Hot Water Freezes Faster Than Cold Water???
Take two pails of water; fill one with hot water and the other one with cold water, and put them in the freezer. The hot one would be frozen before the cold one. But wait, you say, that’s counterintuitive: wouldn’t the hot water have to cool down to the temperature of the cold water before proceeding to freezing temperature, whereas the cold one has “less to go” before freezing?
In 1963, a Tanzanian high-school student named Erasto B. Mpemba was freezing hot ice cream mix in a cooking class when he noticed that a hot mix actually froze faster than a cold mix. When he asked his teacher about this phenomenon, his teacher ridiculed him by saying “All I can say is that is Mpemba’s physics and not universal physics.”
Thankfully, Mpemba didn’t back down – he convinced a physics professor to conduct an experiment which eventually confirmed his observations: in certain conditions, hot water indeed freezes before cold water*.
Actually, Mpemba was in good company. The phenomenon of hot water freezing first, now called the “Mpemba effect” was noted by none other than Aristotle, Francis Bacon and René Descartes.
But how do scientists explain this strange phenomenon? It turns out that no one really knows but there are several possible explanations, including differences in supercooling (see below), evaporation, frost formation, convection, and effects of dissolved gasses between the hot and cold water.
*In reality – of course – it’s much more complex than that: hot water freezes first (it forms ice at a higher temperature than cold water), whereas cold water freezes faster (it takes less time to reach the supercooled state from which it forms ice).