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Revision as of 18:51, 1 March 2008 by The Tutor (talk | contribs) (→Bibliography)(diff) ← Previous revision | Latest revision (diff) | Newer revision → (diff)The Mpemba effect is the observation that, in some specific, fairly common 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.
Origin
The effect is named for the Tanzanian high-school student Erasto B. Mpemba. Mpemba first encountered the phenomenon in the classroom of Eugene Marschall at Mkwawa Secondary (formerly High) School, Iringa, Tanzania, where Mpemba was a student. Eugene Marschall, a member of the Teachers for East Africa/TEA program, taught chemistry and physics at this school from 1965 to 1967. Mpemba first noticed the effect in 1963 after his account of the freezing of hot ice cream mix in cookery classes, and went on to publish experimental results with Dr. Denis G. Osborne in 1979.
Causes
At first sight, the behaviour seems contrary to thermodynamics. However, most thermodynamicists believe that each observation of the Mpemba effect can be explained with standard physical theory. Many effects can contribute to the observation, depending on the experimental set-up:
- 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. Evaporation is endothermic.
- Convection: Accelerating heat transfers. 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 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 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. However, supercooling tends not to be significant where there are particles, which act as nuclei for ice crystals, thus precipitating rapid freezing.
- Solutes: The effects of calcium, magnesium carbonate among others.
- The effect of boiling on dissolved gases.
According to an article by Monwhea Jeng, there is no unique explanation yet for why, in some specific circumstances, hotter water freezes faster than colder water.
Historical observations
Similar behavior may have been observed by ancient scientists such as Aristotle, and Early Modern scientists such as Francis Bacon and René Descartes. 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
- How to Fossilise Your Hamster: And Other Amazing Experiments For The Armchair Scientist, ISBN 1846680441
- Mpemba, Erasto B.; Osborne, Denis G. (1979), "Cool?" (PDF), Physics Education, 14, Tanzania: Institute of Physics, doi:10.1088/0031-9120/14/7/312
- Jeng, Monwhea (2006). "Hot water can freeze faster than cold?!?" (PDF). Am. J. Phys. 74: 514.
- Aristotle, Metereology, 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.
- Novum Organum, 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"
- Descartes, 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.
Bibliography
- Ball, P. (April 2006). "Does hot water freeze first?". Physics World. 19(4): 19–21.
- Dorsey, N. Ernest (1948). "The freezing of supercooled water". Trans. Am. Phil. Soc. 38: 247–326. An extensive study of freezing experiments.
- Auerbach, David (1995). "Supercooling and the Mpemba effect: when hot water freezes quicker than cold". American Journal of Physics. 63(10): 882–885. Auerbach attributes the Mpemba effect to differences in the behaviour of supercooled formerly hot water and formerly cold water.
- Knight, Charles A. (May 1996). "The MPEMBA effect: The freezing times of hot and cold water". American Journal of Physics. 64, Issue 5: 524.
- Monwhea, Jeng (2006). "The Mpemba effect: When can hot water freeze faster than cold?". American Journal of Physics. 74, number 6: 514.
- Chown, Marcus (June 2006). "Why water freezes faster after heating". New scientist.
External links
- Adams, Cecil (1996). "Which freezes faster, hot water or cold water?" (html). The Straight Dope. Chicago Reader, Inc. Retrieved January 2008.
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suggested) (help) - "Heat questions" (html). HyperPhysics. Georgia State University.
- Kurtus, Ron (October 2002). "The Mpemba Effect: Hot Water Freezes before Cold" (html). School for Champions.
- assabs.harvard.edu article collection.
- ""The Mpemba Effect" article". on the online Cherwell (newspaper)
- Katz, Jonathan (April 2006). "When hot water freezes before cold" (html).
- Esposito, de Risi, Somma (February 2008). "Mpemba effect and phase transitions in the adiabatic cooling of water before freezing". Physica A, Volume 387, Issue 4, p. 757-763.