The Teenager Who Disproved Thermodynamics: The Mpemba Effect

The Retro Bliss

This is a bit of a riddle. It turns out that if you have a glass of hot water and you have a glass of cold water, and you put them in a freezer, one of them freezes faster than the other. The hot water actually freezes more quickly. This is called the Mpemba effect.

The Mpemba effect was in fact named after a Tanzanian schoolboy who thought against all logical reasoning that ice cream would freeze quicker if it was first heated. His classmates and peers mocked him until he partnered up with his instructor and they performed an experiment.

Erasto Bartholomeo Mpemba was a teenager at the time and he went to school in Tanzania in the 1960s. He had lessons in cookery and that’s what led him to the ice cream angle. He was taught to make ice cream by heating up milk and sugar together on the stove. Then you’re supposed to leave the mixture to cool to room temperature before you put it in the freezer to make delicious ice cream. Maybe because he was just a rebel without a cause or a rebel without a cream, he put his hot mix of milk and sugar straight into the freezer. He found that it froze faster than it should have been. Everybody assumed it would take longer. That’s just common sense, right? It’s strange because it turned out that was not the case.

From what we understand, Mpemba was kind of the black sheep of his class. He wasn’t super popular. That’s because he stuck to his guns and some of his quirks were weird ones in 1963. When Mpemba was in form three at Makamba secondary school, he was fond of making ice cream. Recall that he had decided to put his hot mixture of ice cream into the freezer. He came back an hour and a half later to check on it and found that his tray of milk had frozen into ice cream completely but that of his pals, who had put a cooled mixture, was still not frozen. It became a thick liquid, but it wasn’t quite ice cream. He was surprised and he asked his physics teacher about it. His physics teacher thought he was confused because what he was describing was the impossible in the 1960s.

So, he kind of let it go assumably because he thought his teacher knew better. But later that year, he met a pal who had his own little side hustle, selling ice cream. He told Mpemba that when he made his ice cream, he put the hot liquids in the fridge to make them freeze faster.

This reminds me a little bit of when you hear stories of Western Europeans discovering something like smallpox vaccines and stuff, and this is no shade to Edward Jenner. It typically turns out that somewhere else in the world, there’s some ancient culture that knew about this for 1000s of years. And in the case of the Mpemba effect, it wasn’t just the ice cream vendors who knew about this. It turns out that people to some degree knew about what we call the Mpemba effect as far as 2000 years ago.

The Mpemba effect has already been observed for a fair amount of time, dating back to the 4th century BC. Some scientists such as Francis Bacon noted that “slightly tepid water freezes more easily than that which is utterly cold.” Various scientists have argued that it occurs for reasons such as evaporation and supercooling, and people have been freezing warm water for a long time to test out this effect. However, this weird phenomenon wasn’t brought to the discussion table until the 1960s.

Mpemba is the one who brought this back into the conversation. Fast forward to when he’s in high school, he learns about something called Newton’s law of cooling. It essentially lays out the rules for how hot bodies are supposed to cool and they have a couple of assumptions, you know, to keep it simple. So, he asked his teacher again who would end up making fun of him but thankfully, Mpemba was not so easily cowed. He was tenacious, and he kept going.
Erasto B MpembaTEDxDar

Eventually, he teamed up with a man named Dr. Denis G. Osborne, a professor of physics at the University of Dar es Salaam. He met him when he went to speak at Makawa High School. And at this event at the high school, he kind of fielded questions from the student body about like, what should I do with my life?, You know, what do I want to be when I grow up? and all that stuff. Mpemba asked, Why does hot water freeze more quickly than cold?

According to Mpemba’s recollection, Professor Osborne smiled and asked him to repeat his question. After the repetition, the professor asked if he had tried it then admitted to not knowing the answer but promised to try the experiment when he was back in Dar es Salaam. This question stuck with Osborne.

Osborne actually was pretty surprised because there was a tech at the University College in Dar es Salaam who tried the same thing and had the same results. The professor didn’t believe the technician either. So, he experimented with it. He took one glass of water at 95 degrees Fahrenheit and took another glass of boiling water, and put them both in a freezer. The hotter one did turn to ice at a faster rate. So in 1969, Mpemba and Osborne published a paper on this experiment in physics.

At this time, Mpemba was studying at the College of African Wildlife Management, but he never gave up his fascination with the Mpemba effect. And how cool is it to have an effect named after you?

Let’s be honest, he did the least amount of work on the Mpemba effect. He was rewarded for his tenacity, standing up, and asking the question.

Scientists have come up with a bunch of different explanations, but none of them 100% solves the case. The most straightforward explanation is the idea that hot water evaporates more than cold water. So when boiling water cools down, it’s losing a little bit of its mass, because it’s turning to steam, right? It floats away. If that is the case, hot water freezes faster than cold water because there’s less of it. It sounds like an elegant answer to the question, or is it? Because there’s a problem with this theory? Remember, not all of them are 100%? What’s the problem with this one?

The problem is that most of those explanations only apply in a limited set of cases. It’s tough. There are a lot of variables at play. And of course, you know, scientists, at the edge of research can be somewhat oppositional and passionate, and they want to argue back and forth to find the correct answer. There was a paper published in 2016 by Henry C Burridge and Paul F Linden, in which they argue that there is no meaningful evidence to support the Mpemba effect. They think it’s misleading. They think the Mpemba effect is a red herring, and that they’re just slight inaccuracy is a measurement.

So that’s the thing. What are we measuring, when we say freezes faster? Right, that’s the issue. We talked about the time it takes to get to the freezing point, or the time it takes for it to become completely frozen over. And there are different time frames where these things can happen. If you froze two things in a freezer, even if it was two glasses of water, at the same exact temperature? Wouldn’t they potentially go through the same exact process at the same exact time? Could it have to do with the particular zone that they’re in the freezer? It does occur to me that in my fridge, for example, if I have something closer to the back of the fridge, it’s going to freeze a little bit more than stuff closer to the front. There are definitely variations in temperature in what would be a consumer fridge, which is what Mpemba was using when he originally observed this phenomenon.

In that paper they argue there’s no such thing as the Mpemba effect, the scientists say there’s a 10% inaccuracy when you measure the height of a thermometer when you’re taking the water’s temperature, and that could trick someone into thinking that they have observed this effect. That paper's argument is still pretty controversial.

Mpemba eventually achieved his goal of studying abroad. He got a degree in Australia at the Canberra College of Advanced Education in Canberra. He became a principal game officer in the Tanzanian Ministry of Natural Resources and Tourism in the wildlife division. And after a great career, he retired. When he was asked if any of his children were physicists, he said, ‘no, they are not doing well at all in physics’.


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I tell conversational style stories of some of the most ridiculous events that have occurred in the past.

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