Hot Water Can Freeze Faster Than Cold Water - And Science Still Debates Why

#1. The Tanzanian Schoolboy Who Challenged Physics

In 1963, a 13-year-old Tanzanian student named Erasto Mpemba was making ice cream during a cookery class in Magamba Secondary School. Rushing to secure space in the dormitory freezer, Mpemba skipped the standard step of cooling his boiled milk-and-sugar mixture to room temperature, placing the steaming mixture directly into the icebox alongside his classmates' cold mixtures.

To the astonishment of the class, Mpemba's hot mixture froze into ice cream first. When he questioned his teachers, he was mocked for having 'Mpemba's physics.'

Undeterred, Mpemba later asked visiting physicist Denis Osborne from the University of Dar es Salaam to explain the observation. Osborne tested it in his laboratory, confirmed the phenomenon, and in 1969 co-authored a foundational scientific paper with Mpemba, officially introducing the 'Mpemba Effect' to modern thermodynamics.

#2. Historical Precedents: From Aristotle to Descartes

While named after Mpemba, the puzzle is centuries old. In the 4th century BCE, Aristotle noted in Meteorologica that 'the fact that the water has previously been warmed contributes to its freezing quickly; for it cools sooner.'

In the 17th century, both Francis Bacon in Novum Organum and Rene Descartes in Discourse on Method reported similar observations. Yet because the phenomenon contradicts simple Newtonian cooling - which posits that hot water must pass through the cold water's initial temperature before freezing - many modern scientists remained skeptical.

#3. Competing Scientific Explanations

Decades of controlled experiments have proposed several competing and complementary thermodynamic mechanisms:

1. Evaporative Mass Loss: Hot water evaporates vigorously. By the time it reaches freezing temperature, there is significantly less mass of water remaining to freeze, while the latent heat of vaporization carries away immense thermal energy.

2. Convection Currents: Hot water establishes vigorous internal thermal convection currents that distribute heat rapidly to the container edges, accelerating cooling efficiency compared to stagnant cold water.

3. Dissolved Gases: Heating water expels dissolved carbon dioxide and oxygen. Degassed water has altered thermal conductivity and altered nucleation kinetics for ice crystal formation.

4. Hydrogen Bond Covalency: In 2013, researchers at Nanyang Technological University proposed that heating water stretches and relaxes anomalous covalent O-H bonds, storing potential energy that releases rapidly upon cooling, facilitating faster crystallization.

#4. Non-Equilibrium Thermodynamics and Beyond

In 2020, researchers at Simon Fraser University used optical tweezers to monitor microscopic colloidal particles cooling in water baths, demonstrating an exact mathematical analogue of the Mpemba effect in microscopic non-equilibrium systems.

Their findings proved that a system starting in an agitated, highly disordered high-temperature state can take a geometric shortcut through thermodynamic phase space, reaching equilibrium faster than a system starting closer to the target.

No. The Mpemba effect is sensitive to boundary conditions: container geometry, air currents, evaporation rates, and dissolved minerals all influence whether the hot sample will overtake the cold sample.

Yes. By placing identical thin glass beakers - one with freshly boiled water and one with tap water at room temperature - into a home freezer, many experimenters can observe the hot beaker forming ice crystals first, though convection air currents in home freezers can cause variance.

Yes, physicists have documented Mpemba-like anomalous cooling shortcuts in carbon nanotubes, magnetic alloys, and granular fluids.