The Debunker: Does Hot Water Freeze Faster Than Cold Water?

by Ken Jennings

 

T. S. Eliot wrote that “April is the cruelest month,” but January brings the Northern Hemisphere its cruelest temperatures of the year. We’ve asked ex-Jeopardy! champ Ken Jennings to come in from the cold and put a chill on some of the most persistent cold-weather myths he could think of. You shall know the truth and the truth shall set you freeze (yes, we stole that from Arnold Schwarzenegger’s dialogue in Batman and Robin.)

Icy Myth #4: Hot Water Freezes Faster Than Cold Water.

It’s a common bit of scientific trivia that, for some mysterious reason, hot water freezes faster than cold water. In general, that’s not true—and would violate the laws of thermodynamics if it were. Think about it this way: for a container of hot water to freeze before a cold one does, it would have to be losing its heat at a faster rate. Why would it do so? And even if it did lose enough heat to “catch up” to the cold sample, the two containers would then be identical, right? Why would the initially-hotter water “remember” its past and continue to cool off faster?

However, repeated trials have found that, under certain limited conditions, it’s possible to observe warmer water freezing before cooler water does. This effect has been described since Aristotle, but it wasn’t widely believed by modern scientists until a Tanzanian high schooler named Erasto Mpemba noticed it in 1963 while making ice cream in a home ec class. The phenomenon is still called the Mpemba Effect in his honor, but it’s not completely understood to this day.

The problem is that there are so many initial variables that can affect the way water cools that it’s difficult to isolate the one (or more) that may best explain the Mpemba effect. At least eight possible mechanisms have been proposed, most arising from the realization that the average water temperature of the two containers may not give a full picture of what’s really going on in there. These explanations range from the elementary (hot water will evaporate faster as it cools, leaving a smaller volume of water to freeze) to the more exotic (the cold water may be more likely to “supercool,” meaning that its temperature can fall below the freezing point without ice forming). Convection currents in the water, dissolved gases, the way the water conducts heat to the surface it’s sitting on—all these have been suggested. In the meantime, hundreds of grade-school science fair projects will continue to confirm what everyone already knows: cold water freezes faster than hot water. (Except when it doesn’t.)

Quick Quiz: What fictitious mineral is used to “freeze” Han Solo at the end of The Empire Strikes Back?

Ken Jennings is the author of Because I Said So!, Brainiac, Ken Jennings's Trivia Almanac, and Maphead. He's also the proud owner of an underwhelming Bag o' Crap. Follow him at ken-jennings.com or on Twitter as @KenJennings.

Photo by Flickr member Kevin Saff. Used under a Creative Commons License.