The Mpemba Effect: Why Hot Water Can Freeze Faster Than Cold

Put a cup of hot water and a cup of cold water in the freezer, and common sense says the cold one freezes first. Sometimes it does not. The strange case of hot water beating cold water to ice is called the Mpemba effect, after a Tanzanian schoolboy who refused to let his teacher dismiss it, and it has kept physicists arguing for more than sixty years.

Erasto Mpemba and the ice cream mixture

In 1963 Erasto Mpemba was a pupil at Magamba Secondary School in Tanzania. During a cookery class he noticed that an ice cream mixture he had put into the freezer while still hot froze before the mixtures his classmates had cooled first. His teachers told him he must be mistaken.

Mpemba kept asking. When the physicist Dr Denis Osborne visited the school to give a lecture, Mpemba put the question to him directly. Osborne went back to his laboratory, tested it, and found that warm water in beakers inside a domestic refrigerator could indeed freeze first. The two published their results together in 1969 in the journal Physics Education, and the name Mpemba effect stuck.

An old puzzle, from Aristotle to Bacon

Mpemba was not the first to notice. More than two thousand years earlier, Aristotle noted that water which has been warmed beforehand cools more quickly. In the early 17th century the English philosopher Francis Bacon wrote that slightly tepid water freezes more easily than water that is utterly cold. In 1775 the Scottish chemist Joseph Black found that previously boiled water froze faster than unboiled water, even when he controlled for evaporation.

Why hot water might freeze faster

Scientists have proposed a long list of explanations, and more than one may be at work in a given freezer:

• Evaporation. Hot water loses some of its mass as vapour, leaving less water to freeze.
• Convection. Hot water circulates strongly, carrying heat to the surface, where most of it escapes.
• Dissolved gases. Heating drives out dissolved air, which changes how the water conducts heat and freezes.
• Supercooling. Cold water can drop well below 0 °C without turning to ice. If the cold sample supercools longer, the hot one can reach solid ice first.
• Frost and contact. A hot container can melt the frost beneath it and sit in better contact with the cold freezer shelf.
• Hydrogen bonds. In 2013 researchers at Nanyang Technological University in Singapore proposed that heating stretches hydrogen bonds between water molecules and stores energy in the covalent bonds inside them, which is then released faster on cooling.

In 2012 the Royal Society of Chemistry offered £1,000 for the best explanation. It received about 22,000 entries, and in January 2013 Erasto Mpemba himself announced the winner, Nikola Bregović, a chemist at the University of Zagreb in Croatia.

What recent experiments show

The effect is notoriously hard to reproduce. In 2016 Henry Burridge and Paul Linden of the University of Cambridge carefully measured how long water samples took to cool to 0 °C. Their study in Scientific Reports found no evidence of a meaningful Mpemba effect under those conditions. Small changes in where a thermometer sits, in container shape, or in what counts as "frozen" can turn a result around.

The story took another twist in 2020. Avinash Kumar and John Bechhoefer of Simon Fraser University in Canada published a study in Nature showing a strong Mpemba effect in a tiny colloidal system, a single microscopic particle in water whose motion was controlled with laser light. Under the right conditions, a hotter system really did cool exponentially faster than a colder one. Physicists now study Mpemba-like effects in many other systems, from magnetic materials to quantum particles.

Try it at home

For a kitchen experiment, use identical containers with the same amount of water, place them on the same shelf, and decide beforehand what you are timing: the first ice crystals, or the moment the water is frozen solid. Repeat it several times. Results vary from freezer to freezer, which is exactly why the puzzle has lasted so long.

Related reading

• Hot Water in Extreme Cold: What the Pale Cloud Shows
• Soap Bubbles in a Thompson, Manitoba Winter
• Frozen Bubbles at Mount Washington Observatory
• Supercooled water forming ice
• Falls Creek’s snow-depth story

Sources