Almost every physical process runs the way intuition expects. Hot things cool. Cold things warm. Something that starts closer to a target reaches it first.
Which is why this particular claim causes so much trouble. If you take two identical containers, fill one with hot water and one with cold, and put both into the same freezer, the hot one will sometimes freeze first.
That should not happen. The hot water has to pass through the temperature of the cold water on its way down, at which point it is in the same state as the cold sample and has additional cooling still to do.
The observation is old and the argument about it is unresolved. What makes it interesting is not the explanation but the disagreement over whether there is anything to explain.
Where the Claim Comes From
The historical record is longer than most people expect.
Aristotle recorded the observation, and it appears intermittently in the writings of natural philosophers over subsequent centuries. It was folk knowledge among people who worked with water and ice.
It re-entered modern attention in 1963 through a Tanzanian secondary school student named Erasto Mpemba, who noticed while making ice cream that a hot mixture placed in a freezer set before a cooled one. He was told he was mistaken. He persisted, later put the question to a visiting physicist, and the two published on it together in 1969.
The effect carries his name, which is a reasonable outcome for someone who kept asking a question after being told it was wrong.
There is a preliminary point worth making about what the claim even is, because it is frequently stated too loosely.
Nobody argues that hot water always freezes faster. The claim is that under some conditions it can, which is a much weaker statement and correspondingly harder to test — a claim about specific circumstances requires identifying those circumstances, and no agreed specification of them exists.
That vagueness is part of the problem. An effect that appears sometimes, in unspecified conditions, is extremely difficult to distinguish from an effect that does not exist.
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The Explanations Proposed
A large number of mechanisms have been suggested, and several are physically plausible under the right conditions.
Evaporation is the oldest. Hot water loses mass to evaporation as it cools, so by the time it reaches the temperature of the cold sample there is less of it to freeze. This works and is generally too small an effect to explain the observed differences on its own.
Dissolved gases are another. Heating drives dissolved air out of water, and water with less dissolved gas may behave differently on freezing. The magnitude of the effect is disputed.
Convection currents are proposed. Hot water develops stronger circulation as it cools, which distributes heat to the surface more efficiently and may accelerate cooling. The temperature at the surface, rather than the average temperature, is what governs heat loss.
The frost layer explanation concerns the container rather than the water. A hot container placed on a frosted surface melts the frost beneath it and makes direct thermal contact with the shelf, while a cold container sits on an insulating layer of frost. This is a real effect and depends entirely on experimental setup.
Supercooling has been raised. Water can drop below freezing without solidifying, and if cold samples supercool more reliably than hot ones, the hot sample may begin freezing at a higher temperature and appear to win.
Hydrogen bonding accounts have been proposed, suggesting the arrangement of bonds in previously heated water differs in a way that affects freezing. These are contested and have not achieved acceptance.
The Problem With All of Them
The number of explanations is itself informative, and not in a good way.
Several of these mechanisms are real. Evaporation removes mass. Frost insulates. Convection affects heat transfer. None of that is in doubt.
The difficulty is that they depend on the experimental arrangement — the container, the surface it sits on, whether it is covered, the freezer, where the temperature is measured and what counts as frozen. Change any of those and the effect appears, disappears or reverses.
That is the signature of an artefact rather than a phenomenon. A robust physical effect should survive changes in apparatus; one that requires a particular setup is generally telling you about the setup.
There is a further definitional problem. What counts as frozen? First ice crystal appearing? Surface completely solid? Entire volume solid? Different criteria produce different answers from the same experiment, and studies have used different ones.
Why the Question Is Harder Than It Looks
There is a reason this has resisted resolution that has nothing to do with the physics and everything to do with what counts as evidence.
The claim is easy to test badly and difficult to test well. Anybody can put two containers in a freezer, and a substantial proportion of people who do will observe the effect at least once, because uncontrolled setups contain exactly the variables that produce it.
That produces a very large body of casual confirmation, none of which bears on the underlying question. A thousand kitchen observations of an artefact are still an artefact.
At the same time, the sceptical position is difficult to establish conclusively. Demonstrating that an effect fails to appear under controlled conditions does not prove it never occurs, only that it did not occur in those conditions – and defenders can reasonably reply that the controls removed the very circumstances the effect requires.
That is an unfalsifiable shape of argument, and it is where the discussion has been stuck for some time.
The Argument That It Is Not Real
This is where the subject becomes interesting, because the sceptical case is strong.
Careful experimental work has repeatedly failed to reproduce the effect under controlled conditions. When containers are matched, positions are controlled, evaporation is prevented, and freezing is defined consistently, the hot sample reliably takes longer — as thermodynamics predicts.
One influential analysis argued that the effect has never been demonstrated in a properly controlled experiment and that reported observations reflect uncontrolled variables. That paper generated substantial discussion and was not universally accepted.
Others maintain the effect is real under specific conditions and that demanding it appear under all conditions misunderstands the claim.
The honest position is that the phenomenon is really disputed among people who have studied it, that many reported instances are explicable by experimental variables, and that no single mechanism has been established. Anyone stating confidently that hot water freezes faster is going beyond the evidence, and so is anyone stating confidently that it never does.
What a Good Experiment Would Need
It is worth setting out what would settle this, because the requirements explain why it has not been settled.
The two containers would need to be identical in material, shape, mass and surface condition, since any difference alters heat transfer.
They would need to sit in identical thermal environments, which is harder than it sounds — a domestic freezer has substantial temperature variation between positions, and swapping the samples between runs is necessary to control for it.
Evaporation would need to be prevented or measured, since mass loss is a real mechanism and an uncontrolled variable.
The surface beneath would need to be identical for both and free of frost, eliminating the thermal-contact effect.
Freezing would need a single unambiguous definition applied identically to both, with temperature measured at the same depth and position in each.
And the whole thing would need repeating many times, since a rare effect appearing occasionally is not distinguishable from noise in a handful of runs.
Assemble all of that and the effect largely stops appearing, which is the sceptical case in a sentence. Assemble some of it and results vary, which is why the literature is inconsistent.
That is not a satisfying resolution, and it is a reasonable description of where the evidence stands.
Why This Case Is Worth Knowing
The value here is not the answer. It is the illustration.
A phenomenon can be reported for two thousand years, be widely believed, be reproducible by amateurs, and still be uncertain — because casual reproduction under uncontrolled conditions is not evidence about the underlying physics.
It also shows how a question can become harder rather than easier with attention. Better controlled experiments made the effect less reproducible, which is the opposite of what happens with a real effect and is exactly what happens with an artefact.
And it demonstrates that “several plausible mechanisms exist” is not the same as “the phenomenon is explained.” A collection of explanations, each valid under conditions that differ from each other, is frequently a sign that the thing being explained is not one thing.
There is a further point about how science handles this kind of case. Nobody is claiming the observers are lying. People do see hot containers freeze first, in kitchens, repeatedly. The dispute is about what that observation is about, and the answer may be the freezer rather than the water.
Which is an unsatisfying place to leave it, and is where the evidence currently sits. Aristotle noticed something, a schoolboy in Tanzania noticed it again, a great many people have noticed it since, and the physics community has not been able to agree on whether there is anything there.
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