Mpemba effect
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2008-07-11T17:06:48Z
Drgao
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/* Add: Dr Martin Chaplin (London South Bank University) page on Mpemba effect and other anomalies of water */
The '''Mpemba effect''' is the observation that, in certain specific circumstances, warmer [[water]] freezes faster than colder water. [[New Scientist]] recommends starting the experiment with containers at 35°C and 5°C to maximise the effect.<ref>How to Fossilise Your Hamster: And Other Amazing Experiments For The Armchair Scientist, ISBN 1846680441</ref>
==Origin==
The effect is named for the [[Tanzania]]n [[high school|high-school]] student Erasto B. Mpemba. Mpemba first encountered the phenomenon in 1963 in Form 3 of Magamba Secondary School, Tanzania when freezing hot [[ice cream]] mix in cookery classes and noticing that they froze before cold mixes. After passing his [[O levels|O-level examinations]], he became a student at Mkwawa Secondary (formerly High) School, [[Iringa]], [[Tanzania]]. The headmaster invited Dr. [[Denis G. Osborne]] from the University College in [[Dar Es Salaam]] to give a lecture on [[physics]]. After the lecture, Erasto Mpemba asked him the question "If you take two similar containers with equal volumes of water, one at 35°C and the other at 100°C, and put them into a refrigerator, the one that started at 100°C freezes first. Why?" only to be ridiculed by his classmates. After initial consternation, Dr. Osborne confirmed Erasto's finding and they published the results together in 1969.<ref>{{citation | last = Mpemba | first = Erasto B. | last2 = Osborne | first2 = Denis G. | title = Cool? | journal = Physics Education | publisher = Institute of Physics | year = 1969 | volume = 4 | pages = 172–175 | doi = 10.1088/0031-9120/4/3/312}}</ref><ref>{{citation | title = The Mpemba effect | journal = Physics Education | publisher = Institute of Physics | year = 1979 | volume = 14 | pages =410–412 | url = http://www.iop.org/EJ/article/0031-9120/14/7/312/pev14i7p410.pdf | doi = 10.1088/0031-9120/14/7/312 | author = Mpemba, E B}}</ref> Erasto Mpemba currently works for the [[Food and Agriculture Organization|African Forestry and Wildlife Commission]].<ref>{{citation | title = Report of the 14th session of the Working Party on the Management of Wildlife and Protected Areas | url = ftp://ftp.fao.org/docrep/fao/meeting/013/ai572e.pdf}}</ref>
==Causes==
At first sight, the behaviour seems contrary to [[thermodynamics]]. However, the Mpemba effect can be explained with standard physical theory. Many effects can contribute to the observation, depending on the experimental set-up:
*''[[freezing|Definition of frozen]]'': Is it the physical definition of the point at which water forms a visible surface layer of ice, or the point at which the entire volume of water becomes a solid block of ice?
*''[[Evaporation]]'': Reducing the volume to be frozen.<ref>{{citation | author = Kell, G. S. | title = The freezing of hot and cold water | journal = Am. J. Phys. | year = 1969 | volume = 37 | pages = 564–565 | doi = 10.1119/1.1975687}}</ref> Evaporation is [[endothermic]].
*''[[Convection]]'': Accelerating [[heat transfer]]s. Reduction of water density below 4°C tends to suppress the convection currents cooling the lower part of the liquid mass; the lower density of hot water would reduce this effect, perhaps sustaining the more rapid initial cooling.
*''[[Frost]]'': Has [[Thermal insulation|insulating]] effects. The lower temperature water will tend to freeze from the top, reducing further heat loss by radiation and air convection, while the warmer water will tend to freeze from the bottom and sides because of water convection. This is disputed as there are experiments which account for this factor.
*''[[Supercooling]]'': It is [[hypothesis|hypothesized]] that cold water, when placed in a freezing environment, supercools more than hot water in the same environment, thus solidifying slower than hot water.<ref>{{citation | author = S. Esposito, R. De Risi and L. Somma | title = Mpemba effect and phase transitions in the adiabatic cooling of water before freezing | journal = Physica A | year = 2008 | volume = 387 | pages = 757–763 | url = http://arxiv.org/abs/0704.1381 | doi = 10.1016/j.physa.2007.10.029}}</ref> However, supercooling tends to be less significant where there are particles that act as nuclei for ice crystals, thus precipitating rapid freezing.
*''[[Solute]]s'': The effects of [[calcium carbonate|calcium]], [[magnesium carbonate]] among others.<ref>{{cite web|url=http://arxiv.org/abs/physics/0604224|title=When hot water freezes before cold|last=Katz|first=Jonathan|date=April 2006 }}</ref>
*The effect of heating on [[solvation|dissolved]] [[gas]]es.
===Scalar functionality===
According to an article by Monwhea Jeng: "Analysis of the situation is now quite complex, since we are no longer considering a single parameter, but a scalar function, and [[computational fluid dynamics]] (CFD) is notoriously difficult."<ref name='Jeng'> {{cite journal|title=Hot water can freeze faster than cold?!?|journal=American Journal of Physics|date=2006|first=Monwhea|last=Jeng| volume=74|issue=6|pages=514|id= arXiv:physics/0512262v1 |url=http://arxiv.org/abs/physics/0512262v1|accessdate=2008-03-18|doi=10.1119/1.2186331 }}</ref>
This effect is a heat transfer problem,<ref name=Knight> {{cite journal|title=The Mpemba effect: the freezing times of hot and cold water|journal=American Journal of Physics|date=1996-05|first=Charles A.|last=Knight|coauthors=|volume=64|issue=5|pages=524|id= |url=|format=|accessdate=2008-03-18|doi=10.1119/1.18275 }}</ref><ref name=Auerbach> {{cite journal|title=Supercooling and the Mpemba effect: when hot water freezes faster than cold|journal=American Journal of Physics|date=1995|first=David|last=Auerbach|coauthors=|volume=63|issue=10|pages=882–885|id= |url=|format=|accessdate=2008-03-18|doi=10.1119/1.18059 }}</ref><ref name='Dorsey1948'> {{cite journal|title=The Freezing of Supercoold Water|journal=Transactions of the American Philosophical Society|date=1948-11|first=N. Ernest|last=Dorsey|coauthors=|volume=38|issue=3|pages=247–328|doi= 10.2307/1005602|url=|format=|accessdate=2008-03-18 }}</ref><ref name='Dorsey1940'>{{cite book | last = Dorsey | first = N. Ernest | authorlink = | coauthors = | title = Properties of ordinary water-substance in all its phases: water vapor, water, and all the ices | publisher = Reinhold Publishing Corporation | date = 1940 | location = New York | pages = | url = | doi = | id = | isbn = }}</ref> and therefore well suited to be studied from a [[transport phenomena]] viewpoint, based on [[continuum mechanics]]. When heat transfer is analyzed in terms of [[partial differential equations]], whose solutions depend on a number of conditions, it becomes clear that measuring only a few [[lumped parameters]], such as the water average temperature is generally insufficient to define the system behaviour, since conditions such as geometry, fluid properties and temperature and flow fields play an important role. The counterintuitiveness of the effect, if analyzed only in terms of simplified thermodynamics illustrates the need to include all the relevant variables and use the best available theoretical tools when approaching a physical problem.<ref name=Knight /><ref name=Auerbach /><ref name='Dorsey1940' /><ref name='Dorsey1948' />
==Recent view of the Mpemba effect==
A reviewer for [[Physics World]] writes, "Even if the Mpemba effect is real - if hot water can sometimes freeze more quickly than cold - it is not clear whether the explanation would be trivial or illuminating."<ref>{{cite journal|last=Ball|first=P.|date=April 2006|url=http://physicsweb.org/articles/world/19/4/4|title=Does hot water freeze first?|journal=Physics World|volume=19(4)|pages=19–21}}</ref>
==Historical observations==
Similar behavior may have been observed by ancient scientists such as [[Aristotle]]<ref>Aristotle, Metereology, [http://classics.mit.edu/Aristotle/meteorology.1.i.html Book 1] "The fact that the water has previously been warmed contributes to its freezing quickly: for so it cools sooner. Hence many people, when they want to cool hot water quickly, begin by putting it in the sun. So the inhabitants of Pontus when they encamp on the ice to fish (they cut a hole in the ice and then fish) pour warm water round their reeds that it may freeze the quicker, for they use the ice like lead to fix the reeds". Trans. by E. W. Webster.</ref>, and [[Early Modern]] scientists such as [[Francis Bacon]]<ref>Novum Organum, [http://www.thelatinlibrary.com/bacon/bacon.liber2.shtml Lib. II], L, "slightly tepid water freezes more easily than that which is
utterly cold". In the original [[Latin]] "aqua parum tepida facilius conglacietur quam omnino frigida"</ref> and [[René Descartes]]<ref>
Descartes, [http://posner.library.cmu.edu/Posner/books/pages.cgi?call=423_D44D_1637&layout=vol0/part0/copy0&file=0247 Les Meteores], Discours Premier "One can see by experience that water that has been kept on a fire for a long time freezes faster than other, the reason being that those of its particles that are least able to stop bending evaporate while the water is being heated". In the original French "Et on peut voir aussy par experience que l'eau qu'on a tenue longuement sur le feu se gele plutot que d'autre, dont la raison est que celles de ses parties, qui peuvent le moins cesser de se plier, s'evaporent pendant qu'on la chauffe." Descartes' explanation here relates to his theory of vortices.
</ref>. Aristotle's explanation involved an erroneous property he called ''[[antiperistasis]]'', defined as "the supposed increase in the intensity of a quality as a result of being surrounded by its contrary quality".
==References==
{{reflist}}
==Bibliography==
*{{cite journal|last=Dorsey |first= N. Ernest |date=1948|title = The freezing of supercooled water| journal=Trans. Am. Phil. Soc.|volume= 38|pages=247–326|doi = 10.2307/1005602}} An extensive study of freezing experiments.
*{{cite journal |first=David|last=Auerbach|url=http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=AJPIAS000063000010000882000001&idtype=cvips&gifs=yes|title=Supercooling and the Mpemba effect: when hot water freezes quicker than cold|journal=[[American Journal of Physics]]|volume=63(10)|date=1995 |pages = 882–885}} Auerbach attributes the Mpemba effect to differences in the behaviour of supercooled formerly hot water and formerly cold water.
*{{cite journal|first=Charles A.|last=Knight|url=http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=AJPIAS000064000005000524000001&idtype=cvips&gifs=Yes|title=The MPEMBA effect: The freezing times of hot and cold water|journal=American Journal of Physics|date=May 1996|volume=64, Issue 5|pages=524}}
*{{cite journal|last=Monwhea|first=Jeng|date=2006|title=The Mpemba effect: When can hot water freeze faster than cold?|journal=American Journal of Physics|volume=74, number 6|pages=514|url=http://lanl.arxiv.org/abs/physics/0512262}}
*{{cite journal|last=Chown|first=Marcus|journal=New scientist|url=http://www.eurekalert.org/pub_releases/2006-05/ns-wwf053106.php|title=Why water freezes faster after heating|date=June 2006}}
==External links==
*{{cite web|last=Adams|first=Cecil|coauthors=Mary M.Q.C.|title = Which freezes faster, hot water or cold water?|work = The Straight Dope|publisher = Chicago Reader, Inc|date = 1996|url = http://www.straightdope.com/classics/a2_098b.html|format = html|accessdate = January 2008}}
*{{cite web|url=http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/freezhot.html|publisher=[[Georgia State University]]|work=HyperPhysics|format=html|title=Heat questions}}
*{{cite web|url=http://www.school-for-champions.com/science/mpemba.htm|title=The Mpemba Effect: Hot Water Freezes before Cold|last=Kurtus|first=Ron|date=October 2002|publisher=School for Champions|format=html}}
*[http://adsabs.harvard.edu/cgi-bin/nph-abs_connect?db_key=AST&db_key=PHY&db_key=PRE&sim_query=YES&aut_xct=NO&aut_logic=OR&obj_logic=OR&author=&object=&start_mon=&start_year=&end_mon=&end_year=&ttl_logic=OR&title=&txt_logic=OR&text=Mpemba+&nr_to_return=100&start_nr=1&jou_pick=ALL&ref_stems=&data_and=ALL&group_and=ALL&start_entry_day=&start_entry_mon=&start_entry_year=&end_entry_day=&end_entry_mon=&end_entry_year=&min_score=&sort=SCORE&data_type=SHORT&aut_syn=YES&ttl_syn=YES&txt_syn=YES&aut_wt=1.0&obj_wt=1.0&ttl_wt=0.3&txt_wt=3.0&aut_wgt=YES&obj_wgt=YES&ttl_wgt=YES&txt_wgt=YES&ttl_sco=YES&txt_sco=YES&version=1 assabs.harvard.edu] article collection.
*{{cite web|url=http://cherwell.larrytech.com/index.php?news=2322|title="The Mpemba Effect" article}} on the online [[Cherwell (newspaper)]].
*{{cite web|url=http://math.ucr.edu/home/baez/physics/General/hot_water.html|title=Can hot water freeze faster than cold water?}} in the [[University of California]] Usenet Physics FAQ.
*{{cite web|url=http://www.lsbu.ac.uk/water/explan.html#mpemba|title=The Phase Anomalies of Water: Hot Water may Freeze Faster than Cold Water}} An analysis of the Mpemba effect. [[London South Bank University]].
[[Category:Physical paradoxes]]
[[Category:Thermodynamics]]
[[Category:Phase changes]]
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