Can hot water freeze faster than cold? Physicists still disagree.
In 1963 a Tanzanian schoolboy, Erasto Mpemba, noticed that his hot ice cream mixture froze before a cooler one. A visiting physicist doubted him, tested it and was surprised. Six decades on, the 'Mpemba effect' is real in some systems but notoriously hard to pin down in water.
Erasto Mpemba was a student at Magamba Secondary School in Tanganyika when he saw hot ice cream mix freeze faster than a cold one in a cookery class. Later, at another school, he asked the visiting physicist Denis Osborne why water starting at 100°C would freeze before water at 35°C. Osborne thought he was mistaken, but tried it in his lab and found the same thing. They published together in 1969.
The idea is far older than Mpemba. Aristotle noted that people warmed water in the sun to make it cool faster, Francis Bacon and René Descartes wrote about similar observations, and in 1775 Joseph Black found that previously boiled water froze more quickly than unboiled water. Proposed explanations for water include evaporation, dissolved gases, minerals coming out of solution when boiled, convection currents, hot containers melting an insulating layer of frost, and differences in how deeply the water supercools before it freezes.
The trouble is that results in water are inconsistent. Researchers do not even agree on what counts: when the first ice appears, or when the sample is frozen solid. Experiments must control container shape, impurities, dissolved gas, thermometer position and the freezer itself. A 2016 review of cooling to 0°C concluded there was no evidence for meaningful observations of the effect, while a 2024 study of small droplets found hot drops consistently froze faster.
Meanwhile, physicists have broadened the question to whether a hotter system can reach equilibrium faster than a cooler one when both are plunged into the cold. The answer there is yes in certain cases: a 'strong' version was demonstrated in a single colloidal particle in 2020, and similar effects have since been reported in quantum systems. A schoolboy's question has become a research field.
Source: Wikipedia — Mpemba effect · Text summarised from Wikipedia (CC BY-SA 4.0)
