How the Mpemba Effect Makes Hot Water Freeze Faster Than Cold

A Tanzanian Student's Ice Cream Question That Stumped Physicists

In 1963, a 13-year-old Tanzanian student named Erasto Mpemba noticed something strange during a cooking class. Students were making ice cream by boiling milk with sugar and then freezing it. Mpemba, rushing to claim the last spot in the freezer, skipped the cooling step and placed his hot mixture directly inside. His batch froze before his classmates' cooled mixtures did. When he asked his physics teacher to explain, the teacher told him he must be confused. Mpemba was not confused. He kept asking.

Years later, physicist Denis Osborne visited Mpemba's school. Mpemba asked the question again. Osborne tested it in his lab, confirmed the result, and in 1969 the two published a joint paper in the journal Physics Education. The Mpemba effect had a name. It still does not have a universally accepted explanation.

The Basic Observation

Under specific conditions, a sample of hot water placed in a freezer will form ice before an identical sample of initially cooler water. This contradicts the intuitive expectation that the cooler sample, having less thermal energy to lose, should always freeze first. The effect is not guaranteed—it depends on container geometry, water volume, freezer temperature, and dissolved gas content. But when it occurs, the result is unmistakable.

• Hot water (around 70-90°C) can freeze before lukewarm water (around 25-35°C)
• The effect is more pronounced in open containers than sealed ones
• Results vary significantly between experimental setups
• Aristotle described the same phenomenon around 350 BCE
• Francis Bacon and René Descartes also noted it in the 17th century

Five Competing Explanations

No single theory has achieved consensus. Each proposed mechanism explains part of the observation but fails under certain conditions.

The Hydrogen Bond Hypothesis

In 2013, a team from Nanyang Technological University in Singapore proposed that the key lies in the peculiar behavior of hydrogen bonds in water. Water molecules form a dynamic network of hydrogen bonds that constantly break and reform. When water is heated, the covalent O-H bonds within each molecule stretch and store energy as the hydrogen bonds between molecules weaken.

During cooling, this stored energy is released faster than expected, effectively giving hot water a shortcut through the cooling curve. The team published their work in the journal Scientific Reports. Critics argued that the energy differences were too small to account for the macroscopic freezing time difference. Supporters countered that billions of molecules acting collectively could amplify the effect.

A 2020 quantum mechanical simulation added support. Researchers showed that the distribution of hydrogen bond configurations in heated water differed from never-heated water even at the same temperature—a form of structural memory.

The Reproducibility Problem

The Mpemba effect's biggest challenge is inconsistency. A 2016 study by Burridge and Linden at the University of Cambridge found that the effect occurred in some trials but not others, even with identical setups. Their statistical analysis suggested the effect was real but depended on initial conditions with extreme sensitivity—a hallmark of chaotic systems.

• Container material (glass vs. metal vs. plastic) changes results
• Freezer temperature and airflow patterns matter significantly
• Water purity and dissolved mineral content affect nucleation
• Sample volume alters the balance between evaporation and conduction
• Precise definition of "freezing" (first ice crystal vs. fully solid) changes outcomes

In 2012, the Royal Society of Chemistry held a competition offering £1,000 for the best explanation of the Mpemba effect. They received over 22,000 entries. The winning entry emphasized supercooling as the dominant mechanism, but the judges noted that no entry provided a complete explanation.

Historical Observations Before Mpemba

Mpemba was not the first to notice. He was simply the most persistent.

That the observation spans over two millennia without resolution says something about both the complexity of water and the difficulty of defining "freezing" precisely enough to design conclusive experiments.

What the Mpemba Effect Reveals About Water

Water is the most studied substance on Earth and one of the least understood. It has over 70 known anomalous properties—expanding when it freezes, reaching maximum density at 4°C, having an unusually high heat capacity. The Mpemba effect may be another expression of water's fundamental strangeness, rooted in the quantum behavior of hydrogen bonds that makes water unlike any other liquid. Solving it fully may require understanding water itself at a level physics has not yet reached.