Editing Boiling

{{short description|Physical process}}
[[Image:Boilingkettle.jpg|thumb|right|Rolling boil of water in an electric kettle]]
{{Use dmy dates|date=May 2023}}
{{Use British English|date=May 2023}}

'''Boiling''' or '''ebullition''' is the rapid [[phase transition]] from [[liquid]] to [[gas]] or [[vapor|vapour]]; the reverse of boiling is [[condensation]]. Boiling occurs when a liquid is heated to its [[boiling point]], so that the [[vapour pressure]] of the liquid is equal to the pressure exerted on the liquid by the [[Standard atmosphere (unit)|surrounding atmosphere]]. Boiling and [[evaporation]] are the two main forms of liquid [[vapourization]].

There are two main types of boiling: [[nucleate boiling]], where small bubbles of vapour form at discrete points; and [[critical heat flux]] boiling, where the boiling surface is heated above a certain critical temperature and a film of vapour forms on the surface. [[Transition boiling]] is an intermediate, unstable form of boiling with elements of both types. The boiling point of water is 100&nbsp;°C or 212&nbsp;°F but is lower with the decreased atmospheric pressure found at higher altitudes.

Boiling water is used as a method of making it potable by killing [[Microorganism|microbes]] and [[Virus|viruses]] that may be present. The sensitivity of different micro-organisms to heat varies, but if water is held at {{convert|100|C}} for one minute, most micro-organisms and viruses are inactivated. Ten minutes at a temperature of 70&nbsp;°C (158&nbsp;°F) is also sufficient to inactivate most [[bacteria]].<ref>{{cite report |access-date = 2023-05-16 |publisher = World Health Organization |url=https://www.who.int/publications/i/item/WHO-FWC-WSH-15.02 |title=Boil Water - Technical Brief WHO/FWC/WSH/15.02}}</ref>

Boiling water is also used in several [[cooking]] methods including boiling, [[Blanching (cooking)|blanching]], [[steaming]], and [[Poaching (cooking)|poaching]].

== Types ==

=== Free convection ===

The lowest heat flux seen in boiling is only sufficient to cause [[Convection|natural convection]], where the warmer fluid rises due to its slightly lower [[density]]. This condition occurs only when the superheat is very low, meaning that the hot surface near the fluid is nearly the same temperature as the boiling point.

=== Nucleate ===
[[File:Boiling water - close up (short shutter speed).ogv|thumb|A video showing water being boiled. As boiling proceeds, more nucleation sites (where bubbles are formed) can be seen.]]
[[Nucleate boiling]] is characterised by the growth of bubbles or pops on a heated surface (heterogeneous nucleation), which rises from discrete points on a surface, whose temperature is only slightly above the temperature of the liquid. In general, the number of [[nucleation]] sites is increased by an increasing surface temperature.

An irregular surface of the boiling vessel (i.e., increased surface roughness) or additives to the fluid (i.e., surfactants and/or [[nanoparticle]]s) facilitate nucleate boiling over a broader temperature range,<ref>{{Cite journal|last1=Doretti|first1=L.|last2=Longo|first2=G. A.|last3=Mancin|first3=S.|last4=Righetti|first4=G.|last5=Weibel|first5=J. A.|date=2017|title=Nanoparticle Deposition During Cu-Water Nanofluid Pool Boiling|journal=Journal of Physics: Conference Series|language=en|volume=923|issue=1|pages=012004|doi=10.1088/1742-6596/923/1/012004|issn=1742-6596|bibcode=2017JPhCS.923a2004D|doi-access=free}}</ref><ref name="sciencedirect.com">{{cite journal|title= Pool boiling of nanofluids: Comprehensive review of existing data and limited new data|journal= International Journal of Heat and Mass Transfer|volume= 52|issue= 23–24|pages= 5339–5347|doi= 10.1016/j.ijheatmasstransfer.2009.06.040|year= 2009|last1= Taylor|first1= Robert A.|last2= Phelan|first2= Patrick E.|bibcode= 2009IJHMT..52.5339T}}</ref><ref>Robert A Taylor, Patrick E Phelan, Todd Otanicar, Ronald J Adrian, Ravi S Prasher, ''[http://digitalcommons.lmu.edu/cgi/viewcontent.cgi?article=1017&context=mech_fac Vapor generation in a nanoparticle liquid suspension using a focused, continuous laser]'', Applied Physics Letters, Volume 95,  Issue 16, 2009</ref> while an exceptionally smooth surface, such as plastic, lends itself to [[superheating]]. Under these conditions, a heated liquid may show [[boiling delay]] and the temperature may go somewhat above the boiling point without boiling.

Homogeneous nucleation, where the bubbles form from the surrounding liquid instead of on a surface, can occur if the liquid is warmer in its center, and cooler at the surfaces of the container. This can be done, for instance, in a microwave oven, which heats the water and not the container.

===Critical heat flux===
[[Critical heat flux]] (CHF) describes the thermal limit of a phenomenon where a phase change occurs during heating (such as bubbles forming on a metal surface used to heat [[water]]), which suddenly decreases the efficiency of [[heat transfer]], thus causing localised overheating of the heating surface. As the boiling surface is heated above a critical temperature, a film of vapour forms on the surface. Since this vapour film is much less capable of carrying heat away from the surface, the temperature rises very rapidly beyond this point into the [[transition boiling]] regime. The point at which this occurs is dependent on the characteristics of boiling fluid and the heating surface in question.<ref name="sciencedirect.com"/>

===Transition===
[[Transition boiling]] may be defined as the unstable boiling, which occurs at surface temperatures between the maximum attainable in nucleate and the minimum attainable in film boiling.

The formation of [[Liquid bubble|bubbles]] in a heated liquid is a complex physical process which often involves [[cavitation]] and acoustic effects, such as the broad-spectrum hiss one hears in a kettle not yet heated to the point where bubbles boil to the surface.

===Film===
{{main|Leidenfrost effect}}

If a surface heating the liquid is significantly hotter than the liquid then film boiling will occur, where a thin layer of vapour, which has low [[thermal conductivity]], insulates the surface. This condition of a vapour film insulating the surface from the liquid characterises ''film boiling''.

== Influence of geometry ==

=== Pool boiling ===

"Pool boiling" refers to boiling where there is no forced convective flow. Instead, the flow occurs due to [[Density gradient|density gradients]]. It can experience any of the regimes mentioned above.

=== Flow boiling ===

"Flow boiling" occurs when the boiling fluid circulates, typically through pipes.<ref name="JamesHoldenFlowBoiling">{{cite journal | last1=Holden | first1=James B. | last2= Rowzee | first2= E. Ralph  | title=Adiabatic flow of boiling water through a horizontal pipe. | journal=Massachusetts Institute of Technology | publisher=Department of Chemical Engineering | year=1931}}</ref> Its movement can be powered by pumps, such as in power plants, or by density gradients, such as in a [[thermosiphon]] or a heat pipe. Flows in flow boiling are often characterised by a void fraction parameter, which indicates the fraction of the volume in the system that is vapor. One can use this fraction and the densities to calculate the [[vapor quality]], which refers to the mass fraction that is in the gas phase. Flow boiling can be very complex, with heavy influences of density, flow rates, and heat flux, as well as surface tension. The same system may have regions that are liquid, gas, and two-phase flow. Such two phase regimes can lead to some of the best heat transfer coefficients of any system.

=== Confined boiling ===

Confined boiling refers to boiling in confined geometries, typically characterized by a [[Bond number]] that compares the gap spacing to the capillary length. Confined boiling regimes begin to play a major role when Bo < 0.5. This boiling regime is dominated by "vapour stem bubbles" left behind after vapour departs.<ref name="Alsaati Warsinger Weibel Marconnet 2021 p=121520">{{cite journal | last1=Alsaati | first1=A.A. | last2=Warsinger | first2=D.M. | last3=Weibel | first3=J.A. | last4=Marconnet | first4=A.M.|author4-link=Amy Marconnet | title=Vapor stem bubbles dominate heat transfer enhancement in extremely confined boiling | journal=International Journal of Heat and Mass Transfer | publisher=Elsevier BV | volume=177 | year=2021 | issn=0017-9310 | doi=10.1016/j.ijheatmasstransfer.2021.121520 | page=121520| bibcode=2021IJHMT.17721520A | url=https://docs.lib.purdue.edu/coolingpubs/384 }}</ref> These bubbles act as seeds for vapor growth. Confined boiling typically has higher heat transfer coefficient but a lower CHF than pool boiling. CHF occurs when the vapor momentum force at the two-phase interface balances the combined surface tension and hydrostatic forces, leading to irreversible growth of the dry spot.<ref name="Alsaati Warsinger Weibel Marconnet 2023 p. 104542">{{cite journal | last1=Alsaati | first1=Albraa A. | last2=Warsinger | first2=David M. | last3=Weibel | first3=Justin A. | last4=Marconnet | first4=Amy M.|author4-link=Amy Marconnet | title=A mechanistic model to predict saturated pool boiling Critical Heat Flux (CHF) in a confined gap | journal=International Journal of Multiphase Flow | publisher=Elsevier BV | volume=167 | year=2023 | issn=0301-9322 | doi=10.1016/j.ijmultiphaseflow.2023.104542 | page=104542| bibcode=2023IJMF..16704542A }}</ref> Confined boiling is particularly promising for electronics cooling.

==Physics==
The boiling point of an [[Chemical element|element]] at a given pressure  is a characteristic attribute of the element. This is also true for many simple compounds including water and simple [[Alcohol (chemistry)|alcohol]]s. Once boiling has started and provided that boiling remains stable and the pressure is constant, the temperature of the boiling liquid remains constant. This attribute led to the adoption of boiling points as the definition of 100&nbsp;°C.

===Distillation===
Mixtures of volatile liquids have a boiling point specific to that mixture producing vapour with a constant mix of components - the [[constant boiling mixture]]. This attribute allows mixtures of liquids to be separated or partly separated by boiling and is best known as a means of separating [[ethanol]] from water.

==Uses==

===Refrigeration and air conditioning===
Most types of [[refrigeration]] and some type of [[air-conditioning]] work by compressing a gas so that it becomes liquid and then allowing it to boil. This adsorbs heat from the surroundings cooling the fridge or freezer or cooling the air entering a building. Typical liquids include [[propane]], [[ammonia]], [[carbon dioxide]] or [[nitrogen]].

=== For making water potable ===
<!-- This section is linked from Boiled water -->
As a method of disinfecting water, bringing it to its boiling point at {{convert|100|°C}}, is the oldest and most effective way since it does not affect the taste, it is effective despite contaminants or particles present in it, and is a single step process which eliminates most [[microbes]] responsible for causing [[intestine]] related diseases.<ref name=Oxfordjournals>{{cite journal|title=Water Disinfection for International and Wilderness Travelers|journal=Clinical Infectious Diseases|volume=34|issue=3|publisher= Oxford journals|author=Howard Backer|pages=355&ndash;364|doi=10.1086/324747|pmid=11774083|year=2002|doi-access=free}}</ref> The boiling point of water is {{convert|100|C}}  at sea level and at normal barometric pressure.<ref>{{Cite web|url=http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch14/melting.php|title=Melting Point, Freezing Point, Boiling Point|website=chemed.chem.purdue.edu|access-date=2019-01-11}}</ref> In places having a proper [[water purification]] system, it is recommended only as an [[Boil-water advisory|emergency treatment method]] or for obtaining potable water in the wilderness or in rural areas, as it cannot remove [[toxins|chemical toxins]] or impurities.<ref>{{Cite web|url=https://www.epa.gov/ground-water-and-drinking-water/emergency-disinfection-drinking-water|title=Emergency Disinfection of Drinking Water|last=US EPA|first=OW|date=2015-11-18|website=US EPA|language=en|access-date=2019-01-11}}</ref><ref name="backpackers field manual">{{cite web|url=http://www.princeton.edu/~oa/manual/water.shtml|title=OA Guide to Water Purification, The Backpacker's Field Manual|last=Curtis|first=Rick|date=March 1998}}</ref>

The elimination of micro-organisms by boiling follows [[first-order kinetics]]—at high temperatures, it is achieved in less time and at lower temperatures, in more time. The heat sensitivity of micro-organisms varies, at {{convert|70|°C}}, ''[[Giardia]]'' species (which cause [[giardiasis]]) can take ten minutes for complete inactivation, most intestine affecting microbes and ''[[Escherichia coli|E.&nbsp;coli]]'' ([[gastroenteritis]]) take less than a minute; at boiling point, ''[[Vibrio cholerae]]''  ([[cholera]]) takes ten seconds and [[hepatitis A virus]] (causes the symptom of [[jaundice]]), one minute. Boiling does not ensure the elimination of all micro-organisms; the bacterial spores ''[[Clostridium]]'' can survive at {{convert|100|°C}} but are not water-borne or intestine affecting. Thus for human health, complete [[sterilization (microbiology)|sterilization]] of water is not required.<ref name="Oxfordjournals"/>

The traditional advice of boiling water for ten minutes is mainly for additional safety, since microbes start getting eliminated at temperatures greater than {{convert|60|°C}} and bringing it to its boiling point is also a useful indication that can be seen without the help of a [[thermometer]], and by this time, the water is disinfected. Though the [[Atmospheric pressure boiling point|boiling point decreases]] with increasing altitude, it is not enough to affect the disinfecting process.<ref name="Oxfordjournals"/><ref>{{Cite web|url=https://www.cdc.gov/healthywater/emergency/drinking/making-water-safe.html|title=Making Water Safe in an Emergency|last=CDC|date=2019-09-06|website=Centers for Disease Control and Prevention|language=en-us|access-date=2020-01-07}}</ref>

===In cooking===
<!-- This section is linked from [[Boiling in cooking]] -->
{{stack|[[File:Flickr - cyclonebill - Pasta (1).jpg|thumb|Boiling [[pasta]]]]}}
''Boiling'' is the method of [[cooking]] food in boiling [[water]] or other water-based liquids such as [[Stock (food)|stock]] or [[milk]].<ref>{{Cite book|url=https://books.google.com/books?id=ORmzCwAAQBAJ&q=Boiling+is+the+method+of+cooking+food+in+boiling+water,+or+other+water-based+liquids+such+as+stock+or+milk&pg=PT100|title=AQA GCSE Food Preparation and Nutrition|last1=Rickus|first1=Alexis|last2=Saunder|first2=Bev|last3=Mackey|first3=Yvonne|date=2016-08-22|publisher=Hodder Education|isbn=9781471863653|language=en}}</ref> [[Simmering]] is gentle boiling, while in [[poaching (cooking)|poaching]] the cooking liquid moves but scarcely bubbles.<ref>{{Cite book|url=https://books.google.com/books?id=ZAM3TJfP09cC&q=Simmering+is+gentle+boiling,+while+in+poaching,+the+cooking+liquid+moves&pg=PA306|title=The Cook's Book: Techniques and tips from the world's master chefs|last=Publishing|first=D. K.|date=2005-08-29|publisher=Penguin|isbn=9780756665609|language=en}}</ref>

The boiling point of water is typically considered to be {{convert|100|°C|°F K}}, especially at sea level. Pressure and a change in the composition of the liquid may alter the boiling point of the liquid. High [[elevation]] cooking generally takes longer since boiling point is a function of [[atmospheric pressure]]. At an elevation of about {{convert|1|mile|m|spell=in}}, water boils at approximately {{convert|95|°C|°F K}}.<ref>{{cite web|url=http://www.iapws.org/faq1/boil.htm|author=IAPWS|author-link=IAPWS|work=FAQs About Water and Steam|title=What is the effect of pressure on the boiling of water? Why does water boil at a lower temperature at high altitudes?|access-date=2009-12-05|url-status=dead|archive-url=https://web.archive.org/web/20090806060431/http://www.iapws.org/faq1/boil.htm|archive-date=2009-08-06}}</ref> Depending on the type of food and the elevation, the boiling water may not be hot enough to cook the food properly.<ref>{{Cite web|url=https://www.fsis.usda.gov/wps/portal/fsis/topics/food-safety-education/get-answers/food-safety-fact-sheets/safe-food-handling/high-altitude-cooking-and-food-safety/ct_index|title=High Altitude Cooking and Food Safety|date=2013-06-15 |website=United States Department of Agriculture|access-date=2020-02-10 }}</ref> Similarly, increasing the pressure as in a [[pressure cooker]] raises the temperature of the contents above the open air boiling point.{{citation needed|date=April 2022}}

===Boil-in-the-bag===
{{main|Boil-in-bag}}
{{See also|Sous-vide}}
Also known as "boil-in-bag", this involves heating or cooking ready-made foods sealed in a thick plastic bag. The bag containing the food, often frozen, is submerged in boiling water for a prescribed time.<ref>{{cite web|url=http://dictionary.reference.com/browse/boil-in-bag|title=Boil-in-bag - Define Boil-in-bag at Dictionary.com|work=Dictionary.com}}</ref> The resulting dishes can be prepared with greater convenience as no pots or pans are dirtied in the process. Such meals are available for camping as well as home dining.

==Contrast with evaporation==

At any given temperature, the molecules in a liquid have varying kinetic energies. Some high energy particles on the liquid surface may have enough energy to escape the intermolecular forces of attraction of the liquid and become a gas. This is called evaporation.

Evaporation only happens on the surface while boiling happens throughout the liquid.
When a liquid reaches its boiling point bubbles of gas form in it which rise into the surface and burst into the air. This process is called boiling. If the boiling liquid is heated more strongly the temperature does not rise but the liquid boils more actively.

This distinction is exclusive to the liquid-to-gas transition; any transition directly from solid to gas is always referred to as [[Sublimation (phase transition)|sublimation]] regardless of whether it is at its boiling point or not.

== See also ==
* [[Phase transition]]
* [[Phase diagram]]
* [[Enthalpy of vaporization]]
* [[Explosive boiling]]
* [[Recovery time (culinary)]]

== References ==
{{Commons category|Boiling}}
{{Wiktionary}}
{{cookbook}}
{{Reflist}}
{{Cooking Techniques}}

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[[Category:Cooking techniques]]
[[Category:Phase transitions]]
[[Category:Heat transfer]]
[[Category:Gases]]
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