Can Hot Water Freeze Faster Than Cold?

1. Mpemba’s Original Observation Was Ice-Cream Mix, Not Plain Water

The student was Erasto Mpemba. He saw that a mixture of milk and sugar froze faster if it went into the freezer hot[1]. Notice what he was actually cooling: ice cream mix, not water. The idea later became famous as “hot water freezes faster,” but it started with dessert.

A physics professor, Denis Osborne, met Mpemba and worked with him on experiments. In 1969 they published a paper together in the journal Physics Education. Osborne himself described the tests as crude[2]. Today the puzzle carries the student’s name: the Mpemba effect.

2. People Had Reported the Puzzle for Centuries

Mpemba was not the first to wonder whether hot water can freeze faster. Aristotle wrote about something similar more than 2,300 years ago, and Francis Bacon, centuries later, wrote that water a little warmed is more easily frozen than water that is quite cold[1][5]. Descartes is also named among those who noticed it[2].

So the question was old, the evidence was messy, and nobody had a clean test of whether hot water really beats cold. That is where the trouble starts.

3. Why Water Experiments Are So Hard to Compare

In 2016, two researchers, Henry Burridge and Paul Linden, looked over the earlier evidence and ran their own carefully controlled cooling experiments with water. They concluded that there is no evidence to support meaningful observations of the Mpemba effect. For two samples identical except for starting temperature, cooled under the same conditions to a set temperature, the hotter one took longer[3].

One reason the effect can seem real: where you put the thermometer matters. If the readings were compared at the same height in both cups, the effect did not appear, but measurements off by even a centimeter (0.4 in) could give false evidence of it[2].

The other slippery part is freezing itself. Water can dip below 0°C (32°F) and stay liquid, which is called supercooling. A researcher who disagreed with Burridge and Linden’s definition argued that the real effect is about which sample starts to form ice first, and that a warm sample might do that before a cooler one that is still supercooled[4]. Scientists disagree here, and it is a fair disagreement: the two sides are measuring slightly different things.

4. A Model System Where Mpemba-Like Cooling Is Reproducible

If water is too messy, try something simpler. In 2020, Avinash Kumar and John Bechhoefer of Simon Fraser University used lasers to position tiny glass beads[2] in water, which acted as a heat bath, and suddenly cooled the system to see how fast it settled[5]. Their results were reproducible and matched a theory proposed a few years before. With carefully chosen settings they saw cooling that was exponentially faster than with typical settings[5].

Be careful with the words “hot” and “cooling” here. Nobody heated a glass bead like a cup of tea and put it in a freezer, and nothing froze. The experiment is a “thermal quench”: a sudden change of the surrounding water’s temperature, after which the scientists watched how quickly the beads’ behavior settled into its final state[5]. “Starting hotter” describes the starting state of that carefully controlled system, not a hot object in everyday life.

In plain words: in this tiny, controlled system, starting from a “hotter” state could mean reaching the final state sooner. The authors say this supports the idea that the Mpemba effect is not only about how water freezes, but an example of a wider kind of unusual settling-down behavior[5]. It shows the pattern can exist. It does not show that water freezing works the same way.

5. Does the Effect Really Happen in Water?

That is still open. The bead trials give insight into how the effect could arise in systems with certain kinds of in-between states, but whether that is the only way, or whether it happens in water at all, is an open question[2]. For water itself, sloppy setups and supercooling appear to explain decades of confusion[1], while the careful 2016 tests found no effect. Writers differ in how much weight they put on the doubt, so it is worth reading the sources and deciding how strong the claims should be.

So the answer to the title question is not simply “yes” or “no.” The teacher’s flat “that cannot happen” was too strong as a description of what scientists know, and “hot water always freezes first” has no good support either. What remains is a question that can be tested, argued over, and split into better questions.

6. Try It Yourself: Do Two Identical Cups of Cold Water Freeze Together?

No hot water is needed. Ask an adult to help, and use cold tap water in two identical cups (plastic or paper is fine) with the same amount in each. Put them side by side in the freezer, and check a few times, about every 15 to 20 minutes, writing down when you first see ice at the surface. Then repeat the exact same test on another day.

Do both cups freeze at the same moment? Does the answer change on the second try? If the same test gives different times, think about what that means for anyone trying to compare hot and cold water with a stopwatch. Then look up the “Sources” below and see what the researchers did about it.

Sources

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