Trivia Junction

A Compendium of trivia I find interesting
...for the curious

Does hot water freeze faster then cold?

It may seem impossible, but sometimes hot water does freeze faster than cold. The key word is “sometimes,” which makes the phenomenon puzzling.

The earliest known reference to this effect was by Aristotle in 350 B.C. It was documented and discussed until the 17th century but then forgotten as modern notions about temperature and energy were developed.

In the 1960s, Erasto Mpemba, a high school student in Tanzania, reintroduced the phenomenon to the scientific community. He first observed the “Mpemba Effect,” as it is now known, while making ice cream with fellow students. He placed boiled milk mixture in the freezer without waiting for it to cool, and later observed that it froze faster than mixtures that had cooled before being placed in the freezer.

Confirmations of the effect have since appeared in scientific journals, along with a plethora of possible explanations. For instance, hot water evaporates faster than cold, and a smaller mass of hot water can freeze faster than a larger mass of cold. But evaporation cannot explain why the effect has been observed in experiments using sealed containers.

The Mpemba Effect may also occur because the hot water melts some of frost on the freezer floor beneath it, and the container settles into the ice. Tighter contact between the container and the ice limits heat loss. Again, this may explain some everyday observations of the effect, but most experiments described in scientific journals used thermal insulators.

The composition of the water is a factor. Some of the dissolved mineral salts in water may precipitate out when the water is heated. They form the crusty deposits at the bottom of a teakettle. Although small amounts of impurities help to seed the formation of ice crystals, larger amounts make freezing more difficult. The fact that heating removes some of these impurities may cause heated water to freeze faster than cold.

Scientists have not entirely figured out why the Mpemba Effect is observed in some experiments but not in others. What seems like a simple problem depends on many factors: the initial temperatures of the hot and cold water; air currents in the freezer; the shape and material of the containers; and the composition of the water. Science fair project, anyone?

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