This winter was really, really cold. Outside of drought-stricken California, most of the U.S. suffered a severe and snowy winter. Temperatures in some states were briefly colder than the surface of Mars.
Mixed in with all the media stories about the polar vortex and its blame in this deep freeze was a short-lived meme: Folks showing off just how cold it was outside by throwing boiling water out of a pot into the air and watching it transform into powdery ice. (Warning, this has also led to injuries). Local TV news reporters were especially fond of this trick.
Part of the fascination with this experiment is that the air is so cold that even extremely hot water immediately turns to snow. This is presumably more impressive than cold water freezing as quickly. Turns out the exact opposite is true.
Certainly some of the effect, regardless of the temperature of the water, is due to the fact that throwing it into the air spreads the water out and exposes more of its surface to the cold, allowing it to quickly freeze.
But there’s another extremely bizarre effect going on that deserves a closer look: Something about hot water makes it freeze faster than cold water.
This counterintuitive phenomenon is part of a 2,000-year-old mystery, having been observed throughout history by scientists such as Aristotle, Francis Bacon, and Rene Descartes. It has the modern name of the Mpemba Effect, named for Erasto Mpemba, a Tanzanian high school student with an interesting story. In 1963, Mpemba’s class was dissolving sugar in boiling milk to make ice cream. Before churning, cooks are generally supposed to cool the milk but Mpemba was impatient and put his concoction in the churner while still hot. To his surprise, he found his ice cream formed faster than any of his classmates. His teacher didn’t believe him and told him he must have been confused.
When a professor of physics later came to visit the class, Mpemba asked him if it was possible for hot things to freeze faster than cold ones. The professor didn’t believe him either but, together, they performed an experiment. They put two glasses of water, one at 95 degrees Fahrenheit and the other at boiling, in a freezer and noted that the hotter one turned faster to ice. They published a paper on the experiment in 1969. The result has since been replicated many times, though it doesn’t always work and depends on the relative temperatures of the two liquids.
How Does It Work?