does hot water or cold water freeze faster experiment

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

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Key Takeaways

The Mpemba effect suggests hot water can freeze faster than cold water under certain conditions, a phenomenon first observed by Aristotle.
Erasto B. Mpemba, a Tanzanian student, rediscovered this effect in 1963 while making ice cream, providing the first documented instance that led to further scientific investigation.
Although still debated among scientists, recent studies suggest the Mpemba effect may occur due to differences in how hot and cold water reach thermal equilibrium.

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?

What Is the Mpemba Effect?

Understanding the freezing process, how can hot water freeze faster than cold water, the study that may prove the mpemba effect, is the mpemba effect a proven scientific fact, the mpemba effect in context.

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.

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.

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 (potentially) 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.

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.

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.

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?

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.

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.

However other physicists, like Raúl Rica Alarcón of Spain’s University of Granada, believe these new datasets, such as those offered by Bechhoefer and Kumar, are significant. "My view is that the Mpemba effect can take place under some special circumstances," says Rica Alarcón, "but we are still trying to figure out what are the minimal conditions for this to happen."

Rica Alarcón notes that observances of the Mpemba Effect always involve drastic differences in temperature between a vessel of water and its surrounding environment. And, he adds, you can observe equally intriguing phenomena when you reverse the temperatures and place frozen ice into a hot environment.

The Mpemba Effect, says Rica Alarcón, "seems to be one of a large group of anomalous thermalization effects, which take place when a system is suddenly put in contact with a thermal bath at a different temperature." The Mpemba Effect describes a hot-to-cold phase change like "when you take a hot cup and you put it in the fridge or in the freezer." But cold-to-hot phase changes also invoke unusual results. "Interesting effects take place when you perform temperature quenches from cold to hot," says Rica Alarcón, "like when you put an ice cube into boiling water."

We know that many generations of scientists have observed hot water freeze with surprising speed. Rica Alarcón urges us to regard this process more holistically, and think about the Mpemba Effect as part of a broader phenomenon. "Thermalization," he explains, "can follow counterintuitive paths due to the fact that the processes take place out of equilibrium."

Just like fresh water, ocean water can freeze — it just does it at a lower temperature. Sea water freezes at about 28.4 degrees Fahrenheit (minus 2 degrees Celsius) because of its salt content, while fresh water freezes at 32 degrees Fahrenheit (0 degrees Celsius).

Frequently Asked Questions

What practical applications might the mpemba effect have, why isn't the mpemba effect consistently reproducible.

Please copy/paste the following text to properly cite this HowStuffWorks.com article:

does hot water or cold water freeze faster experiment

New Research

The Physics of Why Hot Water Sometimes Freezes Faster Than Cold Water

For decades, physicists have debated whether the phenomenon exists and how to study it

Theresa Machemer

Correspondent

The story goes that in 1963, Tanzanian high school student Erasto Mpemba was making ice cream with his class when he impatiently put his sugar and milk concoction into the ice cream churner when it was still hot, instead of letting it cool first. To his surprise, the confection cooled faster than his classmates’ had.

With the help of a physics professor, Mpemba performed additional experiments by putting two glasses of water, one just-boiled and one warm, in a freezer, and seeing which one reached the freezing finish line first. Often, the water with a higher starting temperature was the first to freeze. Their observations set off a decades-long discussion over the existence and details of the counterintuitive phenomenon, now called the Mpemba effect.

Now, new research published on August 5 in the journal Nature not only shows that the Mpemba effect does exist, but also sheds light on how it occurs, Emily Conover reports for Science News .

Rather than experiment on freezing water, which is surprisingly complicated to study, physicists Avinash Kumar and John Bechhofer of Simon Fraser University focused their sights—and lasers—on microscopic glass beads. They measured how the glass beads moved under very specific conditions in water and saw that in some circumstances, beads that started off very hot cooled faster than those that didn’t.

“It’s one of these very simple setups, and it already is rich enough to show this effect.” University of Virginia theoretical physicist Marija Vucelja tells Science News . The experiment also suggests that the effect might show up in materials other than water and glass beads. Vucelja says, “I would imagine that this effect appears quite generically in nature elsewhere, just we haven’t paid attention to it.”

If the freezing point is the finish line, then the initial temperature is like the starting point. So it would make sense if a lower initial temperature, with less distance to the finish line, is always the first to reach it. With the Mpemba effect, sometimes the hotter water reaches the finish line first.

But it gets more complicated. For one thing, water usually has other stuff, like minerals, mixed in. And physicists have disagreed over the what exactly the finish line is: is it when the water in a container reaches the freezing temperature, begins to solidify, or completely solidifies? These details make the phenomenon hard to study directly, Anna Demming writes for Physics World .

The new experiment does away with the details that make the Mpemba effect so murky. In each test, they dropped one microscopic glass bead into a small well of water. There, they used a laser to exert controlled forces on the bead, and they measured the bead’s temperature, per Science News . They repeated the test over 1,000 times, dropping the beads in different wells and starting at different temperatures.

Under certain forces from the laser, the hottest beads cooled faster than the lower temperature beads. The research suggests that the longer path from a higher temperature to the freezing point might create shortcuts so that the hot bead’s temperature can reach the finish line before the cooler bead.

Bechhoefer describes the experimental system as an “abstract” and “almost geometrical” way to picture the Mpemba effect to Physics World . But using the system, he and Kumar identified the optimal “initial temperatures” for a Mpemba cooling effect.

“It sort of suggested that all the peculiarities of water and ice – all the things that made the original effect so hard to study – might be in a way peripheral,” Bechhoefer tells Physics World .

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Theresa Machemer | READ MORE

Theresa Machemer is a freelance writer based in Washington DC. Her work has also appeared in National Geographic and SciShow. Website: tkmach.com

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Does hot water freeze faster than cold water?

It’s an age-old question with a simple answer: no.

Since the time of Aristotle, researchers and amateur scientists alike have batted about the counterintuitive theory that hot water freezes faster than cold. The notion even has a name: the Mpemba effect, named for a Tanzanian schoolboy who in 1963 noticed that the ice cream he and his classmates made from warm milk froze quicker than that made from cool milk.

“No matter what the initial temperature of water is, it must be brought to the freezing point before it will change state and become ice,” says Prakash Govindan, a postdoctoral associate in MIT’s mechanical engineering department. It will actually take more time and/or energy to freeze hot water because it must be brought down further in temperature until it reaches the freezing point, about 0 ° C.

Govindan suggests conducting a simple experiment to demonstrate that hot and cold water will behave as logic predicts. “Fill two identical containers with hot and cold tap water from the kitchen sink and see which freezes first,” he says. Interestingly, he points out, the rates of change in this experiment will not be the same. “When you set them in the freezer, the freezer will work harder to bring the temperature of the hot water down, so initially the rate of heat transfer will be faster in the hot water.” However, the other container will be cooling at the same time (if not at quite the same rate).

When the temperature of the water in each container reaches just about 0 ° C it will undergo the same changes as it moves from a liquid to a solid, and it will take the same amount of time to begin forming tiny ice crystals. At that point, each mixture of liquid and ice will be at a uniform temperature, and as more heat is taken from the mixtures, the thermodynamic principle of latent heat kicks in: The water continues to convert to a solid state, but no longer changes in temperature. “As long as you have a mixture of liquid water and solid ice, the temperature will remain at 0 until all the water is frozen,” says Govindan.

It’s never been convincingly proven than hot water and cold water behave differently from each other at any step of the freezing process, despite the ongoing fascination with the Mpemba effect. In early 2013, Europe’s Royal Society of Chemistry even held a competition for the best explanation of the theory. The winner speculated that hot water indeed freezes more quickly if the cold water is first supercooled. But logic triumphs when it comes the plain ordinary water that comes from the household faucet. Most likely to impact the freezing point of water is the presence of impurities such as salt, dissolved solids and gases — and the ingredients of homemade ice cream.

Thanks to Khubaib Mukhtar of Pakistan for this question.

April 30, 2013

An editorially independent publication supported by the Simons Foundation.

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Controversy Continues Over Whether Hot Water Freezes Faster Than Cold

June 29, 2022

Whether hot or cold water freezes faster remains unknown.

Francis Chee / SPL / Science Source

Introduction

It sounds like one of the easiest experiments possible: Take two cups of water, one hot, one cold. Place both in a freezer and note which one freezes first. Common sense suggests that the colder water will. But luminaries including Aristotle, Rene Descartes and Sir Francis Bacon have all observed that hot water may actually cool more quickly. Likewise, plumbers report hot water pipes bursting in subzero weather while cold ones remain intact. Yet for more than half a century, physicists have been arguing about whether something like this really occurs.

The modern term for hot water freezing faster than cold water is the Mpemba effect, named after Erasto Mpemba, a Tanzanian teenager who, along with the physicist Denis Osborne, conducted the first systematic, scientific studies of it in the 1960s. While they were able to observe the effect, follow-up experiments have failed to consistently replicate that result. Precision experiments to investigate freezing can be influenced by many subtle details, and researchers often have trouble determining if they have accounted for all confounding variables.

Over the past few years, as the controversy continues about whether the Mpemba effect occurs in water, the phenomenon has been spotted in other substances — crystalline polymers , icelike solids called clathrate hydrates , and manganite minerals cooling in a magnetic field. These new directions are helping researchers peek into the complicated dynamics of systems that are out of thermodynamic equilibrium. A contingent of physicists modeling out-of-equilibrium systems has predicted the Mpemba effect should occur in a wide variety of materials (along with its inverse, in which a cold substance heats up faster than a warm one). Recent experiments appear to confirm these ideas.

Yet the most familiar substance of all, water, is proving to be the slipperiest.

“A glass of water stuck in a freezer seems simple,” said John Bechhoefer , a physicist at Simon Fraser University in Canada whose recent experiments are the most solid observations of the Mpemba effect to date. “But it’s actually not so simple once you start thinking about it.”

‘That Cannot Happen’

“My name is Erasto B. Mpemba, and I am going to tell you about my discovery, which was due to misusing a refrigerator.” Thus begins a 1969 paper in the journal Physics Education in which Mpemba described an incident at Magamba Secondary School in Tanzania when he and his classmates were making ice cream.

A middle-aged man in a suit holding an ice cube in his palm

The late Erasto Mpemba initiated decades of research into whether hot water freezes faster than cold water.

PA Images / Alamy Stock Photo

Space was limited in the students’ refrigerator, and in the rush to nab the last available ice tray, Mpemba opted to skip waiting for his boiled-milk-and-sugar concoction to cool to room temperature like the other students had done. An hour and a half later, his mixture had frozen into ice cream, whereas those of his more patient classmates remained a thick liquid slurry. When Mpemba asked his physics teacher why this occurred, he was told, “You were confused. That cannot happen.”

Later, Osborne came to visit Mpemba’s high school physics class. He recalled the teenager raising his hand and asking, “If you take two beakers with equal volumes of water, one at 35°C and the other at 100°C, and put them into a refrigerator, the one that started at 100°C freezes first. Why?” Intrigued, Osborne invited Mpemba to the University College in Dar es Salaam, where they worked with a technician and found evidence for the effect that bears Mpemba’s name. Still, Osborne concluded that the tests were crude and more sophisticated experiments would be needed to figure out what might be going on.

A photo of a school auditorium with blue walls and rows of empty wooden desks.

Mpemba first observed the alleged effect that bears his name in the 1960s as a student at Magamba Secondary School in Tanzania. The school’s auditorium is shown in 2009.

imageBROKER / Alamy Stock Photo

Over the decades, scientists have offered a wide variety of theoretical explanations to explain the Mpemba effect. Water is a strange substance, less dense when solid than liquid, and with solid and liquid phases that can coexist at the same temperature. Some have suggested that heating water might destroy the loose network of weak polar hydrogen bonds between water molecules in a sample, increasing its disorder, which then lowers the amount of energy it takes to cool the sample. A more mundane explanation is that hot water evaporates faster than cold, decreasing its volume and thus the time it takes to freeze. Cold water also could contain more dissolved gases, which lower its freezing point. Or perhaps external factors come into play: A layer of frost in a freezer can act as an insulator, keeping heat from leaking out of a cold cup, whereas a hot cup will melt the frost and cool faster.

Those explanations all assume that the effect is real — that hot water really does freeze faster than cold. But not everyone is convinced.

In 2016, physicist Henry Burridge of Imperial College London and mathematician Paul Linden of the University of Cambridge did an experiment that showed how sensitive the effect is to the particulars of measurement. They speculated that hot water might form some ice crystals first but take longer to fully freeze. Both of these events are difficult to measure, so Burridge and Linden instead noted how long it took water to reach zero degrees Celsius. They found that the readings depended on where they placed the thermometer. If they compared the temperatures between hot and cold cups at the same height, the Mpemba effect didn’t appear. But if measurements were off by even a centimeter, they could produce false evidence of the Mpemba effect. Surveying the literature, Burridge and Linden found that only Mpemba and Osborne, in their classic study, saw a Mpemba effect too pronounced to attribute to this kind of measurement error.

The findings “highlight how sensitive these experiments are even when you don’t include the freezing process,” said Burridge.

Strange Shortcuts

Yet a good number of researchers think the Mpemba effect can occur, at least under certain conditions. After all, Aristotle wrote in the fourth century BCE that “many people, when they want to cool water quickly, begin by putting it in the sun,” the benefits of which were presumably noticeable even before the invention of sensitive thermometers. School-age Mpemba was similarly able to observe the unsubtle difference between his frozen ice cream and his classmates’ slurry. Still, Burridge and Linden’s findings highlight a key reason why the Mpemba effect, real or not, might be so hard to pin down: Temperature varies throughout a cup of rapidly cooling water because the water is out of equilibrium, and physicists understand very little about out-of-equilibrium systems.

In equilibrium, a fluid in a bottle can be described by an equation with three parameters: its temperature, its volume and the number of molecules. Shove that bottle in a freezer, and all bets are off. The particles at the outer edge will be plunged into an icy environment while those deeper in will remain warm. Labels like temperature and pressure are no longer well defined but instead constantly fluctuate.

When Zhiyue Lu of the University of North Carolina read about the Mpemba effect in middle school, he snuck into an oil refinery in the Shandong province of China where his mother worked and used precision lab equipment to measure temperature as a function of time in a sample of water (he ended up supercooling the water without it freezing). Later, while studying nonequilibrium thermodynamics as a graduate student, he tried to reframe his approach to the Mpemba effect. “Is there any thermodynamic rule that will forbid the following: Something starting further away from the final equilibrium that would approach equilibrium faster than something starting from close?” he asked.

A pair of portraits of men looking at the camera.

Zhiyue Lu of the University of North Carolina (top) and Oren Raz of the Weizmann Institute of Science in Israel have shown that hot liquids may find “strange shortcuts” to their freezing points.

Robert Filcsik (top); Itai Belson / Weizmann Institute of Science

Zhiyue Lu of the University of North Carolina (left) and Oren Raz of the Weizmann Institute of Science in Israel have shown that hot liquids may find “strange shortcuts” to their freezing points.

Robert Filcsik (left); Itai Belson / Weizmann Institute of Science

Lu met Oren Raz , who now studies nonequilibrium statistical mechanics at the Weizmann Institute of Science in Israel, and they began developing a framework to investigate the Mpemba effect generally, not just in water. Their 2017 paper in the Proceedings of the National Academy of Sciences modeled the random dynamics of particles, showing that in principle there are nonequilibrium conditions under which the Mpemba effect and its inverse could occur. The abstract findings suggested that the components of a hotter system, by virtue of having more energy, are able to explore more possible configurations and therefore discover states that act as a sort of bypass, allowing the hot system to overtake a cool one as both dropped toward a colder final state.

“We all have this naive picture that says temperature should change monotonically,” said Raz. “You start at a high temperature, then a medium temperature, and go to a low temperature.” But for something driven out of equilibrium, “it’s not really true to say that the system has a temperature,” and “since that’s the case you can have strange shortcuts.”

The thought-provoking work drew the interest of others, including a Spanish group that began simulating what are known as granular fluids — collections of rigid particles that can flow like liquids, such as sand or seeds — and showed that these, too, can have Mpemba-like effects. Statistical physicist Marija Vucelja of the University of Virginia started wondering how common the phenomenon might be. “Is this like is a needle in a haystack, or could it be useful for optimal heating or cooling protocols?” she asked. In a 2019 study , she, Raz, and two co-authors found that the Mpemba effect could appear in a significant fraction of disordered materials, such as glass. While water is not such a system, the findings covered an enormous variety of possible materials.

To investigate whether these theoretical hunches had any real-world basis, Raz and Lu approached Bechhoefer, an experimentalist. “Literally, they kind of grabbed me after a talk and said, ‘Hey, we’ve got something we want you to hear about,’” Bechhoefer recalled.

Exploring the Landscape

The experimental setup Bechhoefer and his collaborator Avinash Kumar came up with offers a highly conceptual, stripped-down look at a collection of particles under the influence of different forces. A microscopic glass bead representing a particle is placed in a W-shaped “energy landscape,” created using lasers. The deeper of the two valleys in this landscape is a stable resting place. The shallower valley is a “metastable” state — a particle can fall into it but may eventually get knocked into the deeper well. The scientists submerged this landscape in water and used optical tweezers to position the glass bead within it 1,000 different times; collectively, the trials are equivalent to a system with 1,000 particles.

Merrill Sherman/Quanta Magazine; source: Nature

An initially “hot” system was one where the glass bead could be placed anywhere, since hotter systems have more energy and can therefore explore more of the landscape. In a “warm” system, the starting position was confined to a smaller area close to the valleys. During the cooling process, the glass bead first settled into one of the two wells, then spent a longer period jumping back and forth between them, buffeted by water molecules. Cooling was considered complete when the glass bead stabilized into spending specific amounts of time in each well, such as 20% of its time in the metastable one and 80% in the stable one. (These ratios depended on the water’s initial temperature and the valleys’ sizes.)

For certain initial conditions, the hot system took longer to settle into a final configuration than the warm system, matching our intuitions. But sometimes the particles in the hot system settled into the wells more quickly. When the experimental parameters were tuned just right, the hot system’s particles almost immediately found their final configuration, cooling exponentially faster than the warm system — a situation that Raz, Vucelja and colleagues had predicted and named the strong Mpemba effect. They reported the results in a 2020 Nature paper and published similar experiments showing the inverse Mpemba effect in PNAS earlier this year.

“The results are clear,” said Raúl Rica Alarcón of the University of Granada in Spain, who is working on independent experiments related to the Mpemba effect. “They show that a system that is farther away from the target can reach this target faster than another one that is closer to the target.”

A pair of portraits of men standing next to lab equipment.

Recent experiments with lasers and glass beads by Avinash Kumar (top) and John Bechhoefer of Simon Fraser University indicate that hot liquids can indeed relax to equilibrium faster than cold liquids.

Simon Fraser University (top); Dianne Mar-Nicolle

Recent experiments with lasers and glass beads by Avinash Kumar (left) and John Bechhoefer of Simon Fraser University indicate that hot liquids can indeed relax to equilibrium faster than cold liquids.

Simon Fraser University (left); Dianne Mar-Nicolle

Yet not everyone is entirely persuaded that the Mpemba effect has been demonstrated in any system. “I always read these experiments and I’m not impressed by the write-up,” said Burridge. “I never find a clear physical explanation, and I feel that leaves us with an interesting question as to whether Mpemba-like effects exist in a meaningful way.”

Bechhoefer’s trials appear to offer some insight into how the Mpemba effect could arise in systems with metastable states, but whether it is the only mechanism or how any particular substance undergoes such out-of-equilibrium heating or cooling is unknown.

Determining if the phenomenon occurs in water remains another open question. In April, Raz and his graduate student Roi Holtzman posted a paper showing that the Mpemba effect could happen through a related mechanism that Raz has previously described with Lu in systems that undergo a second-order phase transition, meaning that their solid and liquid forms can’t coexist at the same temperature. Water is not such a system (it has first-order phase transitions), but Bechhoefer described the work as gradually sneaking up on an answer for water.

If nothing else, the theoretical and experimental work on the Mpemba effect has started giving physicists a handhold into nonequilibrium systems that they otherwise lack. “Relaxation towards equilibrium is an important question that, frankly, we don’t have a good theory [for],” said Raz. Identifying which systems might behave in strange and counterintuitive ways “would give us a much better picture of how systems relax towards equilibrium.”

After igniting a decades-long controversy with his teenage interrogations, Mpemba himself went on to study wildlife management, becoming a principal game officer in Tanzania’s Ministry of Natural Resources and Tourism before retiring. According to Christine Osborne, the widow of Denis Osborne, Mpemba passed away around 2020. Science continues to spring from his insistence about the effect that bears his name. Osborne, discussing the results of their investigations together, took a lesson from the initial skepticism and dismissal that the schoolboy’s counterintuitive claim had faced: “It points to the danger of an authoritarian physics.”

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October 21, 1998

Is It True that Hot Water Freezes Faster than Cold Water or that Cold Water Boils Faster than Hot Water?

It seems hard tobelieve, but some people swear that it is so

A woman tosses hot water into the freezing cold air.

Ismail Kaplan Getty Images

This seemingly simple question continues to generate considerable controversy. Takamasa Takahashi, a physicist at St. Norbert College in De Pere, Wis., attempts a definitive answer:

"Cold water does not boil faster than hot water. The rate of heating of a liquid depends on the magnitude of the temperature difference between the liquid and its surroundings (the flame on the stove, for instance). As a result, cold water will be absorbing heat faster while it is still cold; once it gets up to the temperature of hot water, the heating rate slows down and from there it takes just as long to bring it to a boil as the water that was hot to begin with. Because it takes cold water some time to reach the temperature of hot water, cold water clearly takes longer to boil than hot water does. There may be some psychological effect at play; cold water starts boiling sooner than one might expect because of the aforementioned greater heat absorption rate when water is colder.

"To the first part of the question--'Does hot water freeze faster than cold water?'--the answer is 'Not usually, but possibly under certain conditions.' It takes 540 calories to vaporize one gram of water, whereas it takes 100 calories to bring one gram of liquid water from 0 degrees Celsius to 100 degrees C. When water is hotter than 80 degrees C, the rate of cooling by rapid vaporization is very high because each evaporating gram draws at least 540 calories from the water left behind. This is a very large amount of heat compared with the one calorie per Celsius degree that is drawn from each gram of water that cools by regular thermal conduction.

Science News

You really can freeze hot water faster than cold*.

*But only if you’re a clever physicist and you bend the rules

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Why Does Hot Water Freeze Faster Than Cold Water?

Lots of you in the Northern Hemisphere will be in the middle of yet another winter, and some might even be experiencing the sub-zero temperatures to do some cool experiments, such as creating giant frozen marbles for the front yard , snapping soap bubbles , or even tossing boiling water into air to create snow (although that last one requires caution, seriously).

If you remember the dreadfully cold winter of 2013, you might have seen videos like the one we've posted below circulating as 'proof' of the crazy low temperatures. But it turns out this demonstration is actually not that surprising - hot water is in fact known to freeze faster than cold water. There are records of this from as far back as Aristotle's time.

In modern times, this counterintuitive property has been named the Mpemba effect, after a Tanzanian secondary school student who re-discovered this phenomenon back in 1963.

Erasto Mpemba and other kids at his school often made ice cream using the school freezer - they would do this by boiling milk and mixing it with sugar, which then had to be cooled and placed in the freezer. One day, Mpemba rushed the process and stuck the milk in while still hot, and to his surprise, the ice cream formed quicker than for his classmate. Unsurprisingly, none of his teachers believed the 13-year-old.

Mpemba later teamed up with a physics professor who visited his high school, and in 1969 they published a paper , which has since been replicated many times - most often with a similar result. Although if you try this at home and fail, it's probably because the Mpemba effect is not a reliable phenomenon that happens every single time; in fact, there seem to be several factors at play.

So what are they? Well, it's one of those somewhat unsatisfying cases where just because we know something happens, doesn't mean we entirely understand why. But the process of scientific speculation is still interesting, so here are some versions.

The most commonly proposed hypothesis - and one that's probably somewhat responsible for the effect - is that hot water evaporates more quickly, losing mass and therefore needing to lose less heat in order to freeze. However, scientists have also demonstrated the Mpemba effect with closed containers where evaporation doesn't take place.

Another theoretical speculation is that water develops convection currents and temperature gradients as it cools - a rapidly cooling glass of hot water will have greater temperature differences throughout, and lose heat more quickly from the surface, whereas a uniformly cool glass of water has less of a temperature difference, and there's less convection to accelerate the process. But this idea has not been entirely verified either.

Other theories have been put forward, including supercooling, or the effect that dissolved gasses in the water would have on the freezing process. It's likely that several of these actually come into play.

In late 2013, a team of researchers from Singapore proposed in a paper on arXiv.org that the Mpemba paradox stems from the unique properties of chemical bonds in the water. A standard water molecule contains one oxygen atom and two hydrogen atoms joined to it by covalent bonds sharing electron pairs between the atoms.

But when you put several water molecules together, the hydrogen atoms will also form bonds with oxygen atoms in other molecules. These hydrogen bonds are what gives water some of its properties, such as having a relatively high boiling point, and becoming less dense when frozen.

According to the researchers, when water boils, the molecules spread out, lengthening the hydrogen bonds - but because volume is limited, meanwhile the covalent bonds within individual molecules get compressed, storing away energy. If water is frozen at this state, the bonds release energy as an uncoiled spring, cooling down much more quickly than if the covalent bonds were less compressed.

However, even though many headlines heralded this as the definitive explanation for the Mpemba effect, the idea is yet to be published in a peer-reviewed journal. According to the blog of Physics arXiv , the theory is convincing, but lacks predictive power:

"Xi and co need to use their theory to predict a new property of water that conventional thinking about water does not. For example, the shortened covalent bonds might give rise to some measurable property of the water that would not otherwise be present. The discovery and measurement of this property would be the coup de grâce that their theory needs."

As Adam Mann wrote at Wired , this explanation is also not strongly supported in the scientific community. "When we talked to chemist Richard Zare of Stanford University, he was unconvinced of its merits and said he thinks the dominant force at play is evaporation."

Still, it's fun to (carefully) try this at home.

Does Hot Water Freeze Faster Than Cold Water?

ice spike, ices freezes, hot water, cold water

Determining whether or not hot water can freeze faster than cold water may seem like a no-brainer. After all, water freezes at 0 degrees Celsius. And wouldn’t water hot enough to kill E. coli bacteria (about 120 degrees Fahrenheit or 50 degrees Celsius) take a longer path than cooler water at a fall New England beach (about 60 degrees Fahrenheit or 15 degrees Celsius) towards a frigid future as ice? While a logical assumption, it turns out that hot water can freeze before cooler water under certain conditions.

This apparent quirk of nature is the "Mpemba effect," named after the Tanzanian high school student, Erasto Mpemba, who first observed it in 1963. The Mpemba effect occurs when two bodies of water with different temperatures are exposed to the same subzero surroundings and the hotter water freezes first. Mpemba’s observations confirmed the hunches of some of history’s most revered thinkers, such as Aristotle, Rene Descartes and Francis Bacon, who also thought that hot water froze faster than cold water.

Evaporation is the strongest candidate to explain the Mpemba effect. As hot water placed in an open container begins to cool, the overall mass decreases as some of the water evaporates. With less water to freeze, the process can take less time. But this doesn’t always work, especially when using closed containers that prevent evaporated water from escaping.

And evaporation may not be the only reason the water can freeze more quickly. There may be less dissolved gas in the warmer water, which can reduce its ability to conduct heat, allowing it to cool faster. However, Polish physicists in the 1980s were unable to conclusively demonstrate this relationship.

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A non-uniform temperature distribution in the water may also explain the Mpemba effect. Hot water rises to the top of a container before it escapes, displacing the cold water beneath it and creating a "hot top." This movement of hot water up and cold water down is called a convection current. These currents are a popular form of heat transfer in liquids and gases, occurring in the ocean and also in radiators that warm a chilly room. With the cooler water at the bottom, this uneven temperature distribution creates convection currents that accelerate the cooling process. Even with more ground to cover to freeze, the temperature of the hotter water can drop at a faster rate than the cooler water.

So the next time you refill your ice cube tray, try using warmer water. You might have ice cubes to cool your drink even sooner.

This answer is provided by Scienceline , a project of New York University's Science, Health and Environmental Reporting Program.

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Can hot water freeze faster than cold water?

Yes—a general explanation.

Hot water can in fact freeze faster than cold water for a wide range of experimental conditions. This phenomenon is extremely counterintuitive, and surprising even to most scientists, but it is in fact real. It has been seen and studied in numerous experiments . Although this phenomenon has been known for centuries, and was described by Aristotle, Bacon, and Descartes [1–3] , it was not introduced to the modern scientific community until 1969, by a Tanzanian high school pupil named Mpemba. Both the early scientific history of this effect, and the story of Mpemba's rediscovery of it, are interesting in their own right — Mpemba's story in particular providing a dramatic parable against making snap judgements about what is impossible. This is described separately below.

The phenomenon that hot water may freeze faster than cold is often called the Mpemba effect. Because, no doubt, most readers are extremely skeptical at this point, we should begin by stating precisely what we mean by the Mpemba effect. We start with two containers of water, which are identical in shape, and which hold identical amounts of water. The only difference between the two is that the water in one is at a higher (uniform) temperature than the water in the other. Now we cool both containers, using the exact same cooling process for each container. Under some conditions the initially warmer water will freeze first. If this occurs, we have seen the Mpemba effect. Of course, the initially warmer water will not freeze before the initially cooler water for all initial conditions. If the hot water starts at 99.9°C, and the cold water at 0.01°C, then clearly under those circumstances, the initially cooler water will freeze first. But under some conditions the initially warmer water will freeze first: if that happens, you have seen the Mpemba effect. But you will not see the Mpemba effect for just any initial temperatures, container shapes, or cooling conditions.

This seems impossible, right? Many sharp readers may have already come up with a common proof that the Mpemba effect is impossible. The proof usually goes something like this. Say that the initially cooler water starts at 30°C and takes 10 minutes to freeze, while the initially warmer water starts out at 70°C. Now the initially warmer water has to spend some time cooling to get to get down to 30°C, and after that, it's going to take 10 more minutes to freeze. So since the initially warmer water has to do everything that the initially cooler water has to do, plus a little more, it will take at least a little longer, right? What can be wrong with this proof?

What's wrong with this proof is that it implicitly assumes that the water is characterized solely by a single number — its average temperature. But if other factors besides the average temperature are important, then when the initially warmer water has cooled to an average temperature of 30°C, it may look very different than the initially cooler water (at a uniform 30°C) did at the start. Why? Because the water may have changed when it cooled down from a uniform 70°C to an average 30°C. It could have less mass, less dissolved gas, or convection currents producing a non-uniform temperature distribution. Or it could have changed the environment around the container in the refrigerator. All four of these changes are conceivably important, and each will be considered separately below. So the impossibility proof given above doesn't work. And in fact the Mpemba effect has been observed in a number of controlled experiments [5,7–14]

It is still not known exactly why this happens. A number of possible explanations for the effect have been proposed, but so far the experiments do not show clearly which, if any, of the proposed mechanisms is the most important one. While you will often hear confident claims that X is the cause of the Mpemba effect, such claims are usually based on guesswork, or on looking at the evidence in only a few papers and ignoring the rest. Of course, there is nothing wrong with informed theoretical guesswork or being selective in which experimental results you trust; the problem is that different people make different claims as to what X is.

Why hasn't modern science answered this seemingly simple question about cooling water? The main problem is that the time it takes water to freeze is highly sensitive to a number of details in the experimental setup, such as the shape and size of the container, the shape and size of the refrigeration unit, the gas and impurity content of the water, how the time of freezing is defined, and so on. Because of this sensitivity, while experiments have generally agreed that the Mpemba effect occurs, they disagree over the conditions under which it occurs, and thus about why it occurs. As Firth [7] wrote "There is a wealth of experimental variation in the problem so that any laboratory undertaking such investigations is guaranteed different results from all others."

Finally, supercooling may be important to the effect. Supercooling occurs when the water freezes not at 0°C, but at some lower temperature. One experiment [12] found that its initially hot water supercooled less than its initially cold water. This would mean that the initially warmer water might freeze first because it would freeze at a higher temperature than the initially cooler water. If true, this would not fully explain the Mpemba effect, because we would still need to explain why initially warmer water supercools less than initially cooler water.

In short, hot water does freeze sooner than cold water under a wide range of circumstances. It is not impossible, and has been seen to occur in a number of experiments. But despite claims often made by one source or another, there is no well-agreed explanation for how this phenomenon occurs. Different mechanisms have been proposed, but the experimental evidence is inconclusive. For those wishing to read more on the subject, Jearl Walker's article in Scientific American [13] is very readable and has suggestions on how to do home experiments on the Mpemba effect, while the articles by Auerbach [12] and Wojciechowski [14] are two of the more modern papers on the effect.

History of the Mpemba Effect

The fact that hot water freezes faster than cold has been known for many centuries. The earliest reference to this phenomenon dates back to Aristotle in 300 B.C. The phenomenon was later discussed in the medieval era, as European physicists struggled to come up with a theory of heat. But by the 20th century the phenomenon was only known as common folklore, until it was reintroduced to the scientific community in 1969 by Mpemba, a Tanzanian high school pupil. Since then, numerous experiments have confirmed the existence of the "Mpemba effect", but have not settled on any single explanation.

The earliest known reference to this phenomenon is by Aristotle, who wrote:

"The fact that water has previously been warmed contributes to its freezing quickly; for so it cools sooner. Hence many people, when they want to cool hot water quickly, begin by putting it in the sun. . ." [1,4]

He wrote these words in support of a mistaken idea which he called antiperistasis. Antiperistasis is defined as "the supposed increase in the intensity of a quality as a result of being surrounded by its contrary quality, for instance, the sudden heating of a warm body when surrounded by cold" [4] .

Medieval scientists believed in Aristotle's theory of antiperistasis, and also sought to explain it. Not surprisingly, scientists in the 1400s had trouble explaining how it worked, and could not even decide whether (as Aristotle claimed in support of antiperistasis), human bodies and bodies of water were hotter in the winter than in the summer [4] . Around 1461, the physicist Giovanni Marliani, in a debate over how objects cooled, said that he had confirmed that hot water froze faster than cold. He said that he had taken four ounces of boiling water, and four ounces of non-heated water, placed them outside in similar containers on a cold winter day, and observed that the boiled water froze first. Marliani was, however, unable to explain this occurrence [4] .

Later, in the 1600s, it was apparently common knowledge that hot water would freeze faster than cold. In 1620 Bacon wrote "Water slightly warm is more easily frozen than quite cold" [2] , while a little later Descartes claimed "Experience shows that water that has been kept for a long time on the fire freezes sooner than other water" [3] .

In time, a modern theory of heat was developed, and the earlier observations of Aristotle, Marliani, and others were forgotten, perhaps because they seemed so contradictory to modern concepts of heat. But it was still known as folklore among many non-scientists in Canada [11] , England [15–21] , the food processing industry [23] , and elsewhere.

It was not reintroduced to the scientific community until 1969, 500 years after Marliani's experiment, and more than two millennia after Aristotle's "Meteorologica I" [1] . The story of its rediscovery by a Tanzanian high school pupil named Mpemba is written up in the New Scientist [4] . The story provides a dramatic parable cautioning scientists and teachers against dismissing the observations of non-scientists and against making quick judgements about what is impossible.

In 1963, Mpemba was making ice cream at school, which he did by mixing boiling milk with sugar. He was supposed to wait for the milk to cool before placing it the refrigerator, but in a rush to get scarce refrigerator space, put his milk in without cooling it. To his surprise, he found that his hot milk froze into ice cream before that of other pupils. He asked his physics teacher for an explanation, but was told that he must have been confused, since his observation was impossible.

Mpemba believed his teacher at the time. But later that year he met a friend of his who made and sold ice cream in Tanga town. His friend told Mpemba that when making ice cream, he put the hot liquids in the refrigerator to make them freeze faster. Mpemba found that other ice cream sellers in Tanga had the same practice.

Later, when in high school, Mpemba learned Newton's law of cooling, that describes how hot bodies are supposed to cool (under certain simplifying assumptions). Mpemba asked his teacher why hot milk froze before cold milk when he put them in the freezer. The teacher answered that Mpemba must have been confused. When Mpemba kept arguing, the teacher said "All I can say is that is Mpemba's physics and not the universal physics" and from then on, the teacher and the class would criticize Mpemba's mistakes in mathematics and physics by saying "That is Mpemba's mathematics" or "That is Mpemba's physics." But when Mpemba later tried the experiment with hot and cold water in the biology laboratory of his school, he again found that the hot water froze sooner.

Earlier, Dr Osborne, a professor of physics, had visited Mpemba's high school. Mpemba had asked him to explain why hot water would freeze before cold water. Dr Osborne said that he could not think of any explanation, but would try the experiment later. When back in his laboratory, he asked a young technician to test Mpemba's claim. The technician later reported that the hot water froze first, and said "But we'll keep on repeating the experiment until we get the right result." But repeated tests gave the same result, and in 1969 Mpemba and Osborne wrote up their results [5] .

In the same year, in one of the coincidences so common in science, Dr Kell independently wrote a paper on hot water freezing sooner than cold water. Kell showed that if one assumed that the water cooled primarily by evaporation, and maintained a uniform temperature, the hot water would lose enough mass to freeze first [11] . Kell thus argued that the phenomenon (then a common urban legend in Canada) was real and could be explained by evaporation. But he was unaware of Osborne's experiments, which had measured the mass lost to evaporation and found it insufficient to explain the effect. Subsequent experiments were done with water in a closed container, eliminating the effects of evaporation, and still found that the hot water froze first [14] .

Subsequent discussion of the effect has been inconclusive. While quite a few experiments have replicated the effect [4,6–13] , there has been no consensus on what causes the effect. The different possible explanations are discussed above . The effect has repeatedly a topic of heated discussion in the "New Scientist", a popular science magazine. The letters have revealed that the effect was known by laypeople around the world long before 1969. Today, there is still no well-agreed explanation of the Mpemba effect.

Evaporation

One explanation of the effect is that as the hot water cools, it loses mass to evaporation. With less mass, the liquid has to lose less heat to cool, and so it cools faster. With this explanation, the hot water freezes first, but only because there's less of it to freeze. Calculations done by Kell in 1969 [11] showed that if the water cooled solely by evaporation, and maintained a uniform temperature, the warmer water would freeze before the cooler water.

This explanation is solid, intuitive, and undoubtedly contributes to the Mpemba effect in most physical situations. But many people have incorrectly assumed that it is therefore "the" explanation for the Mpemba effect. That is, they assume that the only reason hot water can freeze faster than cold is because of evaporation, and that all experimental results can be explained by the calculations in Kell's article. But the experiments currently do not bear this belief out. While experiments show evaporation to be important [13] , they do not show that it is the only mechanism behind the Mpemba effect. A number of experimenters have argued that evaporation alone is insufficient to explain their results [5,9,12] ; in particular, the original experiment by Mpemba and Osborne measured the mass lost to evaporation, and found it substantially less that the amount predicted by Kell's calculations [5,9] . And most convincingly, an experiment by Wojciechowski observed the Mpemba effect in a closed container, where no mass was lost to evaporation.

Dissolved Gasses

Another explanation argues that the dissolved gas usually present in water is expelled from the initially hot water, and that this changes the properties of the water in some way that explains the effect. It has been argued that the lack of dissolved gas may change the ability of the water to conduct heat, or change the amount of heat needed to freeze a unit mass of water, or change the freezing point of the water by some significant amount. It is certainly true that hot water holds less dissolved gas than cold water, and that boiled water expels most dissolved gas. The question is whether this can significantly affect the properties of water in a way that explains the Mpemba effect. As far as I know, there is no theoretical work supporting this explanation for the Mpemba effect.

Indirect support can be found in two experiments that saw the Mpemba effect in normal water which held dissolved gasses, but failed to see it when using degassed water [10,14] . But an attempt to measure the dependence of the enthalpy of freezing on the initial temperature and gas content of the water was inconclusive [14] .

One problem with this explanation is that many experiments pre-boiled both the initially hot and initially cold water, precisely to eliminate the effect of dissolved gasses, and yet they still saw the effect [5,13] . Two somewhat unsystematic experiments found that varying the gas content of the water made no substantial difference to the Mpemba effect [9,12] .

It has also been proposed that the Mpemba effect can be explained by the fact that the temperature of the water becomes non-uniform. As the water cools, temperature gradients and convection currents will develop. For most temperatures, the density of water decreases as the temperature increases. So over time, as water cools we will develop a "hot top" — the surface of the water will be warmer than the average temperature of the water, or the water at the bottom of the container. If the water loses heat primarily through the surface, then this means that the water should lose heat faster than one would expect based just on looking at the average temperature of the water. And for a given average temperature, the heat loss should be greater the more inhomogenous the temperature distribution is (that is, the greater the range of the temperatures seen as we go from the top to the bottom).

How does this explain the Mpemba effect? Well, the initially hot water will cool rapidly, and quickly develop convection currents and so the temperature of the water will vary greatly from the top of the water to the bottom. On the other hand, the initially cool water will have a slower rate of cooling, and will thus be slower to develop significant convection currents. Thus, if we compare the initially hot water and initially cold water at the same average temperature, it seems reasonable to believe that the initially hot water will have greater convection currents, and thus have a faster rate of cooling. To consider a concrete example, suppose that the initially hot water starts at 70°C, and the initially cold water starts at 30°C. When the initially cold water is at an average 30°C, it is also a uniform 30°C. But when the initially hot water reaches an average 30°C, the surface of the water is probably much warmer than 30°C, and it will thus lose heat faster than the initially cold water for the same average temperature. Got that? This explanation is pretty confusing, so you might want to go back and read the last two paragraphs again, paying careful attention to the difference between initial temperature, average temperature, and surface temperature.

At any rate, if the above argument is right, then when we plot the average temperature versus time for both the initially hot and initially cold water, then for some average temperatures the initially hot water will be cooling faster than the initially cold water. So the cooling curve of the initially hot water will not simply reproduce the cooling curve of the initially cold water, but will drop faster when in the same temperature range.

This shows that the initially hot water goes faster, but of course it also has farther to go. So whether it actually finishes first (that is, reaches 0°C first), is not clear from the above discussion. To know which one finishes first would require theoretical modelling of the convection currents (hopefully for a range of container shapes and sizes), which has not been done. So convection alone may be able to explain the Mpemba effect, but whether it actually does is not currently known. Experiments on the Mpemba effect have often reported a "hot top" [5,8,10] , as we would expect. Experiments have been done that looked at the convection currents of freezing water [27,28] , but their implications for the Mpemba effect are not entirely clear.

It should also be noted that the density of water reaches a maximum at four° C. So below four°C, the density of water actually decreases with decreasing temperature, and we will get a "cold top." This makes the situation even more complicated.

Surroundings

The initially hot water may change the environment around it in some way that makes it cool faster later on. One experiment reported significant changes in the data simply upon changing the size of the freezer that the container sat in [7] . So conceivably it is important not just to know about the water and the container, but about the environment around it.

For example, one explanation for the Mpemba effect is that if the container is resting on a thin layer of frost, than the container holding the cold water will simply sit on the surface of the frost, while the container with the hot water will melt the frost, and then be sitting on the bottom of the freezer. The hot water will then have better thermal contact with the cooling systems. If the melted frost refreezes into an ice bridge between the freezer and the container, the thermal contact may be even better.

Obviously, even if this argument is true, it has fairly limited utility, since most scientific experiments are careful enough not to rest the container on a layer of frost in a freezer, but instead place the container on a thermal insulator, or in a cooling bath. So while this proposed mechanism may or may not have some relevance to some home experiments, it's irrelevant for most published results.

Supercooling

Finally, supercooling may be important to the effect. Supercooling occurs when water freezes not at 0°C, but at some lower temperature. This happens because the statement that "water freezes at 0°C" is a statement about the lowest energy state of the water: at less than 0°C, the water molecules "want" to be arranged as an ice crystal. This means that they will stop zooming around randomly as a liquid, and instead form a solid ice lattice. But they don't know how to form themselves into an ice lattice, but need some small irregularity or nucleation site to tell them how to arrange themselves. Sometimes, when water is cooled below 0°C, the molecules will not see a nucleation site for some time, and then water will cool below 0°C without freezing. This happens quite often. One experiment found that initially hot water would supercool only a little (say to about −2°C), while initially cold water would supercool more (to around −8°C) [12] . If true, this could explain the Mpemba effect because the initially cold water would need to "do more work"; — that is, get colder — to freeze.

But this also cannot be considered "the" sole explanation of the Mpemba effect. First of all, as far as I know, this result has not been independently confirmed. The experiment described above [12] only had a limited number of trials, so the results found could have been a statistical fluke.

Second, even if the results are true, they do not fully explain the Mpemba effect, but replace one mystery with another. Why should initially hot water supercool less than initially cold water? After all, once the water has cooled to the lower temperature, one would generally expect that the water would not "remember" what temperature it used to be. One explanation is that the initially hot water has less dissolved gas than the initially cold water, and that this affects its supercooling properties (see Dissolved Gasses for more on this). The problem with this explanation is that one would expect that since the hot water has less dissolved gas, and thus fewer nucleation sites, it would supercool more, not less. Another explanation is that when the initially hot water has cooled down to 0°C (or less), its temperature distribution throughout the container varies more than the initially cold water (see Convection for more on this). Since temperature shear induces freezing [26] , the initially hot water supercools less, and thus freezes sooner.

Third, this explanation cannot work in all of the experiments, because many of the experimenters chose to look not at the time to form a complete block of ice, but the time for some part of the water to reach 0°C [7,10,13] (or perhaps the time for a thin layer of frost to form on the top [17] ). While [12] says that it is only a "true Mpemba effect" if the hot water freezes entirely first, other papers have defined the Mpemba effect differently. Since the precise time of supercooling is inherently unpredictable (see e.g. [26] ), many experiments have chosen to measure not the time for the sample to actually become ice, but the time for which the sample's equilibrium ground state is ice; that is, the time when the top of the sample reached 0°C [7,10,13] . The supercooling argument does not apply to these experiments.

Experiments on the Mpemba Effect

General discussion on the mpemba effect, related articles.

Answers to Science Questions
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Does hot water freeze faster than cold water?

Part of the show ban on cheap vapes, and farewell to dolly's 'father'.

Tony asks, 'I heard that water that has previously been boiled freezes faster than water that has not. How is this possible?'

Will - In 1963, 13 year old Tanzanian Erasto Mpemba was making ice cream at school due to time constraints. He decided not to let the mixture cool down and instead put it straight in the freezer, and found to his surprise that the mixture had frozen faster than everyone else's. So was born, the Mpemba effect, the theory that hot water freezes faster than cool water. But what does the science say? I asked an expert in fluid mechanics at Imperial College, Henry Burridge. Fortunately for us, Henry has spent a lot of time studying this theory.

Henry - Well, we started out really trying to prove why this would happen in the hope that it really did happen and looked very much at the cooling process, the idea being that if water cooled in such a way that it gained a lot of momentum, this could potentially cause warmer water to cool at a greater rate than colder water. We did definitely observe that, but we never observed that that greater rate of cooling carried through into lower temperature states. So the hotter samples that we cooled, they could never overtake the cooling process of the colder samples.

Will - So, no, the purely scientific conclusion states that if you change nothing in either experiment other than water temperature, hot water cannot cool or freeze quicker than cold water. So why has it been observed so many times? Well, the answer might be to do with the conditions that the water is frozen in.

Henry - We did some work where we either sandpapered the inside of a beaker or the outside of the beaker so that heat transfer properties were the same in both cases. But in one case, the water was in a relatively smooth container, and in the other case, there were lots of these minute imperfections.

Will - These imperfections create nucleation sites. It sounds complex, but all you need to know is that a rough edge or a dust molecule or something similar makes water molecules align more often. This makes it favourable for the molecules to shift phases, which is to say, turn from water to ice.

Henry - By biassing whether the water was in a container with a smooth inside or a container with a roughened inside, then we were able to get hot water to freeze in less time than cold water.

Will - So a rough container with hot water will freeze faster than a smooth container with cold water. It should be stated, however, that cold water in a rough container will still freeze faster than hot water in a rough container. So perhaps Mr. Mpemba had a rougher ice tray than everyone else.

Henry - I think it comes down to this lovely nuance that it sounds like a really simple question, but you have to very precisely define what you mean. And if you mean that aspects will be identical except for the initial temperature from which you cool the water, then the answer is no, it cannot be done. But if you allow really subtle changes like allowing your hot water samples to be cooled in a container which is only changed by sandpaper on the inside of it, then you can get your hot water samples to freeze in less time than your cold water samples.

Will - Thank you very much to Tony for the question, and Henry Burridge for the answer. Next time we're answering this question from listener Cecilio,

Cecilio - I want to know how the Sun's impact changes on tattooed skin? Could tattoos protect us from getting burned? Thank you.

Will - And if you have a question of your own, please do send it in via our website nakedscientists.com, or email us at [email protected] .

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Does hot water freeze faster than cold water?

August 22, 2024 By Emma Vanstone Leave a Comment

It sounds completely counterintuitive, but whether hot water freezes faster than cold water has been debated for centuries.

The Mpemba Effect

The Mpemba effect is the term used for hot water freezing faster than cold water after a Tanzanian student named Erasto Mpemba found that his mixture of hot milk and sugar froze faster than his classmate’s mixtures that had been left to cool before freezing.

You can test to see if the Mpemba Effect occurs by placing two equal amounts of water in a freezer to investigate which freezes first. One sample should be at room temperature, and the second should be hot or boiling water.

If you’re interested in the quite complex science behind the Mpemba effect, How Stuff Works has a great article explaining it.

Last Updated on August 23, 2024 by Emma Vanstone

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Yes, hot water can freeze faster than cold water. However, it does not always happen, nor has science explained exactly why it can happen.

Key Takeaways: Water Temperature and Rate of Freezing

Sometimes hot water freezes more quickly than cold water. This is called the Mpemba effect after the student who observed it.
Factors that may cause hot water to freeze faster include evaporative cooling, less chance of supercooling, low concentration of dissolved gases, and convection.
Whether hot or cold water freezes more quickly depends on the specific conditions.

The Mpemba Effect

Although Aristotle, Bacon, and Descartes all described hot water freezing faster than cold water, the notion was mostly resisted until the 1960's when a high school student named Mpemba noticed that hot ice cream mix, when placed into the freezer, would freeze before ice cream mix that had been cooled to room temperature before being placed in the freezer. Mpemba repeated his experiment with water rather than ice cream mixture and found the same result: the hot water froze more quickly than the cooler water. When Mpemba asked his physics teacher to explain the observations, the teacher told Mpemba his data must be in error, because the phenomenon was impossible.

Mpemba asked a visiting physics professor, Dr. Osborne, the same question. This professor replied that he did not know, but he would test the experiment. Dr. Osborne had a lab tech perform Mpemba's test. The lab tech reported that he had duplicated Mpemba's result, "But we'll keep on repeating the experiment until we get the right result." (Um... yeah... that would be an example of poor science.) Well, the data was the data, so when the experiment was repeated, it continued to yield the same result. In 1969 Osborne and Mpemba published the results of their research. Now the phenomenon in which hot water may freeze faster than cold water is sometimes called the Mpemba Effect.

Why Hot Water Sometimes Freezes Faster Than Cold Water

There is no definitive explanation for why hot water may freeze faster than cold water. Different mechanisms come into play, depending on the conditions. The main factors appear to be:

Evaporation : More hot water will evaporate than cold water, thus reducing the amount of water remaining to be frozen. Mass measurements lead us to believe this is an important factor when chilling water in open containers, though it isn't the mechanism that explains how the Mpemba Effect occurs in closed containers.
Supercooling : Hot water tends to experience less of a supercooling effect than cold water. When was supercools, it can remain a liquid until it is disturbed, even well below its normal freezing temperature. Water that is not supercooled is more likely to become solid when it reaches the freezing point of water .
Convection : Water develops convection currents as it cools. Water density usually decreases as temperature increases, so a container of cooling water typically is warmer on top than on the bottom. If we assume water loses most of its heat across its surface (which may or may not be true, depending on the conditions), then water with a hotter top would lose its heat and freeze faster than water with a cooler top.
Dissolved Gases : Hot water has less capacity to hold dissolved gases than cold water, which may affect its rate of freezing.
Effect of the Surroundings : The difference between the initial temperatures of two containers of water may have an effect on the surroundings that could influence the rate of cooling. One example would be warm water melting a pre-existing layer of frost, permitting a better cooling rate.

Test It Yourself

Now, don't take my word for this! If you are doubtful that hot water sometimes freezes more quickly than cold water, test it for yourself. Be aware the Mpemba Effect will not be seen for all experimental conditions, so you may need to play around with the size of the water sample and the cooling water (or try making ice cream in your freezer, if you'll accept that as a demonstration of the effect).

Burridge, Henry C.; Linden, Paul F. (2016). "Questioning the Mpemba effect: Hot water does not cool more quickly than cold". Scientific Reports . 6: 37665. doi: 10.1038/srep37665
Tao, Yunwen; Zou, Wenli; Jia, Junteng; Li, Wei; Cremer, Dieter (2017). "Different Ways of Hydrogen Bonding in Water - Why Does Warm Water Freeze Faster than Cold Water?". Journal of Chemical Theory and Computation . 13 (1): 55–76. doi: 10.1021/acs.jctc.6b00735
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Does hot water freeze faster than cold water?

Is it true that hot water freezes faster than cold water and if so, what practical applications have there been found for this phenomenon?

2 I know it does because Peggy Knapp did it on Newton's Apple . That was a great program. – mmyers Commented May 19, 2011 at 15:54
3 I tried this by putting two glasses with the same volume of water in the freezer, one glass had room temperature water the other had hotter, recently boiled water. My outcome was that the cold water froze first as you'd expect. It is a well documented effect though and I'd be interested to know why my experiment gave a negative result. – Stephen Paulger Commented May 20, 2011 at 14:31
3 If evaporation is a key factor, then the surface area will be important for the experiment. Two glasses of water in a freezer will not yield the same result as two shallow amounts of water spread over a square foot each. The hot water will evaporate much faster when it is very shallow and spread out. It has less total mass to retain heat and a lot more surface area to cool it and evaporate it. The hot water will evaporate reducing mass and will then freeze faster than the cold water. The relative humidity in the air will also be a variable to consider. – user2952 Commented May 20, 2011 at 14:51
3 I once heard a rebuttal to this by claiming that hot water must become cold water before it could become frozen water. I find that a good example of completely missing the point. – MrHen Commented May 20, 2011 at 19:42
Related. physics.stackexchange.com/questions/122742/… – Takahiro Waki Commented Jan 17, 2018 at 6:01

4 Answers 4

In certain settings, cold water freezers slower than hot water. This is called the Mpemba effect .

The Mpemba effect is the observation that warmer water sometimes freezes faster than colder water. Although the observation has been verified, there is no single scientific explanation for the effect.

Can hot water freeze faster than cold water? , Monwhea Jeng, University of California, 1998

Hot water can in fact freeze faster than cold water for a wide range of experimental conditions. This phenomenon is extremely counterintuitive, and surprising even to most scientists, but it is in fact real. It has been seen and studied in numerous experiments. While this phenomenon has been known for centuries, and was described by Aristotle, Bacon, and Descartes [1—3], it was not introduced to the modern scientific community until 1969, by a Tanzanian high school student named Mpemba.

Some suggested reasons given in the paper:

Evaporation — As the initially warmer water cools to the initial temperature of the initially cooler water, it may lose significant amounts of water to evaporation. The reduced mass will make it easier for the water to cool and freeze. Then the initially warmer water can freeze before the initially cooler water, but will make less ice. [...] Dissolved Gasses — Hot water can hold less dissolved gas than cold water, and large amounts of gas escape upon boiling. So the initially warmer water may have less dissolved gas than the initially cooler water. [...]

2 I think it is worth giving at least some of the suggested reasons here (e.g. more evaporation of the hot water means less water to freeze). – Oddthinking ♦ Commented May 19, 2011 at 7:07
7 Wouldn't these theories, particularly the evaporation theory easily be tested??? For example measure how much ice is in each sample after the experiment??? – kralco626 Commented May 19, 2011 at 10:33
5 @mplungjan That's actually a different phenomenon called supercooling, where a liquid is cooled to below it's freezing point without it actually freezing (because it lacks a nucleus point from where the freezing should start), and freezes instantly upon applying a shock, or something that makes it non-homogenous. Check out wikipedia for more info: en.wikipedia.org/wiki/Supercooling – Andrei Fierbinteanu Commented May 19, 2011 at 11:15
3 One plausible explanation is that the warmer water has a stronger convection current when cooling. The angular momentum of the convection current sustains the current after dropping to a lower temperature, and so the water cools throughout more rapidly. – Richard Gadsden Commented Jul 1, 2012 at 17:42
5 Interesting answer is at MIT web: Does hot water freeze faster than cold water? Their conclusion is no . – Palec Commented Nov 25, 2013 at 3:58

This was, actually, my 6 th 5 th grade Science Fair experiment. :)

And I'd never heard of this effect before; it was a random experiment I thought of and tried.

My answer: it depends on what you mean by "freeze" .

Cold water starts freezing sooner (entering 0 degrees C), but hot water finishes freezing sooner (leaving 0 degrees C). I measured this with a digital thermometer.

No idea why, but I'm darn sure my experiment was accurate.

I found the data!

I blurred out the years to avoid carbon dating myself. ;)

3 This post does not cite any references or sources. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. – Larian LeQuella Commented Jan 13, 2012 at 21:17
15 @LarianLeQuella: Does original research count? – user541686 Commented Jan 13, 2012 at 21:20
57 Does "publication" and "peer review" at a science fair count for nothing these days? ;p BTW, did you win anything? – Dan Moulding Commented Mar 1, 2012 at 12:51
9 Bravo! Anyway, the fact that you, at age ten, had access to a computer for analysis already partially carbon dates you. :) – JasonSmith Commented Jan 7, 2015 at 14:40
12 Yeah, I guess I silicon-dated myself :) – user541686 Commented Jan 30, 2017 at 0:13

A new paper on this phenomenon has been published recently. It offers yet another explanation and has even caught the attention of popular media.

doi:10.1038/srep03005

arXiv:1310.6514v2 [physics.chem-ph]

They say the interaction between the hydrogen bonds and the stronger bonds that hold the hydrogen and oxygen atoms in each molecule together, known as covalent bonds, is what causes the effect. Normally when a liquid is heated, the covalent bonds between atoms stretch and store energy. The scientists argue that in water, the hydrogen bonds produce an unusual effect that causes the covalent bonds to shorten and store energy when heated. This they say leads to the bonds to release their energy in an exponential way compared to the initial amount stored when they are cooled in a freezer. So hot water will lose more energy faster than cool water. Dr Changqing said: “Heating stores energy by shortening and stiffening the H-O covalent bond. “Cooling in a refrigerator, the H-O bond releases its energy at a rate that depends exponentially on the initially stored energy, and therefore, Mpemba effect happens.” The Royal Society of Chemistry received more than 22,000 responses to its call for a solution to the Mpemba effect and it is still receiving theories despite the competition closing a year ago.

Quoted from Telegraph.co.uk .

It is true, in proper circumstances.

The Scientific explanation for that relates to the fact the freezing temperature may increase with the pressure.

The Mpemba effect is about freezing hot samples faster than cold which may not represent a substantial difference with small pressure variations but phenomenons like supercooling and superheating do have practical applications such as better preservation of organs in medical refrigerators and superconductivity in electrical devices.

You can find more about this in: The Mpemba effect: why hot water can sometimes freeze faster than cold .

6 The freezing temperature actually decreases when the pressure increases... (see the phase diagram of water : 1.bp.blogspot.com/_Ukz5Qzczfbc/TVEUPJxtDfI/AAAAAAAAB54/… ) – Jules Olléon Commented May 20, 2011 at 14:21

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Science project, does hot water freeze faster than cold water.

Have you ever refilled the ice cube tray in your freezer after using the last ice cube in your cup of juice? You probably automatically poured cold water in the ice cube tray without asking the question, "Does hot water freeze faster than cold water?"

It makes sense to believe that cold water would turn to ice before hot water because the hot water would need to cool first before it could freeze; but how do you know if that idea is correct? Test this theory —untested idea—will tell you whether cold water actually freezes faster than hot water.

Does temperature affect how quickly water freezes?

3 bowls of equal size and shape
Sticky labels
Measuring cup
Thermometer
Clear enough room in your freezer for the three bowls. You need to be able to put them in the freezer at exactly the same time, so you don't want to be moving your frozen food and drinks around later.
Think about what you know about ice. What temperature is water right before it freezes? You probably usually take baths in warm water. How quickly does the water turn cold when you're in the tub?
After considering different temperatures of water and ice, make a guess—called a hypothesis —answering the question: Does hot water freeze faster than cold water?
Write your hypothesis in your notebook, including whether you think the hot, warm, or cold water would freeze first and why .
Using your marker, write Hot on one of your sticky labels. Repeat with labels for Warm and Cold.
Place the sticky labels on each of the three bowls, using one per bowl. The labels will help you keep track of which bowl holds which temperature of water.
With your pencil, draw three columns in your notebook. Label the first column Hot, the second one Warm and the third Cold.
With the help of an adult, heat 1 cup of water to 100 degrees Fahrenheit. Pour it into the Hot bowl, being careful not to burn yourself.
Heat 1 cup of water to 70 degrees Fahrenheit, and pour it into the Warm bowl.
Fill the Cold bowl with water that's 40 degrees Fahrenheit.
Immediately place all three bowls in the freezer.
Record the starting temperatures in the correct columns of your notebook.
Open the freezer door every 10 minutes and take the temperature of the water in each bowl with a thermometer. Record the temperature in your notebook.
Repeat Step 13 until all three bowls have frozen over.
Compare the information in each of the three columns in your notebook. Was your hypothesis correct?

The bowls that contain the hot and warm water will freezer faster than the bowl that is filled with cold water.

Hot water freezing more quickly than cold water is known as the Mpemba effect . So, why does the Mpemba effect occur?

First, all water evaporates , which means that the liquid (water) "disappears" and becomes a vapor , or gas. Hot water evaporates at a much faster rate than cold water. This means that the bowl with hot water actually had less water than the bowl with cold water, which helped it freeze more quickly.

Second, convection (the transfer of heat within the water as it moves around) plays a part in helping hot water freeze more quickly than the bowl of cold water. The hot water has more convection currents than cold water, causing it to cool down much more quickly. That's why your bath water always seems to get cold much faster than you'd like!

Now that you know about freezing water at different temperatures, keep the science going by testing other liquids, such as milk or apple juice. Will warm milk freeze faster than cold milk? Or, switch up the project altogether! Does milk freeze faster than water at the same temperature? Science is all about guessing what will happen, then testing to see if you're right. You now know that hot water freezes faster than cold water, so brainstorm a new project that you're interested in. By constantly changing your experiments, you'll continue learning new things—and become a science whiz!

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Add to collection, create new collection, new collection, new collection>, sign up to start collecting.

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Is it true that hot water makes ice cubes faster than cold water?

Asks porterhouse from brooklyn, new york, greg soltis • september 22, 2008.

Does ice freeze more quickly if you start with hot water? <br> [Credit: Molika Ashford]

Does ice freeze more quickly if you start with hot water? [Credit: Molika Ashford]

Should i be scared of my tap water, melissa mahony • july 19, 2006.

Is it true that some frogs can survive being frozen?

Rachel Mahan • June 23, 2008

Deciphering bacteria’s defenses, one gene at a time, adam t. hadhazy • august 27, 2008.

Determining whether or not hot water can freeze faster than cold water may seem like a no-brainer. After all, water freezes at zero degrees Celsius. And wouldn’t water hot enough to kill E. coli bacteria (about 120 degrees Fahrenheit or 50 degrees Celsius) take a longer path than cooler water at a fall New England beach (about 60 degrees Fahrenheit or 15 degrees Celsius) towards a frigid future as ice? While a logical assumption, it turns out that hot water can freeze before cooler water under certain conditions.

This apparent quirk of nature is the “ Mpemba effect ,” named after the Tanzanian high school student, Erasto Mpemba, who first observed it in 1963. The Mpemba effect occurs when two bodies of water with different temperatures are exposed to the same subzero surroundings and the hotter water freezes first. Mpemba’s observations confirmed the hunches of some of history’s most revered thinkers, like Aristotle, Rene Descartes and Francis Bacon, who also thought that hot water froze faster than cold water

Evaporation is the strongest candidate to explain the Mpemba effect . As hot water placed in an open container begins to cool, the overall mass decreases as some of the water evaporates. With less water to freeze, the process can take less time. But this doesn’t always work, especially when using closed containers that prevent evaporated water from escaping.

A non-uniform temperature distribution in the water may also explain the Mpemba effect. Hot water rises to the top of a container before it escapes, displacing the cold water beneath it and creating a “hot top.” This movement of hot water up and cold water down is called a convection current. These currents are a popular form of heat transfer in liquids and gases, occurring in the ocean and radiators that warm a chilly room. With the cooler water at the bottom, this uneven temperature distribution creates convection currents that accelerate the cooling process. Even with more ground to cover to freeze, the temperature of the hotter water can drop at a faster rate than the cooler water.

So the next time you refill your ice cube tray, try using warmer water. You might have ice cubes to cool your drink even sooner.

Also on Scienceline :

Should I be scared of my tap water ?

Deciphering bacteria’s defenses , one gene at a time.

About the Author

Greg Soltis

18 comments.

The negative consequences of your tongue-in-cheek closing sentence gave me pause. Can you imagine the enormous amount of fresh water wasted (the cost of which is already skyrocketing in the US) should any sizeable portion of the populace begin running their taps until hot water pours out just to obtain “warmer (than tap water temp) water” for slightly faster ice cube formation? Add to that the wasted energy caused by heating the water only to cool it down again. Comparing/quantifying the inputs and savings – fresh water costs + energy cost of heating a volume of water – energy saved by not running the freezer compressor quite as long to cool it down again gives me a slurpee-like “brain freeze”!

Oh, it would be terrible! with americans making ice cubes constantly as we all do, it would virtually eliminate all water from the planet in a matter of days! I brushed my teeth this morning, I left the faucet on too. I got news for you, there is no more or less water on the planet now as there was this morning or there will be tomorrow.

Just want to say- I live in an area that has very cold winter temperatures, and the hot water pipes ALWAYS freeze before the cold water pipes.

Ed is correct, the hot water will cause global warming and then we won’t have any ice. I’m totally serial.

A coworker who tried the warm-water method for making ice noted that the ice had absorbed smells/taste from other food in the freezer.

Learned this awhile ago while working as a chef’s assistant. Hot water freezes faster and COLDER water boils faster.

Making water warmer would not make it feeze faster, it has to be boiling. I once left bowls of hot, warm and cold water out in freezing temperatures and found that the hot water froze first, the cold water second and the warm water last.

Hi – I always thought that the hot water ice blocks froze first because of the extent of the difference in temperature between the water and the surrounding cold air, with the freezing being caused by the rapid osmosis – in the same way that evaporation causes cooling.

However, I was wondering, does this just effect the outside edges of the ice-block? If so, how does this effect the time it takes for the whole ice-block to freeze – right through to the core of the block?

Great goods from you, man. I have understand your stuff previous to and you are just extremely wonderful.

This is the most idiotic post and comment thread (aside from the tongue in cheek ones) that I have ever read.

I would like to know about logical difference between ice cold water and simple cold water?

Has anyone thought I how gross the inside of a hot water tank is… I wouldn’t drink or eat anything that comes from a hot water tank. I have seen it it’s not safe to drink… Why make ice cubes from it?!

very nice blog

My never images are never as good :(

I guess the majority of people did not take physics or even simple science in high school. This has been taught for decades. Ha.

wow I never knew this, got landed to this page while searching for something similar on goolge :) will try and compare and come back to post if this is right !!

Back when I worked as a bartender the notion was that making ice using hot water resulted in clearer ice cubes than using cold water.

I have observed this to be true, the cloudiness in conventional ice cubes appears to be trapped and compressed dissolved gasses in the water. Heating the water expels the gasses, so the ice freezes clear.

The rate at which ice cubes melt is faster with hot water. However, whether they freeze faster overall I am going to see for myself. Time for some good old fashioned expiriementation.

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What Freezes first… Hot or Cold Water?

Follow FizzicsEd 150 Science Experiments:

You Will Need:

One cup of cold water, 100mL in volume
One cup of hot water, 100mL in volume
One Stopwatch
One Stirrer
A pen and paper

What freezes first, hot or cold water Science Experiment - setupmaterials

Instruction

What freezes first, hot or cold water Science Experiment - end results

Stir both water cups the same amount of time. Place both cups of water inside your freezer and start the timer.

Keep checking at 5 minute intervals to see which freezes first.

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Record your observations. What happened?

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Why Does This Happen:

Hmmmm, you’ve completed an experiment whose results are quite tricky to explain. Did you find that the hot water froze first? Under some conditions this can happen…

We started with two containers of water, which were identical in shape and held identical amounts of water. The only difference between the two was that the water in one was at a higher (uniform) temperature than the water in the other. Of course, if the hot water had started at 99.9° C, and the cold water at 0.01° C, then clearly under those circumstances, the initially cooler water would have frozen first. However, under some conditions the initially warmer water will freeze first — if that happens, you have seen the Mpemba effect which describes the phenomenon.

How does it work you might ask? Several ideas have been put forward and no-one really is sure as to which effect plays the biggest role:

1. As the initially warmer water cools to the freezer temperature, it may lose significant amounts of water to evaporation. The reduced mass will make it easier for the warmer water to cool and freeze than the colder water.

2. A convection current may have been setup in the warmer water. As the warmer water cooled it lost heat primarily through the surface of the liquid faster than the colder water. This is due to a great temperature difference between the cold freezer air and the warm water. The water from the bottom of the cup then rose to the water surface, bringing more heat energy to the cold freezer air. As the current is greater in the warmer water than the cold water, a greater amount of liquid got exposed to the cold freezer air. Think of a fan forced oven, circulating the hot air through the oven heats the oven faster than just allowing the air to sit still… bakers have known this over a thousand of years!

More on temperature and water rising or falling 3. The surrounding air around the cups may have more movement around the warmer cup, therefore drawing heat energy away from the warmer cup more effectively.

4. Warm water holds less dissolved gas than cold water. There have been some suggestions that the presence of dissolved gases impede the production of convection currents in the colder water.

5. The cold water may have supercooled , therefore not forming a solid as quick as the hot water.

Quote: “I often put boiling water in the freezer. Then whenever I need boiling water, I simply defrost it.” -Gracie Allen.

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28 thoughts on “ What Freezes first… Hot or Cold Water? ”

Very helpful and useful for kids and everyone and it’s easy to understand.

Thanks Amal, we’re glad that you’ve been using these science activities!

can you please answer these 2 questions: What’s the independent, dependent & controlled variable of this experiment? And what equipments have you used for this experiment?

The independent variable for this experiment was the temperature of the water (this was the variable that we could manipulate). The dependent variable was the time it took for the water to change from liquid to solid (as this depends on the initial temperature of the water). The controlled variables were the freezer temperature, the cup size, the volume of the water used in each test and the amount of stirring in each cup. More details on variable testing here

You can find the details on the materials needed at the top of this page. Have fun with this experiment!

what are some risks that can take place when doing this experiment

If the water is close to boiling you really need to be careful. Also, water spills are a slipping hazard.

Is this an actual good experiment that will work in class

Hi Ben, give it go and see what happens! All the best.

is it ok to do in school

Sure thing, just check with your teacher first and let them handle the hot water. Have fun!

i don’t have it yet,but it sounds like its super fun! : )

Its an interesting experiment! Please let us know what your results were… do they conform to the theory above? Have fun!

How hot should the hot water be?

Hi! Try cups with different amounts of heat – you might find that there is a particular temperature where the result can be opposite 🙂 We like to use near-boiling vs room temperature, but you could try every temperature as a series of tests (as long as you’re safe with water over 55 degrees celsius). Have fun!

Hi, can you please describe the science involved in this investigation

Hi Jae! This is based on the Mpemba effect. Check out further details here

The cold water froze first for me, why?

This has been a tough one for us to reproduce too! We’ve found that there can be slight differences within the freezer itself, whereby placement of the cups in relation to the vents being a crucial factor. Did you try it again?

wow, this experiment was really surprising at first but now that I have read the science behind it, it makes a lot more sense.

Great to hear that you were able to confirm this experiment! It’s a strange one to observe 🙂

Just did the experiment with my son. I was fully expecting the hot water to freeze first, but it ended up being the cold water that froze. Cold started to freeze after 20 minutes. At 40 minutes cold is solid and hot is probably a third of the way frozen. Disappointed that the results didn’t turn out as described in the experiment. Maybe there needs to be more controls included in the experiment. I made sure to use the same cup and I put the cups in the same spot of the freezer. It would be good to have a suggested temperature for both.

Hi Alisa, I’m glad to hear you and your son have tried this experiment! Freezing water may seem commonplace, so you’d think that scientists would have this all figured out…but in fact, the Mpemba Effect is still being researched and redefined! From fancy labs where things are monitored and controlled down to the molecular level to high school classrooms, scientists see different results. This is because the effect is only sometimes observed under “some conditions”, as we have stated in the experiment description. The latest research suggests that convection currents, nucleation sites, supercooling and dissolved gases/ions all might play a part, and have complex interactions. This means that even though you have used the same cup and put it in the same spot in the freezer (great variable control, by the way!), our freezers at home may still introduce inconsistencies as it cycles on and off to maintain temperature (usually -10 to -18 degrees Celcius), or have the cooling vent in a location which favours the cold or hot sample (top or bottom may influence the outcome). How we pour the water into the cup and walk it over to the freezer and place it might make a difference. One scientist even wrote that “two samples of water taken from the same bottle may differ significantly”! While these are factors that are very hard to control at home, this is such an interesting experiment to attempt because this is not an experiment where it will “work” or “fail”. At Fizzics we don’t always get the same results either, but we love figuring out why we see each observation! The authors of this article had some success with around 50 degrees Celcius difference in sample temperatures. Fizzics scientists have observed the effect in the past with nearly boiling (80-100 degrees) vs. cold tap water.

I loved this experiment

Glad that you enjoyed it!

Hi this is perfect for my students because I looked it up on Google and it was perfect and like to say thank you guys for putting this up because I need to show it to my students and I love how it’s very good experience

Hi this is perfect for my students

Fantastic! Please let us know how you go 🙂

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Fizzics Education ran a show today at our school and it was wonderful. He was a great facilitator and the show was age appropriate and well done.

I just wanted to pass on how much the staff and students really enjoyed it and how perfect it was to launch our science week activities. The students were enthralled, educated and entertained – a perfect trifecta!

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COMMENTS

The Mpemba Effect: Does Hot Water Really Freeze Faster Than Cold Water
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 ...
The Physics of Why Hot Water Sometimes Freezes Faster Than Cold Water
Under certain forces from the laser, the hottest beads cooled faster than the lower temperature beads. The research suggests that the longer path from a higher temperature to the freezing point ...
A new experiment hints at how hot water can freeze faster than cold
The experiment was inspired by reports of the Mpemba effect, the counterintuitive observation that hot water sometimes freezes faster than cold. But experiments studying this phenomenon have been ...
Does hot water freeze faster than cold water?
It's an age-old question with a simple answer: no. By Sarah Jensen. Since the time of Aristotle, researchers and amateur scientists alike have batted about the counterintuitive theory that hot water freezes faster than cold. The notion even has a name: the Mpemba effect, named for a Tanzanian schoolboy who in 1963 noticed that the ice cream ...
Does Hot Water Freeze Faster Than Cold? Physicists Keep Asking
Or perhaps external factors come into play: A layer of frost in a freezer can act as an insulator, keeping heat from leaking out of a cold cup, whereas a hot cup will melt the frost and cool faster. Those explanations all assume that the effect is real — that hot water really does freeze faster than cold. But not everyone is convinced.
Does Hot Water Really Freeze Faster Than Cold Water?
As the video above explains, the phenomenon of hot water freezing faster than cold water is known as the Mpemba effect, named after Erasto Mpemba, a Tanzanian student who in 1963 was making ice cream as part of a school project. The students were meant to boil a mixture of cream and sugar, let it cool down, and then put it in the freezer.
Is It True that Hot Water Freezes Faster than Cold Water or that Cold
"To the first part of the question--'Does hot water freeze faster than cold water?'--the answer is 'Not usually, but possibly under certain conditions.' ... "You can readily set up an experiment ...
Investigating the 'Mpemba Effect': Can Hot Water Freeze Faster than
Introduction. It may seem counterintuitive, but folk wisdom and a body of published evidence agree that, under some conditions, warmer water can freeze faster than colder water (for an excellent review on the subject, see Jeng, 2005). This phenomenon has been known for a long time, but was rediscovered by a Tanzanian high school student, Erasto ...
You really can freeze hot water faster than cold*
Hot water really can freeze faster than cold water, a new study finds. Sometimes. Under extremely specific conditions. With carefully chosen samples of water. New experiments provide support for a ...
Why Does Hot Water Freeze Faster Than Cold Water?
The most commonly proposed hypothesis - and one that's probably somewhat responsible for the effect - is that hot water evaporates more quickly, losing mass and therefore needing to lose less heat in order to freeze. However, scientists have also demonstrated the Mpemba effect with closed containers where evaporation doesn't take place.
Does Hot Water Freeze Faster Than Cold Water?
Here's how it works. Hot water freezes faster than cold, known as the Mpemba effect.(Image credit: Image released under GNU Free Documentation License.) Determining whether or not hot water can ...
The ice cream that changed physics
That's because it appeared to be proof of a strange and counterintuitive idea: that a hot liquid may freeze faster than a cold one. ... Mpemba repeated the experiment with hot and cold water ...
Can hot water freeze faster than cold water?
Yes—a general explanation. Hot water can in fact freeze faster than cold water for a wide range of experimental conditions. This phenomenon is extremely counterintuitive, and surprising even to most scientists, but it is in fact real. It has been seen and studied in numerous experiments. Although this phenomenon has been known for centuries ...
Does hot water freeze faster than cold water?
Will - So a rough container with hot water will freeze faster than a smooth container with cold water. It should be stated, however, that cold water in a rough container will still freeze faster than hot water in a rough container. So perhaps Mr. Mpemba had a rougher ice tray than everyone else. Henry - I think it comes down to this lovely ...
Why Hot Water Freezes Faster Than Cold—Physicists Solve the Mpemba
The Mpemba effect is the observation that warm water freezes more quickly than cold water. The effect has been measured on many occasions with many explanations put forward.
Mpemba effect
The Mpemba effect is the name given to the observation that a liquid (typically water) which is initially hot can freeze faster than the same liquid which begins cold, under otherwise similar conditions. There is disagreement about its theoretical basis and the parameters required to produce the effect. [1][2]
Does Hot Water Freeze Faster Than Cold Water?
It will actually take more time and energy to freeze hot water because it must be brought down further in temperature until it reaches the freezing point, about 0°C. Govindan suggests conducting a simple experiment to demonstrate that hot and cold water will behave as logic predicts. "Fill two identical containers with hot and cold tap water ...
Does hot water freeze faster than cold water?
It sounds completely counterintuitive, but whether hot water freezes faster than cold water has been debated for centuries. The Mpemba Effect. The Mpemba effect is the term used for hot water freezing faster than cold water after a Tanzanian student named Erasto Mpemba found that his mixture of hot milk and sugar froze faster than his classmate's mixtures that had been left to cool before ...
Is It True Hot Water Freezes Faster Than Cold?
Updated on December 02, 2018. Yes, hot water can freeze faster than cold water. However, it does not always happen, nor has science explained exactly why it can happen. Key Takeaways: Water Temperature and Rate of Freezing. Sometimes hot water freezes more quickly than cold water. This is called the Mpemba effect after the student who observed it.
physics
The hot water will evaporate much faster when it is very shallow and spread out. It has less total mass to retain heat and a lot more surface area to cool it and evaporate it. The hot water will evaporate reducing mass and will then freeze faster than the cold water. The relative humidity in the air will also be a variable to consider ...
Does Hot Water Freeze Faster Than Cold Water?
First, all water evaporates, which means that the liquid (water) "disappears" and becomes a vapor, or gas. Hot water evaporates at a much faster rate than cold water. This means that the bowl with hot water actually had less water than the bowl with cold water, which helped it freeze more quickly. Second, convection (the transfer of heat within ...
Is it true that hot water makes ice cubes faster than cold water?
With the cooler water at the bottom, this uneven temperature distribution creates convection currents that accelerate the cooling process. Even with more ground to cover to freeze, the temperature of the hotter water can drop at a faster rate than the cooler water. So the next time you refill your ice cube tray, try using warmer water.
What Freezes first… Hot or Cold Water? : Fizzics Education
The only difference between the two was that the water in one was at a higher (uniform) temperature than the water in the other. Of course, if the hot water had started at 99.9° C, and the cold water at 0.01° C, then clearly under those circumstances, the initially cooler water would have frozen first. However, under some conditions the ...

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

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Does hot water freeze faster than cold water?

4 Answers 4

This was, actually, my 6 th 5 th grade Science Fair experiment. :)

I found the data!

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Is it true that hot water makes ice cubes faster than cold water?

Does ice freeze more quickly if you start with hot water? [Credit: Molika Ashford]

Rachel Mahan • June 23, 2008

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What Freezes first… Hot or Cold Water?

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