The Mpemba effect is known as the phenomenon whereby hot water can freeze faster than cold water.
The effect is named for its rediscoverer, the Tanzanian high-school student Erasto B. Mpemba. He 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 effect is contrary to thermodynamics in general.
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 & physics at this school 1965 to 1967.
Erroneously, Mpemba's story is often given as a cautionary parable to those who reject theories or experiments solely because they seem counterintuitive, or contradict accepted theories, or because their proponent is not an expert. In the six years between Mpemba's discovery and his publication, his ideas were rejected on a number of occasions by his physics teachers and other authorities.
The effect was known to ancient scientists such as Aristotle, and Early Modern scientists such as Francis Bacon and René Descartes. Aristotle's explanation was that this was due to 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.
To reduce the embellishment of an impossible sounding effect, the History of Science gives us the answer to Mpemba's delima. Some explanations include various thermodynamic or chemistry processes to shed light on how the process is possible or not. Unfortunately, Mpemba never took notes on an uncontrolled experiment (which is taboo).
The only way to understand why Mpemba is inconsistant is to understand latent heat and the three phases of matter—solid, liquid, and gas. The associated materials Mpemba was trying to compare to his ice cream experiment was ice, water, and water vapor.
- 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.