Does hot water freeze faster than cold water?

Posted Friday, June 21, 2013

Erasto Mpemba, a middle school student in Africa, decades ago found indeed that hot water froze faster than cold water: when Mpemba added ice-cream mix to warm water instead of cold water, and shoved those mixes into the freezer, the warm water created ice cream faster than the cold water. A paradox.

During the past several months we found in laboratory experiments that it was not just the ice cream mix that froze faster, but even pure water itself. Chambers of warm water placed next to a cooling block froze faster than chambers of cold water placed next to the block, by 10 – 15 percent. We were excited to confirm this seemingly paradoxical result (dealt with at length in my new water book).

This “Mpemba Effect” rose to prominence recently when the esteemed Royal Society proposed a contest, offering 1,000 pounds sterling to the person who could explain the paradox. They were surprised to receive 22,000 entries. The winner, recently announced, is a young scientist working in Zagreb. If you are interested, check out his explanation. See if you like it.