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

In 1963, a Tanzanian student named Erasto Mpemba was making ice cream at school when he noticed something that contradicted everything his physics classes had taught him. The hot ice cream mixture he placed in the freezer solidified faster than the cold mixture a classmate had placed in at the same time. He brought the observation to his physics teacher, who dismissed it. He later brought it to a visiting professor, Denis Osborne, who found it interesting enough to investigate. Together they published a paper in 1969, and ever since, the phenomenon has been known as the Mpemba effect.

The observation is this: under certain conditions, hot water placed in a freezer will reach the freezing point and solidify before cold water placed in the same freezer at the same time. This directly contradicts the straightforward expectation that a liquid closer to freezing temperature should freeze first, since it has less cooling to undergo. And yet the effect appears to be real — reproducible in some experiments, though not consistently in all conditions — which is precisely why it has fascinated and frustrated physicists for over fifty years.

Why It Might Happen: The Competing Theories

No single explanation has achieved consensus acceptance among physicists, which is itself unusual for a phenomenon observed in everyday conditions. Several mechanisms have been proposed, each supported by some evidence and contested by other researchers.

Evaporation is the most frequently cited mechanism. Hot water evaporates more rapidly than cold water, losing mass in the process. If a container of hot water loses enough mass through evaporation before freezing, it will contain less water to freeze and may therefore solidify faster than a heavier container of cold water. This explanation has the advantage of being quantitatively tractable — the effect is measurable — but critics note that evaporation alone cannot explain all observed instances of the effect.

Convection currents offer another partial explanation. Hot water in a container develops strong convective circulation as it cools, with warm water rising and cool water sinking. This circulation accelerates heat transfer from the water to the container walls and from there to the freezer environment. Cold water, already close to the temperature at which convection weakens (water's maximum density occurs at 4°C, not 0°C), develops weaker circulation and may transfer heat less efficiently to its surroundings.

Dissolved gases represent a third candidate. Hot water that has been boiled contains significantly less dissolved oxygen and carbon dioxide than cold water, since heating drives these gases out of solution. The presence of dissolved gases affects water's thermal properties and freezing characteristics in ways that researchers are still quantifying.

A 2016 Study and Ongoing Controversy

In 2016, a team at the University of Zaragoza published a paper in Physical Chemistry Chemical Physics offering a hydrogen-bond-based explanation. Their argument was that the hydrogen bonds between water molecules store energy in a configuration-dependent way, and that hot water's molecular arrangement places the system in a state from which freezing can proceed more rapidly under certain conditions. The paper generated significant attention and equally significant criticism, with some researchers disputing the theoretical framework and others questioning whether the experimental conditions were sufficiently controlled.

The difficulty in studying the Mpemba effect is that it is highly sensitive to experimental conditions — container shape, volume, freezer temperature, water purity, and many other variables appear to influence whether the effect occurs in any given trial. This sensitivity makes reproducibility challenging and has led some physicists to question whether the effect is a single coherent phenomenon or a collection of distinct mechanisms that can each produce the same surface observation under different conditions.

What Mpemba's Story Tells Us About Science

The human dimension of this story is as interesting as the physics. Erasto Mpemba was a secondary school student in Tanzania when he made his observation. His teacher's dismissal of what he saw reflects a pattern in science history where anomalous observations by non-experts or students are filtered out by authority rather than investigated. Denis Osborne's decision to take the observation seriously and test it rigorously is the behavior that produced a fifty-year research program.

Mpemba went on to study wildlife management. The effect bearing his name remains unsolved. Both of these facts seem right for a story about the way science actually works: a teenager notices something strange, a professor takes it seriously, and generations of researchers argue about it without reaching consensus. The universe, it turns out, has more patience for unresolved puzzles than textbooks suggest.