Accidental Origin of the Mpemba Effect in Freezing: Reversals in the Freezing Order Are Stochastic—Experimentation and Theory

The "Mpemba effect" is the name given to the assertion that hot water may freeze faster than cool water. This counterintuitive statement seemingly breaches fundamental laws of physical chemistry; however, numerous experiments with classical and quantum systems have apparently confirmed it, sparking debate in prominent scientific journals. Here we report on water freezing experiments conducted under carefully controlled conditions; they show that the Mpemba effect in water freezing exists as a stochastic, that is, accidental, rather than deterministic phenomenon, which – as the theory shows – is due to the fundamental uncertainty in the onset time of a first-order phase transition (like freezing) close to the temperature of nucleation of this transition. This uncertainty results in a huge scattering of freezing onset times (both for initially cool and initially hot water), so that, close to the temperature of ice nucleation, initially hot water sometimes freezes faster and sometimes slower than initially cool water (whereas at lower temperatures, this almost never happens). We anticipate our assay to be a starting point for reconsidering the famous Mpemba paradox in freezing water and other systems undergoing first-order phase transitions.