YSM Issue 87.4

www.yalescientific.org
Unsolved Mysteries The Mpemba Effect
?
BY KEVIN SALINAS
Boiling water freezes more quickly than does water at This theory was one
room temperature. Discovered in 1969 and labeled the of the first proposed,
“Mpemba Effect,” this phenomenon remains a mystery to but it became one of
scientists. The Mpemba Effect went relatively unnoticed the first in a long chain
throughout early human history. References by Aristotle of theories to be tested
and René Descartes demonstrate a vague awareness of the since 1970. During initial
effect, but provide no explanation.
experiments conducted,
Since the Mpemba Effect caught the attention of a Mpemba, working with Dr.
Tanzanian high school student nearly 50 years ago, various Osborne, compared various
attempts have been made to understand the phenomenon. heated beakers of water to
Results from different experiments vary, and no single those that were not heated. They
theory has been substantial. Two major explanations found that evaporation causes only
involve the processes of evaporation and supercooling, slight changes in volume. No more than 30 percent of the
though neither has sufficient empirical support.
cooling could be attributed to this volume loss. There had
The discovery of the Mpemba Effect is a story of its to be other causes for the effect, they both concluded.
own, taking place before the great scientific debates that On the other hand, supercooling is a theory based on
this bizarre phenomenon has sparked. Erasto Mpemba was the fact that water temperature sometimes drops below
making ice cream one day in high school. He mixed boiled the freezing point before it turns into ice. Professor James
milk and sugar just as the rest of the class did, but rather Brownridge at Binghamton University found that water in
than waiting until the milk cooled, Mpemba put his hot sealed containers will almost always drop below the freezing
milk in the refrigerator right away. He later checked on his temperature before becoming ice. He also found that the
bag and realized that it became ice cream before any of the magnitude of supercooling is smaller for hot water than it
other bags. His teacher told him that this should not have is for cool water. In other words, hot water does not have to
happened, and dismissed the episode. But Mpemba would drop as far below the freezing point, and thus it will freeze
not take that as an answer.
more quickly than cold water. There are no explanations as
Soon after this incident Dr. Denis Osborne, a physics to why hot water supercools to a lesser extent than water at
professor, visited Mpemba’s school. He did not have an room temperature, which only adds more to this mystery.
answer when Mpemba asked why hot water freezes faster Despite many attempts and explorations, scientists have
than cold water. However, he was so intrigued by the fallen short of finding a definitive explanation for the
question that he began to work with Mpemba to solve the Mpemba Effect.
conundrum. This was more than 40 years ago.
Still, the Mpemba Effect remains a very real phenomenon
Since then, many scientists have searched for an with impacts outside of the lab. Besides Mpemba’s ice
explanation, but have failed to reach a consensus. One cream observation, this effect can be seen in other aspects
theory is based on the process of evaporation. Hot water of daily life. The recent fad of making “instant snow” on
will evaporate more quickly than cold water, with a greater chilly days by throwing boiling water up into the air is a
change in volume in a given time. Evaporation causes heat manifestation of the Mpemba Effect. Room temperature
loss, and lower mass also makes it easier to reduce the water thrown up into cold air results in nothing more than
temperature of water and to get the water to freeze. Thus, the water falling to the ground. Despite such interesting
some scientists theorized that rate of evaporation may be observations, the Mpemba Effect remains an unsolved
at the root of the Mpemba Effect.
mystery.
October 2014
ART BY RACHEL LAWRENCE
Yale Scientific Magazine
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