The Mpemba Effect: Does Hot Water Really Freeze Faster Than Cold Water?

By: Dylan Ris  | 
Mpemba effect
Throw a pot of boiling water up in the air during the winter and you will get a beautiful result, as long as it's cold enough. It's called the Mpemba effect. makieni/Shutterstock

For centuries, observant scientists from Aristotle to Descartes have harbored a suspicion that — contrary to all conventional wisdom — hot water can somehow freeze faster than cold water. But there was no scientific consensus that this conjecture was actually true.

In 1963, a Tanzanian physics student named Erasto B. Mpemba (pronounced em-pem-ba) rekindled the idea via a fluke accident that occurred when he was making ice cream at his school. He seemed to prove what Aristotle and Descartes had suspected: Hot water reaches a freezing point faster than cold water does. He wrote about his observations in a 1969 paper, titled simply "Cool?" which gave rise to the term "Mpemba effect." But was Erasto Mpemba correct? Does hot water really freeze faster than cold water?

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What Is the Mpemba Effect?

The Mpemba effect is a physics concept that postulates that when hot water and cold water are placed in the identical freezing environment, the hot water will freeze faster than the cold water.

Erasto Mpemba noted that when his class was making ice cream, he placed a near-boiling blend of sugar and milk (which is mostly water) into a freezer, and it froze before other mixtures which had been cooled to room temperature before freezing.

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Mpemba's extrapolation from this observation was that when identical volumes of water — one at 212 degrees Fahrenheit (100 degrees Celsius) and the other at 95 degrees Fahrenheit (35 degrees Celsius) — were placed in identical beakers and put in a freezer, the 212 degree water would turn to ice faster. Mpemba's ice cream observation and water postulation aligned him with many centuries of scientists who had also suspected this unusual property of water.

Understanding the Freezing Process

When water freezes into ice, it undergoes a phase change; it turns from a liquid into a solid. Physicists traditionally declare the phase of a substance when it's at equilibrium. This means the substance is in a stable state, and significant amounts of energy are not flowing from one region to another. It also means that its volume and temperature remain steady. When a substance is not at equilibrium, its energy levels fluctuate, and so does its state of matter.

For water to freeze and stay frozen, individual water particles have to reach equilibrium. If too much energy surges through nonequilibrium water, it will fluctuate between solid and liquid (at low temperatures) or liquid and gas (at higher temperatures). The sooner that water particles reach equilibrium at low energy levels, the sooner they can freeze.

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Mpemba effect
This graph of freezing rates for two water samples demonstrates the Mpemba effect.
Wikimedia Commons (CC BY 3.0)

How Can Hot Water Freeze Faster Than Cold Water?

Physicists are still debating whether hot water consistently freezes faster than cold water. When it does happen, certain conditions have to be met.

When a vessel of water is submerged in a freezing environment, different parts of the water reach equilibrium at different times. Water around the outskirts of the vessel gets colder faster, which means that it may freeze while water in the middle of the vessel stays liquid. And when you specifically place a vessel of hot water in a freezer (like the 212 degree boiling water described by Mpemba), it is also releasing steam from the top of the vessel, and this decreases the total volume of water that needs to freeze.

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Furthermore, cold water (or even room temperature water) often develops a layer of frost on its surface as part of the freezing process. Ironically, this frost temporarily insulates the water (kind of like how an ice igloo insulates its inhabitants against cold air), which can slow down the overall freezing process. Hot water, at least in the early stages, blocks the formation of frost, which allows cold air to penetrate deeper into the vessel.

These are some of the ways that hot water can engender freezing faster than cold water can. But remember that for water to freeze and stay frozen, it must achieve a state of equilibrium.

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The Study That May Prove the Mpemba Effect

If there's proof that the Mpemba effect is real and consistent, it comes from a 2020 study by John Bechhoefer and Avinash Kumar. Published in the journal Nature, the study subjected microscopic glass beads to what they called an "energy landscape" controlled by lasers. The researchers heated beads to different temperatures. They then observed which of the beads first reached a state of equilibrium within that energy landscape.

Bechhoefer and Kumar observed that microscopic beads that started at high temperatures reached equilibrium faster than those that started at lower temperatures. That's interesting enough, but how does reaching equilibrium relate to freezing?

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The connection comes from prior work done by Zhiyue Lu of the University of North Carolina and Oren Raz of the Weizmann Institute of Science in Israel. Their paper, "Nonequilibrium thermodynamics of the Markovian Mpemba effect and its inverse," published in Proceedings of the National Academy of Sciences (PNAS) and described by Quanta Magazine, postulates that hotter systems of matter may be able to skip ahead in the process of reaching equilibrium, thus reaching a stable state faster than a colder system.

If relaxing toward equilibrium is a critical benchmark in the freezing process of water, then the combined work of Bechhoefer and Kumar along with Lu and Raz might prove the existence of a Mpemba effect.

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Is the Mpemba Effect a Proven Scientific Fact?

The Mpemba effect is not uniformly accepted as a proven scientific phenomenon. However, centuries of observation, plus recent work by Bechhoefer, Kumar, Lu, and Raz have convinced many physicists that under the right circumstances, hot water really can reach a freezing point faster than cold water.

Some scientists, like Harry Burridge and Paul Linden, remain skeptical. They acknowledge that while some vessels of hot water can freeze faster than equal-sized vessels of cold water, even the slightest shift in conditions erases the effect. Burridge and Linden's own 2016 study, "Questioning the Mpemba effect: hot water does not cool more quickly than cold," found that any proof of a Mpemba effect depended on the size of a water vessel and the placement of a thermometer. In a separate study, researcher James Brownridge found that impurities in a vessel of water (such as those in Mpemba's ice cream concoction) will alter the liquid's freezing point. While acknowledging there are times when hot water freezes faster than cold water, these scientists argue the phenomenon does not uniformly apply in nature.

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However other physicists, like Raúl Rica Alarcón of Spain's University of Granada, believe Bechhoefer and Kumar achieved a breakthrough. Alarcón and others believe it's significant enough to identify the exact conditions that cause the Mpemba effect. They argue that by describing the conditions in which hot water freezes faster than cold water, the scientific community has proven that the Mpemba effect is a legitimate natural phenomenon.

Mpemba effect
We thought it was worth just one more shot.
Scalia Media/Shutterstock

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