View source for Evaporation

{{short description|Vaporization of a liquid from its surface}}
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[[File:Watervapor cup.jpg|thumb|right|[[Aerosol]] of microscopic water droplets suspended in the air above a cup of hot tea after the water vapor has sufficiently cooled and condensed. Water vapor is an invisible gas, but the clouds of condensed droplets refract and scatter the sunlight and are thus visible.]]
[[File:Condensation droplets.jpg|thumb|Droplets of water vapor in a pan.]]
[[File:10. Ладење при испарување.ogv|thumb|right|280px|Demonstration of evaporative cooling. When the sensor is dipped in [[ethanol]] and then taken out to evaporate, the instrument shows progressively lower temperature as the ethanol evaporates.]]
[[File:Misty road after rain.jpg|thumb|Rain evaporating after falling on hot pavement]]
'''Evaporation''' is a type of [[vaporization]] that occurs on the [[Interface (chemistry)|surface]] of a [[liquid]] as it changes into the gas phase.<ref>{{Cite web|url=http://www.dictionary.com/browse/evaporate|title=the definition of evaporate|website=Dictionary.com|access-date=2018-01-23}}</ref> A high [[concentration]] of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when [[humidity]] affects rate of evaporation of water.<ref>{{Cite web |title=Why Does Humidity & Wind Speed Affect Evaporation? |url=https://sciencing.com/humidity-wind-speed-affect-evaporation-12017079.html |access-date=2022-08-20 |website=Sciencing |language=en}}</ref> When the molecules of the liquid collide, they transfer energy to each other based on how they collide. When a molecule near the surface absorbs enough energy to overcome the [[vapor pressure]], it will escape and enter the surrounding air as a gas.<ref>{{Cite book|url=https://en.wikisource.org/wiki/The_New_Student's_Reference_Work/Evaporation|title=The New Student's Reference Work (1914)|chapter=Evaporation |year=1914|pages=636}}</ref> When evaporation occurs, the energy removed from the vaporized liquid will reduce the temperature of the liquid, resulting in evaporative cooling.<ref>{{Cite news|url=https://www.scientificamerican.com/article/chilling-science-evaporative-cooling-with-liquids/|title=Chilling Science: Evaporative Cooling with Liquids|last=Lohner|first=Science Buddies, Svenja|work=Scientific American|access-date=2018-01-23|language=en}}</ref>
On average, only a fraction of the molecules in a liquid have enough heat energy to escape from the liquid. The evaporation will continue until an equilibrium is reached when the evaporation of the liquid is equal to its condensation. In an enclosed environment, a liquid will evaporate until the surrounding air is saturated.
Evaporation is an essential part of the [[water cycle]]. The sun (solar energy) drives evaporation of water from [[ocean]]s, lakes, [[soil moisture|moisture]] in the soil, and other sources of water. In [[hydrology]], evaporation and [[transpiration]] (which involves evaporation within plant [[stomata]]) are collectively termed [[evapotranspiration]]. Evaporation of water occurs when the surface of the liquid is exposed, allowing molecules to escape and form water vapor; this vapor can then rise up and form clouds. With sufficient energy, the liquid will turn into vapor.
== Theory ==
{{See also|Kinetic theory of gases}}
For [[molecule]]s of a liquid to evaporate, they must be located near the surface, they have to be moving in the proper direction, and have sufficient [[kinetic energy]] to overcome liquid-phase [[intermolecular force]]s.<ref name="Silberberg">{{cite book |first=Martin A. |last=Silberberg |title=Chemistry |url=https://archive.org/details/studentsolutions00mart |url-access=registration |edition=4th |pages=[https://archive.org/details/studentsolutions00mart/page/n435 431]–434 |publisher=McGraw-Hill |location=New York |year=2006 |isbn=0-07-296439-1}}</ref> When only a small proportion of the molecules meet these criteria, the rate of evaporation is low. 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 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, [[atmospheric pressure]] (determines the percent 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 cannot be seen.<ref>{{Cite journal | last1 = Gusarov | first1 = A. V. | last2 = Smurov | first2 = I. | doi = 10.1063/1.1516211 | title = Gas-dynamic boundary conditions of evaporation and condensation: Numerical analysis of the Knudsen layer | journal = Physics of Fluids | volume = 14 | issue = 12 | pages = 4242 | year = 2002 |bibcode = 2002PhFl...14.4242G }}</ref>
Liquids that do not evaporate visibly at a given temperature in a given gas (e.g., cooking oil at room [[temperature]]) have molecules that do not tend to transfer energy to each other in a pattern sufficient to frequently give a molecule the heat energy necessary to turn into vapor. However, these liquids ''are'' evaporating. It is just that the process is much slower and thus significantly less visible.
=== Evaporative equilibrium ===
[[Image:Water vapor pressure graph.svg|thumb|240px|right|Vapor pressure of water vs. temperature. 760&nbsp;[[Torr]] = 1&nbsp;[[Atmosphere (unit)|atm]].]]
If evaporation takes place in an enclosed area, 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_{\rm  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 ==
Note: Air is used here as a common example of the surrounding gas; however, other gases may hold that role.
;[[Concentration]] of the substance evaporating in the air: If the air already has a high concentration of the substance evaporating, then the given substance will evaporate more slowly.
;Flow rate of air: This is in part related to the concentration points above. If "fresh" air (i.e., air which is neither already saturated with the substance nor with other substances) is moving over the substance all the time, then the concentration of the substance in the air is less likely to go up with time, thus encouraging faster evaporation. This is the result of the [[boundary layer]] at the evaporation surface decreasing with flow velocity, decreasing the diffusion distance in the stagnant layer.
;The amount of minerals dissolved in the liquid
;Inter-molecular forces: The stronger the forces keeping the molecules together in the liquid state, the more energy one must get to escape. This is characterized by the [[enthalpy of vaporization]].
;[[Pressure]]: Evaporation happens faster if there is less exertion on the surface keeping the molecules from launching themselves.
;[[Surface area]]: A substance that has a larger surface area will evaporate faster, as there are more surface molecules per unit of volume that are potentially able to escape.
;[[Temperature]] of the substance: the higher the temperature of the substance the greater the kinetic energy of the molecules at its surface and therefore the faster the rate of their evaporation.
;Photomolecular effect: The amount of light will affect the evaporation. When photons hits the surface area of the liquid they can make individual molecules break free and disappear into the air without any need for additional heat.<ref>[https://news.mit.edu/2024/how-light-can-vaporize-water-without-heat-0423 How light can vaporize water without the need for heat]</ref>
In the US, the National Weather Service measures, at various outdoor locations nationwide, the actual rate of evaporation from a [[Pan evaporation|standardized "pan"]] open water surface. Others do likewise around the world. The US data is collected and compiled into an annual evaporation map. The measurements range from under 30 to over {{convert|120|in|mm}} per year.
Because it typically takes place in a complex environment, where 'evaporation is an extremely rare event', the mechanism for the evaporation of water is not completely understood. Theoretical calculations require prohibitively long and large computer simulations. 'The rate of evaporation of liquid water is one of the principal uncertainties in modern climate modeling.'<ref name="naut-sayk">{{cite web |author1=Richard Saykally |title=Five Things We Still Don't Know About Water |url=https://nautil.us/issue/25/water/five-things-we-still-dont-know-about-water |website=Nautilus |access-date=20 October 2021 |date=11 June 2015}}</ref><ref>{{cite journal |author1=Kotaro Ohashi |title=Evaporation coefficient and condensation coefficient of vapor under high gas pressure conditions |journal=Scientific Reports |date=18 May 2020 |volume=10 |issue=8143 |page=8143 |doi=10.1038/s41598-020-64905-5 |publisher=Nature |pmid=32424295 |pmc=7235219 |bibcode=2020NatSR..10.8143O |language=English}}</ref>
==Thermodynamics==
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Evaporation is an [[endothermic process]], since heat is absorbed during evaporation.
== Applications ==
* Industrial applications include many [[printing]] and [[coating]] processes; recovering salts from solutions; and drying a variety of materials such as lumber, paper, cloth and chemicals.
* The use of evaporation to dry or concentrate samples is a common preparatory step for many laboratory analyses such as [[spectroscopy]] and [[chromatography]]. Systems used for this purpose include [[rotary evaporator]]s and [[centrifugal evaporator]]s.
* When clothes are hung on a laundry line, even though the ambient temperature is below the boiling point of water, water evaporates. This is accelerated by factors such as low [[humidity]], heat (from the sun), and wind. In a [[clothes dryer]], hot air is blown through the clothes, allowing water to evaporate very rapidly.
* The [[Matki (earthen pot)|matki/matka]], a traditional Indian porous clay container used for storing and cooling water and other liquids.
* The [[botijo]], a traditional Spanish porous clay container designed to cool the contained water by evaporation.
* [[Evaporative cooler]]s, which can significantly cool a building by simply blowing dry air over a filter saturated with water.
=== Combustion vaporization ===
Fuel [[droplet]]s vaporize as they receive heat by mixing with the hot gases in the combustion chamber. Heat (energy) can also be received by radiation from any hot refractory wall of the combustion chamber.
=== Pre-combustion vaporization ===
Internal combustion engines rely upon the vaporization of the fuel in the cylinders to form a fuel/air mixture in order to burn well.
The chemically correct air/fuel mixture for total burning of gasoline has been determined to be 15 parts air to one part gasoline or 15/1 by weight. Changing this to a volume ratio yields 8000 parts air to one part gasoline or 8,000/1 by volume.
=== Film deposition ===
{{Main|Evaporation (deposition)}}
[[Thin film]]s may be [[Thin-film deposition|deposited]] by evaporating a substance and condensing it onto a substrate, or by dissolving the substance in a solvent, spreading the resulting solution thinly over a substrate, and evaporating the solvent. The [[Hertz–Knudsen equation]] is often used to estimate the rate of evaporation in these instances.
== See also ==
{{NSRW Poster}}
* [[Atmometer]] (evaporimeter)
* [[Boiling point]]
* [[Cryophorus]]
* [[Crystallisation]]
* [[Desalination]]
* [[Distillation]]
* [[Drying]]
* [[Eddy covariance]] flux (a.k.a. eddy correlation, eddy flux)
* [[Evaporator]]
* [[Evapotranspiration]]
* [[Flash evaporation]]
* [[Heat of vaporization]]
* [[Hertz–Knudsen equation]]
* [[Hydrology (agriculture)]]
* [[Latent heat]]
* [[Latent heat flux]]
* [[Pan evaporation]]
* [[Sublimation (phase transition)]] (phase transfer from solid directly to gas)
* [[Transpiration]]
{{Table of phase transitions}}
== References ==
{{Reflist}}
==Further reading==
{{Refbegin}}
* {{cite book |title=Semiconductor Devices: Physics and Technology |first=Simon Min |last=Sze |date=25 September 2001 |publisher=Wiley |isbn=0-471-33372-7}} Has an especially detailed discussion of film deposition by evaporation.
{{Refend}}
==External links==
{{Wiktionary|evaporation}}
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[[Category:Atmospheric thermodynamics]]
[[Category:Meteorological phenomena]]
[[Category:Materials science]]
[[Category:Phase transitions]]
[[Category:Thin film deposition]]
[[Category:Gases]]