The ice cream anomaly
In 1963, a 13-year-old student named Erasto B. Mpemba was in a cooking class at Magamba Secondary School in the Tanga Region of Tanzania. The students were making ice cream, which involved boiling milk, mixing it with sugar, and letting it cool before placing it in a shared refrigerator. With space in the freezer running out, Mpemba skipped the cooling step and placed his hot, boiled milk mixture directly into the icebox. An hour and a half later, he returned to find his mixture had frozen solid, while those of his classmates who had followed the instructions were still liquid.
When Mpemba asked his physics teacher to explain this, he was told it was impossible and that his "physics is not the universal physics". Despite the dismissal, Mpemba's observation was the first step in bringing a curious thermal anomaly to the attention of modern science. This counterintuitive phenomenon, that hot water can freeze faster than cold water, is the Mpemba effect.
From classroom question to scientific paper
A few years later, while attending Mkwawa High School in Iringa, Mpemba had the opportunity to question a visiting physicist, Dr. Denis G. Osborne from the University College in Dar es Salaam. After a lecture, Mpemba raised his hand and asked: "If you take two similar containers with equal volumes of water, one at 35°C and the other at 100°C, and put them into a refrigerator, the one that started at 100°C freezes first. Why?". Though initially skeptical, Osborne was intrigued.
Back at his university lab, Osborne had a technician test the schoolboy's claim. To his surprise, the experiments confirmed the observation. The two collaborated and, in 1969, co-authored a paper titled "Cool?" in the journal Physics Education. The publication detailed their experiments, showing that a 70 ml sample of water starting at 90°C could freeze before an identical sample that started at 25°C. This officially documented the Mpemba effect, a puzzle that still prompts scientific investigation today.
A thermodynamic debate
The Mpemba effect is not guaranteed to happen every time; it depends on a specific set of conditions, and scientists still debate the exact mechanisms behind it. Several physical processes are thought to contribute, often working in combination.
One leading explanation is evaporation. Hotter water loses mass more quickly through evaporation, leaving a smaller volume of water to freeze. Another factor is the presence of dissolved gases, like carbon dioxide and oxygen, which are less soluble in hot water. As water is heated, these gases are expelled, which can slightly alter the freezing point.
Convection also plays a part. A warmer liquid has more vigorous convection currents, which promotes a faster rate of heat transfer from the surface. Finally, supercooling—the process where water remains liquid below its normal freezing point of 0°C—is an important factor. Some studies suggest that previously heated water is less likely to supercool as much as water that was initially cold, allowing it to begin the freezing process sooner. No single theory has been universally accepted, making this accidental classroom discovery a persistent thermodynamic puzzle.