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The Mpemba effect is the observation that very hot liquids or colloids (such as ice cream) can freeze more quickly than colder ones, for similar volumes and surrounding conditions.

It is named after Erasto Mpemba, a Tanzanian teenager who studied it scientifically in the 1960s for the first time, along with Denis Osborne. The Mpemba effect was initially observed in only ice cream and water, and later in other colloids. It has been studied extensively in water, with mixed results, and some experiments finding no reproducible effect.

The definition of the Mpemba effect used in theoretical studies varies, making it difficult to compare experiments.

The effect is named after Tanzanian student Erasto Mpemba, who described it in 1963 in Form 3 of Magamba Secondary School, Tanganyika; when freezing a hot ice cream mixture in a cookery class, he noticed that it froze before a cold mixture.

Various effects of heat on the freezing of water were described by ancient scientists, including Aristotle: "The fact that the water has previously been warmed contributes to its freezing quickly: for so it cools sooner. He discussed the influence of stirring on the results of the experiment, noting that stirring the unboiled water led to it freezing at the same time as the previously boiled water, and also noted that stirring the very cold unboiled water led to immediate freezing.

Modern studies using freezers with well-understood properties have observed the Mpemba effect where water supercools before freezing. Considerable random variation was observed in the time required for spontaneous freezing to start, and Auerbach observed the Mpemba effect more frequently when the ambient temperature was between −6 and −12 °C (21 and 10 °F).

Some researchers have criticized studies of the Mpemba effect for not accounting for dissolved solids and gases, and other confounding factors. Even among experiments that agree on a definition and observe the Mpemba effect for some experimental setups, they often do not observe it for all setups and starting conditions. Their review noted that the large effects observed in early experiments had not been replicated in other studies of cooling to the freezing point, and that studies showing small effects could be influenced by variations in the positioning of thermometers: "We conclude, somewhat sadly, that there is no evidence to support meaningful observations of the Mpemba effect."

The original classroom observations of the Mpemba effect were of fresh ice cream, a colloid, freezing in a freezer. A generalized version of the Mpemba effect is "when a hotter system equilibrates faster than a colder one when both are quenched to the same low temperature." This has been modeled theoretically for simple systems such as single particles under Brownian motion. The possibility of a "strong Mpemba effect" where exponentially faster cooling can occur in a system at particular initial temperatures was predicted in 2019 by Klich, Raz, Hirschberg and Vucelja. In 2020 the strong Mpemba effect was demonstrated experimentally by Avinash Kumar and John Boechhoefer in a single-particle colloidal system. In 2022, that group also demonstrated an "inverse Mpemba effect" in a single-particle colloid where a cold system heats up much faster than a warmer one, under the right conditions.

In 2025, experimental observations by Zhang, et al. found a quantum strong Mpemba effect for a single trapped ion, and Chatterjee, et al. found the Mpemba effect occurs naturally during the cooling of nuclear spin states.

While the definition of the Mpemba effect used in theoretical studies varies, several explanations have been offered for its occurrence. In 2017, two research groups independently and simultaneously found a theoretical Mpemba effect and also predicted a new "inverse" Mpemba effect in which heating a cooled, far-from-equilibrium system takes less time than another system that is initially closer to equilibrium.

Distribution function: Strong deviations from the Maxwell–Boltzmann distribution can result in a Mpemba effect in gases and granular fluids.

Quick Facts

• The Mpemba effect was initially observed in only ice cream and water, and later in other colloids.
• In 2017, two research groups independently and simultaneously found a theoretical Mpemba effect and also predicted a new "inverse" Mpemba effect in which heating a cooled, far-from-equilibrium system takes less time than another system that is initially closer to equilibrium.
• In 2025, experimental observations by Zhang, et al. found a quantum strong Mpemba effect for a single trapped ion, and Chatterjee, et al. found the Mpemba effect occurs naturally during the cooling of nuclear spin states.
• It has been studied extensively in water, with mixed results, and some experiments finding no reproducible effect.
• The definition of the Mpemba effect used in theoretical studies varies, making it difficult to compare experiments.

Source material: Wikipedia - "Mpemba effect". Adapted and summarized for DiscoverScroll. Original contributors are credited through the linked Wikipedia article. Read original on Wikipedia. CC BY-SA 4.0. Changes were made from the original.