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The fact that hot water freezes faster than cold has been known for many centuries. The earliest reference to this phenomenon dates back to Aristotle in 300 B.C. The phenomenon was later discussed in the medieval era, as European physicists struggled to come up with a theory of heat. But by the 20th century the phenomenon was only known as common folklore, until it was reintroduced to the scientific community in 1969 by Mpemba, a Tanzanian high school student. Since then, numerous experiments have confirmed the existence of the "Mpemba effect", but have not settled on any single explanation.

The earliest known reference to this phenomenon is by Aristotle, who wrote:

"The fact that 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. . ." [1,4]

He wrote these words in support of a mistaken idea which he called antiperistasis. Antiperistasis is defined as "the supposed increase in the intensity of a quality as a result of being surrounded by its contrary quality, for instance, the sudden heating of a warm body when surrounded by cold" [4].

Medieval scientists believed in Aristotle's theory of antiperistasis, and also sought to explain it. Not surprisingly, scientists in the 1400's had trouble explaining how it worked, and could not even decide whether (as Aristotle claimed in support of antiperistasis), human bodies and bodies of water were hotter in the winter than in the summer [4]. Around 1461, the physicist Giovanni Marliani, in a debate over how objects cooled, said that he had confirmed that hot water froze faster than cold. He said that he had taken four ounces of boiling water, and four ounces of non-heated water, placed them outside in similar containers on a cold winter day, and observed that the boiled water froze first. Marliani was, however, unable to explain this occurrence [4].

Later, in the 1600's, it was apparently common knowledge that hot water would freeze faster than cold. In 1620 Bacon wrote "Water slightly warm is more easily frozen than quite cold" [2], while a little later Descartes claimed "Experience shows that water that has been kept for a long time on the fire freezes sooner than other water" [3].

In time, a modern theory of heat was developed, and the earlier observations of Aristotle, Marliani, and others were forgotten, perhaps because they seemed so contradictory to modern concepts of heat. However, it was still known as folklore among many non-scientists in Canada [11], England [15-21], the food processing industry [23], and elsewhere.

It was not reintroduced to the scientific community until 1969, 500 years after Marliani's experiment, and more than two millennia after Aristotle's "Meteorologica I" [1]. The story of its rediscovery by a Tanzanian high school student named Mpemba is written up in the New Scientist [4]

                               Modified for private educational use only. Original Source Copyright                           
                  back                  Written Nov, 1998 by Monwhea Jeng (Momo),                             next