Does Hot Water Freeze Before Cold?

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When a secondary school student said his hot ice cream mix froze before the cold one, the class laughed and his teacher called it "your physics, Mpemba physics". Sixty years later the most careful measurements made on water did not vindicate the student — but laser-trapped glass beads and a single ion did. The answer to the question is neither yes nor no; the question itself was asked wrong.

The science channel's own config (`podcast-science.json`: `"language": "en"`, `kaynak_kanal: bilim`) and its hard constraint make this an English translation of today's `bilim.txt`, not a new Turkish text — so I'm delivering it in English, with numbers spelled out (the `en` path skips the Turkish number normalization in `kanal-render.ps1:56`).

In nineteen sixty-three, in the Tanga region of northern Tanzania, a group of students between thirteen and fourteen years old were making ice cream in the cookery room of Magamba Secondary School. The recipe was simple: boil the milk, stir in sugar, wait until it cools to room temperature, then put it in the freezing compartment of the refrigerator. There was only room for a few containers in that compartment, and every week the students went through the same race for space. That day a student named Erasto Mpemba decided not to wait his turn. He mixed the boiling milk with sugar and put the container straight into the freezer while it was still steaming.

An hour and a half later, when the freezer was opened, Mpemba's mixture had frozen. The containers that had been cooled first were still liquid.

This was not supposed to happen. To cool down, a hot body must first pass through the temperature of the cold body; by the time it gets there, you expect it to still be behind, or at best to have caught up. When Mpemba described what he had seen in his physics lesson, his teacher made fun of him. This was not physics, the teacher said; it was, at most, Mpemba's physics. His classmates laughed too. But Mpemba did not let it go. He asked about the method used by an ice cream seller in a nearby town, and the man told him he put the milk in hot, because that way it froze more quickly.

A few years later, while Mpemba was studying at Mkwawa High School, a physicist came to the school from the university in Dar es Salaam. After Denis Osborne finished his talk, during the questions, Mpemba stood up and asked his question: if I take two equal volumes of water, bring one to thirty-five degrees and the other to one hundred degrees, and put them both in the freezer, why does the one at one hundred degrees freeze first? The hall laughed again. Osborne did not. He said he did not know the answer, and decided to try it in the laboratory when he got back. He asked a technician to set up the experiment. When the technician brought him the results, he was surprised: under some conditions, hot water really did freeze first. Osborne repeated the experiment himself, refined the measurements, and in nineteen sixty-nine published a short paper, co-signed with Mpemba, in Physics Education, the journal put out by the Institute of Physics. From that day on, the oddity carried the surname of a schoolboy who had been laughed at in class: the Mpemba effect.

The curious part is that the observation was not new. Aristotle, in his book on the phenomena of the weather, had written that people who wanted to cool water quickly first left it standing in the sun. Francis Bacon, in his work of sixteen twenty, Novum Organum, noted that lukewarm water freezes more readily than ice-cold water. Descartes made a similar remark. So the observation had been drifting around for centuries, had been written down, and had then been forgotten. Once modern physics had settled its concepts of temperature, heat and heat transfer, folk observations of this kind were no longer taken seriously, because the theory said it could not happen.

What Mpemba did was not a discovery; it was an act of persistence. With nobody willing to agree with him, he pushed hard enough to turn his observation into a defensible question. He never became a physicist. He went to the wildlife management college at Mweka, worked as an officer in Tanzania's game and forestry administration, taught, and died in twenty twenty. His name entered the physics literature because, once in his life, he asked a question.

But an effect having a name is not proof that it exists. And for the next fifty years, the Mpemba effect remained one of the most stubbornly elusive phenomena in physics.

The problem was this: whoever ran the experiment, somebody else could not get the same result.

Some researchers reported that hot water froze first. Some found no difference at all. Some found exactly the opposite. Trials carried out in the same laboratory, with the same equipment, in the same week, could contradict one another. That amounted to a lasting failure on the most basic test of whether a phenomenon is real: reproducibility.

Over the years the proposed explanations grew into a list, and every one of them sounded plausible. Evaporation: boiling water evaporates rapidly in a freezer, loses part of its mass, and less water is left to freeze. This is a genuine effect and it can be measured in open containers; but when the containers were sealed the difference usually disappeared, and yet in some experiments it showed up in sealed containers too. Dissolved gases: boiled water holds less dissolved air, which can change how it conducts heat and how it freezes. Measurements showed that this difference was not large enough to account for the effect on its own. Convection: strong currents form inside hot water, the distribution of temperature inside the container is completely different from that in a cold one, the surface stays hotter, and heat loss from the surface speeds up. This was perhaps one of the strongest candidates, but it was not sufficient by itself. The layer of ice: if there is a thin coating of frost on the freezer shelf, the hot container melts the frost beneath it, sticks to the shelf and makes direct contact with the metal, while the cold container goes on sitting on an insulating bed of frost. This explained very well why the effect turns up in certain kitchen experiments, but what it explained was a property of the freezer shelf, not a property of water.

And underneath all of them lay a more insidious problem: what does it mean to freeze? The appearance of the first ice crystal? All of the water dropping to zero degrees? The entire contents of the container turning solid? Water can pull a trick called supercooling: it can go below zero, sometimes down to minus ten degrees, while remaining liquid, and then freeze all at once when something triggers it. When that trigger happens is largely a matter of chance, depending on a speck of dust inside the container, a scratch, a vibration. In other words, which of two identical containers froze first was already close to a throw of the dice, quite independently of the effect anyone was trying to measure.

In twenty twelve the Royal Society of Chemistry turned the question into a public competition, and the call for the best explanation drew more than twenty-two thousand entries from over one hundred countries. The winning submission came from Nikola Bregović, a chemist working in Zagreb, whose analysis combined convection and supercooling. But the competition itself looked rather like a confession: fifty years on, there was still no agreed explanation.

The real blow came in twenty sixteen. Two fluid physicists, Henry Burridge and Paul Linden, working in London and Cambridge, came at the matter from an unusual direction. They began by asking what the precise definition of this effect was. The definition in the literature ran like this: water that starts hotter freezes before water that starts colder. They translated that into a measurable statement and looked for the point where the cooling curves of two samples crossed. Then they ran a large number of experiments.

What they found was less about whether the effect existed than about how fragile the measurement was. The time difference observed between two samples was extremely sensitive to exactly where inside the container the thermocouple measuring the temperature was sitting. Moving the thermometer by one centimeter was enough to wipe out the measured difference, or to flip its sign. In other words, the difference that people had been reporting as the Mpemba effect for decades was, in most cases, of the same order of magnitude as the temperature distribution inside the container, the position of the sensor and the noise in the measurement. They published their results in Scientific Reports and reached this conclusion: under ordinary conditions there is no reliable, reproducible evidence that water freezes faster when it is hot.

This looked like the end of the story. A centuries-old kitchen observation had evaporated under careful measurement.

But at exactly this point the question changed hands. Physicists stopped looking at water and looked at the question itself.

You can ask the following question without any reference to water at all: is it possible, in principle, for a system setting out from a hot initial state to reach equilibrium faster than the same system setting out from a cold initial state?

Posed that way, the issue is not when ice forms, but how a system relaxes towards equilibrium. We are used to representing the state of an object with a single number, its temperature; but the true state of a many-particle system is a probability distribution over the possible arrangements of those particles. Cooling is the journey of that distribution, in an abstract space, from one point towards another point corresponding to cold equilibrium. And just as on a map, a route starting from a distant point can take less time than a route starting from a nearby one, provided it lies along a more direct road.

In twenty seventeen, Zhiyue Lu and Oren Raz turned this intuition into mathematics. As a system goes to equilibrium it does so as the sum of a series of components that decay at different rates, and the slowest-decaying component is the one that holds the process back over the long run. Lu and Raz showed that for a specially chosen initial state the contribution of that slowest component can be zero. The system then falls to equilibrium through the fast components alone, never using the slow road at all. This opened the door that a hot start needed in order to overtake a cold one, and on one side of the scales there was now not a kitchen anecdote but nonequilibrium statistical physics.

Next came the demonstration. At Simon Fraser University in Canada, Avinash Kumar and John Bechhoefer set water and freezers aside entirely. Their experiment was a single micron-scale glass bead suspended in water. They placed the bead inside an optical trap built from laser light, and shaped the intensity distribution of that light so that the bead sat within an energy landscape with two wells, one deeper than the other. In such a landscape the bead moves back and forth between the wells at random and, over the long run, settles into a particular probability distribution. That distribution is what serves as the temperature of this tiny system.

The experiment ran like this: start the bead off with distributions corresponding to different temperatures, then release them all into the same cold bath and measure how long each takes to settle into equilibrium. The path of a single bead is noisy, so they repeated the experiment thousands of times and gathered statistics. The result was published in twenty twenty in Nature: under certain initial conditions the system reached equilibrium many times faster than a system that started colder. What is more, this was not a difference barely picked out of the noise, but a speed-up that separated exponentially, exactly where the theory had said in advance it would be. In twenty twenty-two the same group measured the reverse as well: under certain conditions a system with a cold start can also get ahead while heating up.

After that things moved quickly. In twenty twenty-four, in a quantum simulator built from trapped ions, a similar speed-up was observed in the return of quantum systems to their symmetries. The Mpemba effect is no longer studied as a quirk belonging to water, but as a general behaviour that can arise in a wide variety of systems with relaxation dynamics.

So the honest answer to the question has two layers. In the freezer in your kitchen, you cannot count on hot water freezing before cold; careful measurements show that this is not a reproducible phenomenon, and that the differences people observe come mostly from details such as evaporation, frost, where the container is placed, the randomness of supercooling and the position of the sensor. But the physics behind the question, the question of whether a hot start can reach equilibrium sooner, is not a fake. It has been measured in a properly constructed system, it was calculated in advance, and it has a theory of its own.

This is a kind of success that science rarely talks about. When an observation turns out to be wrong, the story usually ends there. Here the observation, at least in the form in which it was claimed, was not confirmed; and yet the question itself opened a door nobody had expected. If physicists today discuss shortening a system's path to equilibrium, or speeding up cooling through engineered initial conditions, then somewhere in the family tree of that discussion stands a secondary school student who opened a freezer door in nineteen sixty-three and saw that his own container had frozen.

Mpemba was not proved right. But his question outlived its answer.

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