The Mpemba effect, named after Tanzanian student Erasto Mpemba, is the assertion that warmer water can freeze faster than colder water. Although there is anecdotal support for the effect, there is no agreement on exactly what the effect is and under what circumstances it occurs. There have been reports of similar phenomena since ancient times, although with insufficient detail for the claims to be replicated.
The phenomenon seems contrary to thermodynamics, but a number of possible explanations for the effect have been proposed. Further investigations will need to decide on a precise definition of "freezing" and control a vast number of starting parameters in order to confirm or explain the effect.
Historical observations 
Similar behavior was observed by ancient scientists such as Aristotle: "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 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." 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"
Early modern scientists such as Francis Bacon noted that "slightly tepid water freezes more easily than that which is utterly cold." In the original Latin, "aqua parum tepida facilius conglacietur quam omnino frigida."
René Descartes wrote in his Discourse 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." Descartes' explanation here relates to his theory of vortices.
Mpemba's observations 
The effect is named after Tanzanian Erasto Mpemba. He first encountered the phenomenon in 1963 in Form 3 of Magamba Secondary School, Tanganyika when freezing ice cream mix that was hot in cookery classes and noticing that it froze before the cold mix. After passing his 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 (95 °F) and the other at 100 °C (212 °F), and put them into a freezer, the one that started at 100 °C (212 °F) freezes first. Why?" only to be ridiculed by his classmates and teacher. After initial consternation, Dr. Osborne experimented on the issue back at his workplace and confirmed Mpemba's finding. They published the results together in 1969.
Effect definition 
Although widely mentioned there are very few, if any, modern descriptions of exactly what the effect is and how it may be observed. It is not clear whether "freezing" refers to 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. Some experiments have instead measured the time until the water reached 0 °C.
There are no reliable sources that indicate exactly how to demonstrate the effect and under exactly what conditions it occurs.
Suggested explanations 
Osborne observed that the top is warmer than the bottom in a beaker of water being cooled, the difference being sustained by convection. Blocking heat transfer from the top with a film of oil drastically slowed cooling. Also, the effect of dissolved air was accounted for by using boiled water. The beakers were also insulated from the bottom.
At first sight, the behaviour seems contrary to thermodynamics. Many standard physical theory effects contribute to the phenomenon, although no single explanation is conclusive. Several effects may contribute to the observation, depending on the experimental set-up:
- Evaporation: The evaporation of the warmer water reduces the mass of the water to be frozen. Evaporation is endothermic, meaning that the water mass is cooled by vapor carrying away the heat, but this alone probably does not account for the entirety of the effect.
- Convection: Accelerating heat transfers. Reduction of water density below 4 °C (39 °F) tends to suppress the convection currents that cool the lower part of the liquid mass; the lower density of hot water would reduce this effect, perhaps sustaining the more rapid initial cooling. Higher convection in the warmer water may also spread ice crystals around faster.
- 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 that 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 to be less significant where there are particles that act as nuclei for ice crystals, thus precipitating rapid freezing.
- Solutes: The effects of calcium, magnesium carbonate among others.
- Thermal conductivity: The container of hotter liquid may melt through a layer of frost that is acting as an insulator under the container (frost is an insulator, as mentioned above), allowing the container to come into direct contact with a much colder lower layer that the frost formed on (ice, refrigeration coils, etc.) The container now rests on a much colder surface (or one better at removing heat, such as refrigeration coils) than the originally colder water, and so cools far faster from this point on.
- The effect of heating on dissolved gases; however, this was accounted for in the original article by using boiled water.
- Latent heat of condensation: During the cooling phase the cooler container is picking up more condensation than the warm container. This reduces the rate of cooling. 
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."
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." He pointed out that investigations of the phenomenon need to control a large number of initial parameters (including type and initial temperature of the water, dissolved gas and other impurities, and size, shape and material of the container, and temperature of the refrigerator) and need to settle on a particular method of establishing the time of freezing, all of which might affect the presence or absence of the Mpemba effect. The required vast multidimensional array of experiments might explain why the effect is not yet understood.
In 2012, the Royal Society of Chemistry held a competition calling for papers offering explanations to the Mpemba effect. More than 22,000 people entered and Erasto Mpemba himself announced Nikola Bregović's paper as the winner.
See also 
- Leidenfrost effect – lower temperature boilers can sometimes vaporize water faster than higher temperature boilers.
- Aristotle, Meteorology I.12 348b31–349a4
- Francis Bacon, Novum Organum, Lib. II, L
- Descartes, Les Meteores
- Mpemba, Erasto B.; Osborne, Denis G. (1969). "Cool?". Physics Education (Institute of Physics) 4 (3): 172–175. Bibcode:1969PhyEd...4..172M. doi:10.1088/0031-9120/4/3/312.
- Ball, P. (April 2006). "Does hot water freeze first?" (– Scholar search). Physics World 19 (4): 19–21.[dead link]
- Kell, G. S. (1969). "The freezing of hot and cold water". Am. J. Phys. 37 (5): 564–565. Bibcode:1969AmJPh..37..564K. doi:10.1119/1.1975687.
- Jeng, Monwhea (2006). "Hot water can freeze faster than cold?!?". American Journal of Physics 74 (6): 514. arXiv:physics/0512262. doi:10.1119/1.2186331. arXiv:physics/0512262v1.
- CITV Prove It! Series 1 Programme 13
- S. Esposito, R. De Risi and L. Somma (2008). "Mpemba effect and phase transitions in the adiabatic cooling of water before freezing". Physica A 387 (4): 757–763. arXiv:0704.1381. Bibcode:2008PhyA..387..757E. doi:10.1016/j.physa.2007.10.029.
- Katz, Jonathan (April 2006). "When hot water freezes before cold". arXiv:physics/0604224 [physics.chem-ph].
- Physicsforums 
- How to Fossilize Your Hamster: And Other Amazing Experiments For The Armchair Scientist, ISBN 1-84668-044-1
- Dorsey, N. Ernest (1948). "The freezing of supercooled water". Trans. Am. Phil. Soc. (American Philosophical Society) 38 (3): 247–326. doi:10.2307/1005602. JSTOR 1005602. 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. Bibcode:1995AmJPh..63..882A. doi:10.1119/1.18059. 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 (5): 524. Bibcode:1996AmJPh..64..524K. doi:10.1119/1.18275.
- Monwhea, Jeng (2006). "The Mpemba effect: When can hot water freeze faster than cold?". American Journal of Physics 74 (6): 514. arXiv:physics/0512262. doi:10.1119/1.2186331.
- Chown, Marcus (June 2006). "Why water freezes faster after heating". New scientist.
- Mpemba, E B; Osborne, D G. "The Mpemba effect". Institute of Physics.
- Adams, Cecil; Mary M.Q.C. (1996). "Which freezes faster, hot water or cold water?". The Straight Dope. Chicago Reader, Inc. Retrieved January 2008.
- Brownridge, James (2010). "A search for the Mpemba effect: When hot water freezes faster than cold water". arXiv:1003.3185 [physics.pop-ph].
- "Heat questions". HyperPhysics. Georgia State University.
- assabs.harvard.edu article collection.
- "The Mpemba Effect". - History and analysis of the Mpemba Effect.
- Jeng, Monwhea (November 1998). "Can hot water freeze faster than cold water?". in the University of California Usenet Physics FAQ
- "The Phase Anomalies of Water: Hot Water may Freeze Faster than Cold Water". An analysis of the Mpemba effect. London South Bank University.
- "Mpemba effect: Why hot water can freeze faster than cold". A possible explanation of the Mpemba Effect
- "The story of the Mpemba effect told by the protagonists". An historical interview with Erasto Mpemba, Dr Denis Osborne and Ray deSouza