Mpemba effect

The Mpemba effect, named after Erasto Mpemba in 1963, is the observation that, in some circumstances, warmer water can freeze faster than colder water. Although there is evidence of the effect, there is disagreement 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. 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.

Definition

The phenomenon, when taken to mean "hot water freezes faster than cold", is difficult to reproduce or confirm, because this statement is ill-defined.[1] Monwhea Jeng proposes as a more precise wording:

There exists a set of initial parameters, and a pair of temperatures, such that given two bodies of water identical in these parameters, and differing only in initial uniform temperatures, the hot one will freeze sooner.[2]

However, even with this definition it is not clear whether "freezing" refers to the point at which water forms a visible surface layer of ice; the point at which the entire volume of water becomes a solid block of ice; or when the water reaches 0 °C (32 °F).[1] A quantity of water can be at 0 °C (32 °F) and not be ice; after enough heat has been removed to reach 0 °C (32 °F) more heat must be removed before the water changes to solid state (ice), so water can be liquid or solid at 0 °C (32 °F).[3]

With the above definition there are simple ways in which the effect might be observed: For example, if the hotter temperature melts the frost on a cooling surface and thus increases the thermal conductivity between the cooling surface and the water container.[1] On the other hand, there may be many circumstances in which the effect is not observed.[1]

Observations

Historical context

Various effects of heat on the freezing of water were described 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."[4] Aristotle's explanation involved antiperistasis, "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."[5] In the original Latin, "aqua parum tepida facilius conglacietur quam omnino frigida."

René Descartes wrote in his Discourse on the Method, "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."[6] This relates to Descartes' vortex theory.

Joseph Black investigated a special case of this phenomenon comparing previously-boiled with unboiled water;[7] the previously-boiled water froze more quickly. Evaporation was controlled for. He discussed the influence of stirring on the results of the experiment, noting that stirring the unboiled water led to it freezing at the same time as the previously-boiled water, and also noted that stirring the very-cold unboiled water led to immediate freezing. Joseph Black then discussed Fahrenheit's description of supercooling of water (although the term supercooling had not then been coined), arguing, in modern terms, that the previously-boiled water could not be as readily supercooled.

Mpemba's observation

The effect is named after Tanzanian Erasto Mpemba. He described 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. He later became a student at Mkwawa Secondary (formerly High) School in Iringa. 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, Osborne experimented on the issue back at his workplace and confirmed Mpemba's finding. They published the results together in 1969, while Mpemba was studying at the College of African Wildlife Management.[8]

Modern context

Mpemba and Osborne describe placing 70 ml (2.5 imp fl oz; 2.4 US fl oz) samples of water in 100 ml (3.5 imp fl oz; 3.4 US fl oz) beakers in the ice box of a domestic refrigerator on a sheet of polystyrene foam. They showed the time for freezing to start was longest with an initial temperature of 25 °C (77 °F) and that it was much less at around 90 °C (194 °F). They ruled out loss of liquid volume by evaporation as a significant factor and the effect of dissolved air. In their setup most heat loss was found to be from the liquid surface.[8]

David Auerbach describes an effect that he observed in samples in glass beakers placed into a liquid cooling bath. In all cases the water supercooled, reaching a temperature of typically −6 to −18 °C (21 to 0 °F) before spontaneously freezing. Considerable random variation was observed in the time required for spontaneous freezing to start and in some cases this resulted in the water which started off hotter (partially) freezing first.[9]

In studies appearing in Phys.org, James Brownridge, a radiation safety officer at the State University of New York, indicates supercooling is involved.[10]

Suggested explanations

The behaviour seems contrary to natural expectation but many explanations have been proposed.

Recent views

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.[1]

New Scientist recommends starting the experiment with containers at 35 and 5 °C (95 and 41 °F) to maximize the effect.[14] In a related study, it was found that freezer temperature also affects the probability of observing the Mpemba phenomenon as well as container temperature.

In 2012, the Royal Society of Chemistry held a competition calling for papers offering explanations to the Mpemba effect.[15] More than 22,000 people entered and Erasto Mpemba himself announced Nikola Bregović as the winner, suggesting that convection and supercooling were the reasons for the effect.[16]

Similar effects

Other phenomena in which large effects may be achieved faster than small effects are

See also

References

  1. 1 2 3 4 5 Ball, Philip (April 2006). Does hot water freeze first?. Physics World, pp. 19-26.
  2. 1 2 3 4 Jeng, Monwhea (2006). "Hot water can freeze faster than cold?!?". American Journal of Physics 74 (6): 514. arXiv:physics/0512262v1. Bibcode:2006AmJPh..74..514J. doi:10.1119/1.2186331.
  3. ↑ High School Chemistry and Physics
  4. ↑ Aristotle, Meteorology I.12 348b31–349a4
  5. ↑ Francis Bacon, Novum Organum, Lib. II, L
  6. ↑ Descartes, Les Meteores
  7. ↑ Black, Joseph (1 January 1775). "The Supposed Effect of Boiling upon Water, in Disposing It to Freeze More Readily, Ascertained by Experiments. By Joseph Black, M. D. Professor of Chemistry at Edinburgh, in a Letter to Sir John Pringle, Bart. P. R. S.". Philosophical Transactions of the Royal Society of London 65: 124–128. doi:10.1098/rstl.1775.0014.
  8. 1 2 Mpemba, Erasto B.; Osborne, Denis G. (1969). "Cool?". Physics Education (Institute of Physics) 4: 172–175. Bibcode:1969PhyEd...4..172M. doi:10.1088/0031-9120/4/3/312. republished as Mpemba, E B; Osborne, D G (1979). "The Mpemba effect" (PDF). Physics Education (Institute of Physics) 14: 410–412. Bibcode:1979PhyEd..14..410M. doi:10.1088/0031-9120/14/7/312.
  9. ↑ Auerbach, David (1995). "Supercooling and the Mpemba effect: when hot water freezes quicker than cold" (PDF). American Journal of Physics 63 (10): 882–885. Bibcode:1995AmJPh..63..882A. doi:10.1119/1.18059.
  10. ↑ Edwards, Lin (26 March 2010). "Mpemba effect: Why hot water can freeze faster than cold". SUNY: Science X Network, Phys.org.
  11. ↑ 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.
  12. ↑ CITV Prove It! Series 1 Programme 13
  13. ↑ Katz, Jonathan (April 2006). "When hot water freezes before cold". arXiv:physics/0604224 [physics.chem-ph].
  14. ↑ How to Fossilize Your Hamster: And Other Amazing Experiments For The Armchair Scientist, ISBN 1-84668-044-1
  15. ↑ Mpemba Competition. Royal Society of Chemistry. 2012.
  16. ↑ Winner of the Mpemba Competition. Royal Society of Chemistry. 2013.

Bibliography

External links

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