Have you ever thrown boiling hot water in the air on a cold day and watched it freeze before hitting the ground? For those of us who live in colder climates this is common practice especially when trying to impress the kids. But why boiling water? Shouldn’t cold water freeze faster? Well, not necessarily.
What you are dealing with is called the Mpemba effect, and although we know it exists, no one can fully explain it. There are, however, a few plausible explanations.
The Mpemba effect was named after a 13-year-old student in Tanzania who identified the phenomenon while trying to make ice cream faster than his classmates. Rather than wait for his boiled milk mixed with sugar to cool, like the other students, he stuck his mixture in the freezer while the water was still boiling hot—and it froze faster than any of his classmates’ ice cream. Later in life, Mpemba teamed up with a visiting physics professor and they published the first paper on it in 1969.
The most popular hypothesis makes sense, though it’s still not an exhaustive explanation. Hot water evaporates more quickly than cold, which lowers it’s mass. Less mass means there is less to freeze, thus freezing will occur more quickly. But the Mpemba effect was still observable in closed containers where evaporation couldn’t take place.
Another theory is that the variation in temperatures throughout the hot water causes convection currents. The currents’ movement then cause the water to cool more quickly. A uniformly cool glass of water, for example, will have less temperature variations and so less convection to speed up the cooling process.
For now it seems that this unlikely paradox must be left without a full scientific explanation, only adding to the allure of this fascinating experiment. So why not give it a try this winter? (But please do so carefully.) Perhaps you’ll develop the next plausible explanation, or maybe just enjoy the wonder of it.