As a young boy, I learned not to try to solve family arguments between my elders. My father told my aunt (his sister) that he had heard that hot water freezes faster than cold water. She said that could not be true, and that led to a heated argument between them. I thought this was not something worth arguing about, and I suggested that we test the theory by the scientific method. Does hot or cold water freeze faster?
Thinking I could settle the argument, I suggested we fill one ice-cube tray with hot water and one with cold water, put them both in the freezer and see what happens. This suggestion from a young “whippersnapper” brought down the wrath of both sides. I realized that they didn’t want to know the truth. They just wanted to argue. Perhaps they were both afraid they might be proven wrong. I have since learned that believers and skeptics often fear looking at evidence for that very reason.
So, does hot or cold water freeze faster? The short answer is that it depends. Some famous thinkers, including Aristotle, Francis Bacon, and Rene Descartes, noticed that hot water sometimes froze faster than cold water. But it took a Tanzanian secondary student making ice cream in 1963 to finally bring this question into scientific focus.
Young Eraso Mpemba, along with fellow students at Magamba Secondary School, was making ice cream to earn some money. He was in a hurry, so rather than letting it cool, he put the hot ice cream mix into his freezer. He was surprised that his ice cream froze faster than the colder mix of his fellow students. Mpemba’s teacher and classmates laughed at his claim that the hot mix froze faster.
Later, a noted British physicist named Denis Osborne came to the African school, and Mpemba asked him to explain why hot water freezes faster than cold. Not only did his teacher and classmates think Mpemba’s suggestion was absurd, but the physicist was also skeptical. However, Dr. Osborne was open-minded enough to test it experimentally. (Like I suggested to my father and aunt.) In 1969, when Mpemba was in college, he and Osborne published a paper on the phenomenon, which came to be called the Mpemba effect.
So some people, when asked, “Does hot or cold water freeze faster?” were no longer laughing at the “foolish question.” However, nobody knew how hot water could freeze faster. After all, if the water is already closer to freezing temperature, it should freeze in less time. Shouldn’t it? The truth is that the question does not have a simple answer. In 2013, scientists in Singapore proposed a solution. Tomorrow, we will look at their explanation.
— Roland Earnst © 2023