Mpemba effect
The Mpemba effect is a name for the observation that, in some specific circumstances, hotter water freezes faster than colder water.
The effect is named for its rediscoverer, 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 1969.
At first sight, the behavior seems contrary to thermodynamics. However, most thermodynamicians 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:
- Different definition of freezing (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
- Convection, accelerating heat transfers
- The insulating effects of frost
- The effect of boiling on dissolved gases
- Supercooling. It is hypothesized that cold water, when placed in a freezing environment, supercools more than hot water in the same environment, thus solidifying the hot water faster
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 a physical 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". He used the concept of antiperistasis to provide evidence for his conjecture that human bodies and bodies of water were hotter in the winter than in the summer, a theory that was later disproved by Medieval and Renaissance observations.
See also
- Hot ice (turning room temperature water in room temperatures ice using an electric field)
External links and references
- David Auerbach, Supercooling and the Mpemba effect: when hot water freezes quicker than cold, American Journal of Physics, 63(10), 1995. Auerbach attributes the Mpemba effect to differences in the behaviour of supercooled formerly hot water and formerly cold water.
- Can hot water freeze faster than cold water? Nov, 1998 by Monwhea Jeng (Momo), Department of Physics, University of California
- Which freezes faster, hot water or cold water from Marilyn is Wrong, a site that picks Marilyn vos Savant's mistakes
- The Straight Dope: Which freezes faster, hot water or cold water? by Cecil Adams, The Straight Dope, Chicago Reader Inc.
- HyperPhysics: The Mpemba Effect
- Charles A. Knight, The MPEMBA effect: The freezing times of hot and cold water, American Journal of Physics -- May 1996 -- Volume 64, Issue 5, p. 524
- The Mpemba Effect: Hot Water Freezes before Cold
- Other citations to the Mpemba Effect.