Why hot water can freeze faster than cold water
A Tanzanian schoolboy insisted in 1963 that his observation was right. Physicists are still arguing about why.
In 1963 a secondary-school student in Tanzania, Erasto Mpemba, noticed something that looked like a textbook error: his warm ice-cream mixture froze faster than the cold one. His teacher told him he was imagining things. Ten years later the same observation appeared in a physics journal, signed by him and a university physicist.
What is actually happening
The effect is not universal — it appears only under specific conditions. Hot water evaporates more, so there is less water left to cool. Convection currents in the warm liquid carry heat to the surface more efficiently, and dissolved gases escape differently, which changes how ice crystals nucleate.
A fourth, subtler factor is supercooling: cold water can stay liquid below 0 °C without a nucleation point, then freeze abruptly. Measurements show the effect depends on the container, on impurities and on the starting temperature — which is why experiments have contradicted each other since the 1970s.