Why Does Hot Water Sometimes Freeze Faster Than Cold? (Mpemba Effect)

The Mpemba effect, named after Tanzanian student Erasto Mpemba who reported it in 1963, describes situations where hot water freezes more quickly than cold water under specific conditions. Several mechanisms have been proposed: evaporation reducing the mass of hot water, dissolved gases escaping from heated water (changing its freezing behaviour), convection currents being stronger in hot water (distributing heat more efficiently), and differences in supercooling behaviour. A 2020 study suggested that hydrogen-bond dynamics in hot water may store energy differently. Despite numerous experiments, no single explanation is universally accepted, and the effect does not occur under all conditions.