Hot Water Freezes Faster Than Cold Water — The Mpemba Effect

Quick Answer

The Mpemba effect is the observation that, under certain conditions, hot water can freeze faster than cold water. Named after Tanzanian student Erasto Mpemba, who noticed it in 1963 while making ice cream, the effect has been confirmed in numerous experiments but its cause remains debated. Proposed explanations include evaporation reducing the volume of hot water, dissolved gases escaping from hot water, convection currents, and supercooling differences. No single explanation has been universally accepted, making it one of the most surprisingly unresolved questions in physics.

The Student Who Challenged His Teacher

In 1963, a 13-year-old Tanzanian student named Erasto Mpemba was making ice cream in a cooking class at Magamba Secondary School. The recipe called for boiling milk with sugar, letting it cool, and then placing it in the freezer. Impatient and worried about losing his spot in the crowded freezer, Mpemba put his hot mixture directly in without letting it cool first.

To his surprise, his ice cream froze before those of classmates who had followed the instructions and let their mixtures cool. When Mpemba told his teacher about the observation, the teacher dismissed it. "That is not possible," Mpemba recalled being told. "You must have been confused."

Mpemba was not confused. He continued to experiment with the phenomenon and eventually had the chance to ask a visiting physics professor, Dr. Denis Osborne from University College Dar es Salaam, about it. Osborne was initially skeptical but agreed to test the claim. His experiments confirmed Mpemba's observation: under certain conditions, hot water did indeed freeze faster.

In 1969, Mpemba and Osborne published a paper in the journal Physics Education documenting the effect. The paper noted that the phenomenon had actually been described centuries earlier by Aristotle, Francis Bacon, and Rene Descartes, but had been forgotten or ignored by modern science. Mpemba's contribution was to bring it back to scientific attention and put his name on it permanently.

What Experiments Show

The Mpemba effect is not a universal rule — hot water does not always freeze faster than cold water. It occurs under specific conditions, and reproducing it reliably is part of what makes it so maddening for physicists.

In a typical demonstration, two identical containers are filled with water at different temperatures (say, 95 degrees Celsius and 25 degrees Celsius), placed in the same freezer, and the time to reach 0 degrees Celsius or to fully solidify is measured. In many trials, the hot water reaches the freezing point or solidifies first.

However, the effect is sensitive to experimental conditions: the container shape, the volume of water, the freezer temperature, whether the containers are covered, and the mineral content of the water all influence the outcome. Some experimenters reproduce it easily; others struggle to observe it at all. This inconsistency has fueled decades of debate.

A 2016 study found that the effect was statistically significant in about one-third of their experimental trials. This is neither rare enough to dismiss nor consistent enough to fully explain, which is exactly the kind of result that keeps scientists arguing.

The Competing Explanations

Multiple mechanisms have been proposed to explain the Mpemba effect, and the honest answer is that the true explanation is probably a combination of several factors, with different ones dominating under different conditions.

Evaporation. Hot water evaporates faster than cold water. This evaporation reduces the mass of the hot water sample, meaning there is less water to freeze. Less water freezes faster. In an uncovered container, this can be a significant effect — hot water can lose 10 to 15 percent of its mass before reaching the temperature of the initially cold sample.

Dissolved gases. Hot water holds less dissolved gas than cold water (this is why a pot of water releases bubbles as it heats, well before boiling). Less dissolved gas might change the water's thermal properties or nucleation behavior during freezing. Some researchers have found that degassed water behaves differently during the freezing process, though the mechanism is not fully understood.

Convection currents. Hot water develops strong convection currents as different parts of the volume cool at different rates. These currents increase heat transfer within the water and between the water and the cold environment, potentially accelerating the cooling process. Cold water, being closer to a uniform temperature, has weaker convection and may cool less efficiently despite starting closer to the freezing point.

Supercooling. Water does not always freeze at exactly 0 degrees Celsius — it can sometimes cool below 0 without forming ice crystals, a phenomenon called supercooling. Cold water may be more prone to supercooling than water that started hot (possibly because the hot water lost dissolved gases that would otherwise interfere with ice crystal nucleation). If the cold water supercools while the hot water does not, the hot water may form ice first.

Hydrogen bond effects. A 2013 paper by researchers at Nanyang Technological University in Singapore proposed that the anomalous behavior relates to the unique properties of hydrogen bonds in water. They argued that when water is heated, the covalent O-H bonds within water molecules store energy that is released rapidly during cooling, effectively giving hot water a faster cooling rate. This explanation generated significant attention but remains controversial.

Why It Is Still Unresolved

The Mpemba effect touches on something that seems like it should be simple — when does water freeze? — but is actually deeply complicated. Water is one of the most anomalous substances in chemistry. It expands when it freezes (most substances contract). Its solid form is less dense than its liquid form (ice floats). It has an unusually high specific heat capacity. It exists in at least 15 known crystalline forms of ice.

These anomalies arise from water's hydrogen bonding network, and they make water's behavior near the freezing point more complex than the behavior of almost any other common substance. The Mpemba effect likely involves the interplay of multiple anomalous properties simultaneously, which is why no single, clean explanation has emerged.

It is also a reminder that familiar everyday phenomena can harbor genuine scientific mysteries. We put humans on the Moon, we detected gravitational waves from colliding neutron stars, and we sequenced the human genome — but we still cannot fully explain why hot water sometimes freezes faster than cold water. Science is not a finished project.

Practical Implications

The Mpemba effect has limited practical application, but there are a few situations where it is relevant.

Some people claim that using hot water in ice cube trays produces clearer ice, because the dissolved gases that create cloudy ice are driven off by the heat before freezing. This does seem to work in practice, though the clarity improvement may owe more to the degassing than to the Mpemba effect per se.

In cold climates, there is a tradition of throwing boiling water into extremely cold air to create instant snow or fog (the water evaporates and instantly refreezes into tiny ice crystals). This is dramatic and visually spectacular, but it is driven by rapid evaporation rather than the Mpemba effect.

For most practical purposes — making ice, freezing food, cooling drinks — starting with cold water remains faster and more reliable. The Mpemba effect is real, but it is not reliable enough to change your kitchen habits. It is a scientific curiosity, not a life hack.