The Mpemba Effect: The Bizarre Phenomenon Where Hot Water Freezes Faster Than Cold

The answer seems obvious, right? It’s gotta be the cold glass – the water inside has fewer degrees to drop. And even if you take into account the greater temperature difference between the surroundings and the hot water, eventually it’s going to reach the cold water’s temperature anyway – at which point it’s just going to do what that did, but later. Gotta be the cold water that freezes first.

So, uh… why doesn’t it?

What is the Mpemba effect?

“Water [which] has previously been warmed […] cools sooner.” So wrote Aristotle, sometime around 340 BCE. It was, he thought, common knowledge: “[M]any people, when they want to cool hot water quickly, begin by putting it in the sun,” he pointed out.

Francis Bacon was similarly convinced of this thermodynamical hack: “Aqua parum tepida facilius conglacietur quam omnino frigida,” he wrote in his 1620 opus Novum Organum – or, in English: “Slightly lukewarm water freezes more easily than completely cold water.” Even Descartes – he of the thinking and the therefore amming – was aware of the concept, claiming in his Discourse on the Method that “water that has been kept on a fire for a long time freezes faster than other waters.”

It wasn’t until the 20th century, though – 1969, to be precise – that this phenomenon became associated with the name it now bears: Mpemba. 

“My name is Erasto B Mpemba, and I am going to tell you about my discovery, which was due to misusing a refrigerator,” begins the paper that reintroduced to the world this counterintuitive physical phenomenon. It goes on to recount a botched attempt at ice cream making: “After buying milk from the local women, I started boiling it,” Mpemba wrote. “Knowing that if I waited for the boiled milk to cool before placing it in the refrigerator I would lose the last available ice-tray, I decided to risk ruin to the refrigerator on that day by putting hot milk into it.”

But he wasn’t alone in his dairy-based shenanigans that day. “Another boy, who had bought some milk for making ice-cream, ran to the refrigerator when he saw me boiling up milk and quickly mixed his milk with sugar and poured it into the ice-tray without boiling it; so that he may not miss his chance,” Mpemba explained. “The other boy and I went back an hour and a half later and found that my tray of milk had frozen into ice-cream while his was still only a thick liquid, not yet frozen.”

Naturally, this puzzled the boys, and Mpemba asked his physics teacher why it happened… only to be told he was wrong. “You were confused,” he recalled his instructor saying. “That cannot happen.”

Years of mockery and repudiation later, Mpemba had a chance to pose his question to an actual physics professor – and he got a slightly warmer reception.

“I confess that I thought he was mistaken,” Denis Osborne, the physicist from the University of Dar Es Salaam who Mpemba finally put his challenge to, wrote. “But fortunately [I] remembered the need to encourage students to develop questioning and critical attitudes. No question should be ridiculed.”

Osborne’s open-mindedness was to his credit: he tasked one of his technicians with repeating the experiment, and they confirmed that the hot water did indeed freeze faster. In fact, quite a steep gradient was found: the hottest water, at around 90°C (194°F), took just 30 minutes to freeze, while water with an initial temperature of 70°C (158°F) took more than double that time. The coldest water tested of all, at 20°C (68°C) – chilly enough that you wouldn’t want to swim in it, for sure – took a whole 100 minutes to freeze over.

The evidence was in – and it seemed to support Mpemba’s assertion. But why?

How does it work?

The results were surprising – and the explanation was elusive. “A number of possible explanations for the effect have been proposed,” then-University of California Riverside physicist Monwhea Jeng wrote in 1998, “but so far the experiments do not show clearly which, if any, of the proposed mechanisms is the most important one.”

For Descartes, the answer was simple: “the reason” for the phenomenon, he stated, was “that those of [the water’s] particles that are least able to stop bending evaporate while the water is being heated.” In other words: boiling the water first gets rid of the hot bits. Or something. Look, it’s obviously not quite right – but old René might not have been too far off the mark. 

“As the initially warmer water cools to the initial temperature of the initially cooler water, it may lose significant amounts of water to evaporation,” Jeng pointed out. “The reduced mass will make it easier for the water to cool and freeze.”

“This explanation is solid, intuitive, and evaporation is undoubtedly important in most situations,” he wrote. “But it is not the only mechanism.”

Modern-day scientists have presented a range of potential explanations for the Mpemba effect. Perhaps, physicists suggested in 2013, it has something to do with the effect of heating on the water’s covalent bonds. Maybe it’s got something to do with the levels of gases within the water. 

“Hot water can hold less dissolved gas than cold water, and large amounts of gas escape upon boiling,” Jeng explained. “It has been speculated that this changes the properties of the water in some way, perhaps making it easier to develop convection currents (and thus making it easier to cool), or decreasing the amount of heat required to freeze a unit mass of water, or changing the boiling point.”

Cheekiest of all: maybe it’s not to do with the water itself at all. Stick some warm water in a bog-standard freezer, and the heat may melt some of the frost surrounding it, connecting the container with the more conductive materials of the appliance. Colder water, meanwhile, is insulated by the frost layer around it – as well as the layer that formed oh-so-easily on the surface thanks to the lower temperature.

More recent experiments have yielded even less intuitive solutions. “We all have this naive picture that says temperature should change monotonically,” Oren Raz, a physicist who studies nonequilibrium statistical mechanics at the Weizmann Institute of Science in Israel, told Quanta in 2022. “You start at a high temperature, then a medium temperature, and go to a low temperature.” 

But for an object driven out of equilibrium – say, by forcing it to shed its heat energy as fast as possible – “it’s not really true to say that the system has a temperature,” he explained. “Since that’s the case you can have strange shortcuts.”

New explanations of the Mpemba effect have strayed into quantum theory, laser physics, and Markovian dynamics of solids. The phenomenon has even been found working backwards, with cold systems heating up faster than warm ones.

But as much as we learn about the phenomenon, we still don’t have a full explanation for it – and there might be a very good reason for that.

Is the Mpemba effect actually real?

Could it be that the Mpemba effect is so inexplicable… because it simply doesn’t exist? As counterintuitive as the phenomenon first seemed, the idea seems silly to suggest after all this evidence in support of it – but technically, it’s still an open question.

“The Mpemba effect is not observable in any meaningful way,” was the conclusion of a 2016 study by Imperial College London physicist Henry Burridge and University of Cambridge mathematician Paul Linden. Not only had their analysis of previous results convinced them that almost all examples of the Mpemba effect were “marginal”, but attempting to recreate the result on their own had revealed a surprisingly large confounding variable: the height of the thermometer.

“Much of the data reporting to be observations of the Mpemba effect were from studies not reporting the height at which temperatures were measured,” the pair pointed out. Worse still for the Mpemba effect, they found that by taking those temperatures at different heights – even those separated only by a centimeter – they could “produce” an effect that wasn’t present if they measured from the same place. 

“Despite our best efforts, we were not able to make observations of any physical effects which could reasonably be described as the Mpemba effect,” they concluded – and previous experiments in support of the phenomenon “could have been altered by simply recording temperatures without precisely monitoring the height.”

“We are not gladdened by such a conclusion, indeed quite the opposite,” they added. “The Mpemba effect has proved to be a wonderful puzzle with which to engage and interest people of all ages and backgrounds in the pursuit of scientific understanding. However, the role of scientists is to objectively examine facts and further knowledge by reporting the conclusions, and as such we feel compelled to disseminate our findings.”

Still, many other physicists remain convinced of the effect’s validity. We may not fully understand it yet, but that’s to be expected: nonequilibrium thermodynamics is a weird and unintuitive field, and there’s a lot we still haven’t unraveled about it.

“Interesting effects take place when you perform temperature quenches from cold to hot,” Raúl Rica Alarcón, a postdoctoral researcher in the University of Granada’s Department of Applied Physics, told HowStuffWorks – and the Mpemba effect, he said, is likely “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.”

“My view is that the Mpemba effect can take place under some special circumstances,” he said. “But we are still trying to figure out what are the minimal conditions for this to happen.”