Advent Calendar of Science Stories 21: Hot and Cold

Another weekend day, another story I'm going to outsource a bit. In this case, to the original scientist, who at the time of his discovery was a 13-year-old schoolboy in Tanzania:

In 1963, when I was in form 3 in Magamba Secondary School, Tanzania, I used to make ice-cream. The boys at the school do this by boiling milk, mixing it with sugar and putting it into the freezing chamber in the refrigerator, after it has first cooled nearly to room temperature. A lot of boys make it and there is a rush to get space in the refrigerator.

One day after buying milk from the local women, I started boiling it. 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 icetray without boiling it; so that he may not miss his chance. 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. 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.

I asked my physics teacher why it happened like that, with the milk that was hot freezing first, and the answer he gave me was that “You were confused, that cannot happen”. Then I believed his answer.

Fortunately for science, though, the schoolboy in question, Erasto Mpemba, didn't forget this incident, and kept after the problem. He asked friends and acquaintances who also made ice cream about it, and they reported seeing the same phenomenon. He kept pestering his physics teachers about it, as well, to no avail; one replied in class "that is Mpemba's physics, not the universal physics," which became a mocking shorthand any time he made a mistake-- "That's Mpemba's ____..."

Eventually, he asked a visiting university professor from Dar es Salaam, Denis Osborne, the same question. Unlike Mpemba's past teachers, Osborne treated the question seriously, and promised to investigate when he got back to his lab. Where, to his great surprise, he found that Mpemba was right-- if you put equal volumes of hot and cold water into a freezer at the same time, the hot water will sometimes freeze faster than the cold.

It turns out that there are references to this odd phenomenon dating back to the days of Aristotle. Mpemba is the first to launch a really systematic study, though, and he and Osborne published a paper about the "Mpemba Effect" in 1969 (PDF available from the Royal Society of Chemistry's history). The effect still defies explanation, though-- in 2013, the Royal Society of Chemistry ran a contest looking for the best explanation, but none of the 22,000 was truly conclusive.

Mpemba's story does, in fact, appear in Eureka: Discovering Your Inner Scientist, but it's too good not to re-use. It's an absolutely wonderful demonstration of both the simplicity and universality of science: discovering the Mpemba effect didn't require sophisticated and expensive apparatus, just a careful eye and persistence. It's also a reminder of the importance of keeping an open mind-- any of Mpemba's teachers could've done the experiment and thus claimed a share of the credit, but they weren't willing to test what they thought they knew.

The core lesson here is just to keep following the scientific process: keep looking at the world around you, thinking about the things you see, testing anything weird you run across, and telling people what you find. Even if the first couple make fun of you, because sometimes anomalous physics turns out to be universal physics after all.


(Part of a series promoting Eureka: Discovering Your Inner Scientist, available from Amazon, Barnes and Noble, IndieBound, Powell’s, and anywhere else books are sold.)

("Featured image" above taken from this Times (UK) article.)

(There's a nice exploration of the original Mpemba Effect paper by Dr. SkySkull if you want more detail.)


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