undetermined. Because this layer is only a few molecules thick, at a macroscopic scale a clear phase transition interface can be seen.
=== Evaporative equilibrium ===
[[Image:Water vapor pressure graph.jpg|thumb|240px|right|Vapor pressure of water vs. temperature. 760 [[Torr]] = १ [[Atmosphere (unit)|atm]].]]
If evaporation takes place in a closed vessel, the escaping molecules accumulate as a [[vapor]] above the liquid. Many of the [[molecules]] return to the liquid, with returning molecules becoming more frequent as the [[density]] and [[pressure]] of the vapor increases. When the process of escape and return reaches an [[Thermodynamic equilibrium|equilibrium]],<ref name="Silberberg" /> the vapor is said to be "saturated," and no further change in either [[vapor pressure]] and density or liquid temperature will occur. For a system consisting of vapor and liquid of a pure substance, this equilibrium state is directly related to the vapor pressure of the substance, as given by the [[Clausius-Clapeyron relation]]:
: <math>\ln \left( \frac{ P_2 }{ P_1 } \right) = - \frac{ \Delta H_{ vap } }{ R } \left( \frac{ 1 }{ T_2 } - \frac{ 1 }{ T_1 } \right)</math>
<!-- ## Original Equation ## ## DO NOT DELETE UNLESS THE ABOVE EQUATION IS VERIFIED TO BE CORRECT ## [[Natural logarithm|ln]] P<sub>2</sub>/P<sub>1</sub> = −[[standard enthalpy change of vaporization|ΔH<sub>vap</sub>]]/[[Universal gas constant|R]]((1/T<sub>2</sub>)-(1/T<sub>1</sub>)) -->
where ''P''<sub>1</sub>, ''P''<sub>2</sub> are the vapor pressures at temperatures ''T''<sub>1</sub>, ''T''<sub>2</sub> respectively, Δ''H''<sub>vap</sub> is the [[enthalpy of vaporization]], and ''R'' is the [[universal gas constant]]. The rate of evaporation in an open system is related to the vapor pressure found in a closed system. If a liquid is heated, when the vapor pressure reaches the ambient pressure the liquid will [[boiling|boil]].
The ability for a molecule of a liquid to evaporate is based largely on the amount of [[kinetic energy]] an individual particle may possess. Even at lower temperatures, individual molecules of a liquid can evaporate if they have more than the minimum amount of kinetic energy required for vaporization.
== Factors influencing the rate of evaporation ==
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