What Is the Mpemba Effect and Why Does Hot Water Freeze Faster Than Cold?

The 1963 Ice Cream Experiment: Erasto Mpemba’s Discovery

In 1963, a 13-year-old Tanzanian schoolboy named Erasto Mpemba was making ice cream with his classmates at Magamba Secondary School [1,2]. To secure the last spot in the school freezer before it filled up, Mpemba skipped the standard cooling step and placed his boiled, hot milk-sugar mixture directly into the freezer next to his classmates’ pre-cooled mixtures [1,2].

An hour and a half later, Mpemba opened the freezer and discovered that his hot mixture had completely frozen into solid ice cream, while the colder mixtures remained liquid slush [1,2]. When Mpemba questioned his physics teacher, he was mocked: "That is Mpemba’s physics, not universal physics" [1,2]. Unshaken, Mpemba repeated the experiment with pure water and later challenged visiting University of Dar es Salaam physics professor Denis G. Osborne, leading to their seminal 1969 paper in Physics Education that formalized the Mpemba Effect [1,2].

"Mocked by his teacher for claiming hot milk froze faster than cold milk, 13-year-old Erasto Mpemba proved the effect in published peer-reviewed physics."

Historical Precedents: Aristotle, Bacon, and Descartes

While modern physics treated Mpemba’s claim with skepticism, the phenomenon had been observed by the foundational thinkers of Western science for millennia [2,3]:

1. Aristotle (c. 350 BC) noted in Meteorology: "The fact that the water has previously been warmed contributes to its rapid freezing; for it cools more quickly... thus many people, when they want to cool water quickly, place it first in the sun" [3].

2. Francis Bacon (1620) observed in Novum Organum: "Water slightly tepid freezes more easily than that which is quite cold" [3,4].

3. René Descartes (1637) wrote in Discourse on the Method (Les Météores): "We can also see by experiment that water which has been kept on the fire for a long time freezes faster than other water" [3,4].

Despite these centuries of observation, standard Newton cooling laws seemed to forbid the effect: if hot water at 80°C must cool down to 20°C on its journey to 0°C, how could it possibly overtake water that started at 20°C? [1,2,4]

"Aristotle in 350 BC, Francis Bacon in 1620, and René Descartes in 1637 all documented that pre-heated water froze faster than cold water."

The Modern Physics Solution: Supercooling and Non-Equilibrium Relaxation

For decades, scientists proposed various macroscopic mechanisms to explain the paradox: rapid evaporation (reducing the mass of water to be frozen), degassing of dissolved carbon dioxide and oxygen, and strong convection currents promoting faster heat transfer [2,4,5].

However, the definitive breakthrough came from modern condensed matter physics and non-equilibrium thermodynamics [5,6]:

1. Supercooling Differentials: Water rarely freezes at exactly 0°C; it supercools until a nucleation seed forms. Heated water degasses and alters hydrogen-bond cluster networks, often triggering ice crystallization at -2°C, whereas unheated cold water can remain supercooled down to -6°C without freezing [4,5].

2. Microscopic Optical Tweezer Proof (2020): In a landmark 2020 Nature study, physicists Avinash Kumar and John Bechhoefer demonstrated the Mpemba effect in a single microscopic colloidal bead trapped in an energy landscape [6,7]. When heated, particles explore higher-energy states that allow them to take a shortcut trajectory through thermal phase space, relaxing to equilibrium faster than particles that started closer to the cold state [6,7].

"In 2020, Nature experiments proved that pre-heated systems take topological shortcuts through energy phase space, relaxing to freezing faster."