Hot Water Freezes Faster Than Cold Water — And We Don't Fully Know Why
A Boy, a Teacher, and a Mystery
In 1963, a Tanzanian student named Erasto Mpemba was making ice cream in class. He noticed that hot milk mixture placed in the freezer solidified faster than the cold mixture made by his classmates. When he reported this to his physics teacher, he was ridiculed. But he was right.
Mpemba later worked with physicist Denis Osborne to study the phenomenon, and in 1969 they published a paper formally describing what is now called the Mpemba effect.
What Makes It So Puzzling?
Under standard intuition, a colder liquid should freeze faster — it has less distance to travel to reach 0°C. Yet experiments repeatedly show that under certain conditions, water starting at 70°C or even 90°C will freeze before water starting at 30°C.
It defies the most basic expectation of thermodynamics, which is why it took so long to be taken seriously.
Proposed Explanations
Over the decades, several explanations have been put forward:
- Evaporation: Hot water loses mass through evaporation, leaving less water to freeze.
- Dissolved gases: Heating drives out dissolved gases that might otherwise slow freezing.
- Convection currents: Hot water develops stronger convection patterns that may accelerate cooling.
- Hydrogen bonding: A 2016 study suggested hot water's hydrogen bonds stretch in a way that stores more energy, releasing it faster during cooling.
No single explanation has been universally accepted. The effect may not even be universal — it appears under specific conditions and not others, adding to its mystery.
