Erasto Mpemba and the Physics of Freezing
Newton had an apple, Erasto Mpemba had ice cream!
“Strawberry ice cream cone (5076899310)” by TheCulinaryGeek from Chicago, USA – Strawberry Ice Cream ConeUploaded by Mindmatrix. Licensed under CC BY 2.0 via Wikimedia Commons – http://commons.wikimedia.org/wiki/File:Strawberry_ice_cream_cone_(5076899310).jpg#mediaviewer/File:Strawberry_ice_cream_cone_(5076899310).jpg
If you asked most westerners where the big discoveries in Science have happened over the last 50 years, they’d probably point to Switzerland’s large hadron collider, a NASA facility pushing the frontiers of space exploration, or research universities like Stanford and MIT. But one of the most puzzling—and until recently, unexplained—scientific phenomena of recent years was discovered in a very humble setting: a middle school classroom in Tanzania.
In 1963, Erasto Mpemba was just 13 years old, and was a student at the Magamba Secondary School in eastern Tanzania. Making ice cream in class, he noticed that the recipes that were hot seemed to freeze faster than those that were cold. Excited, he told his physics teacher what he’d observed…
…and was promptly laughed out of the room. Not only did the teacher not believe Erasto, he taunted him, saying it wasn’t real physics, but “Mpemba physics.”
Little did he know then that Mpemba physics was very much real.
A few years later, Erasto had moved on to the Mkwawa High School in Iringa. Dr. Denis G. Osbourne visited the school from the University College in Dar es Salaam in order to give a lecture on physics. After the lecture, Erasto went up to Dr. Osbourne and asked him about the strange effect he’d observed, hoping the scientist could explain the phenomenon he’d observed (and was still being laughed at for believing). The doctor had no explanation, but agreed to test the results with Mpemba. He was fascinated to find that Mpemba was right; heated water froze faster than an equal volume of chilled water. They published the results of their study together in 1969, lending Mpemba’s name to the strange effect they were observing.
Mpemba moved on to study wildlife management, eventually attaining the position of Principal Game Officer for the Tanzanian Ministry of Natural Resources and Tourism, but his discovery remained a mystery. Yes, heated liquids seemed to freeze faster, but why? Did frost on the chilled water somehow “barricade” the liquid inside from freezing? Was evaporation sufficiently reducing the volume of the heated water to allow it to freeze faster? Or was something else entirely going on?
Recently, scientists based out of Singapore think they’ve found the answer, one that depends on the relative elasticity of the hydrogen bonds that form between nearby molecules vs. that of the covalent bonds that hold the oxygen and hydrogen atoms within a single water molecule together.
It was an important answer to a puzzling question; after all, the Mpemba effect seems to fly in the face of long-accepted science, specifically Newton’s Law of Cooling.
Fortunately for science, one Tanzanian student wouldn’t let ridicule, or accepted tenets, keep him from questioning the world around him. Thanks to Erasto Mpemba and his eponymous effect, we understand a little bit more about the world than we did before!