The Mpemba Effect

Under certain conditions, a container of hot water appears to freeze faster than an identical container of cold water placed in the same freezer. The observation is named after Erasto Mpemba, a Tanzanian student who noticed in the 1960s that his hot ice-cream mixture froze before a classmate's cooler one. What makes the effect unusual is that, more than sixty years later, physicists still disagree about whether it is even real. Some experiments seem to confirm it; others, run under controlled conditions, find no advantage for hot water at all. Several explanations have been proposed rather than settled on. Hot water evaporates faster, leaving less liquid to freeze. It can also lose dissolved gases more readily and develop stronger convection currents, both of which might speed up cooling. A more recent hypothesis focuses on hydrogen bonds: heating water may rearrange these bonds in a way that leaves the liquid slightly better positioned to form ice crystals once cooling begins. Part of the difficulty is that the outcome seems extremely sensitive to small details -- container shape and material, the exact definition of "frozen," even trace impurities in the water. Because those details are rarely controlled identically across experiments, researchers studying the same basic setup can still reach opposite conclusions.