Why Does Hot Water Freeze Faster Than Cold?

Boiling hot water has been known to freeze faster than cold water since Aristotle's day, but we've never been able to explain why. There's plenty of videos like the one above that show cold water just splashing to the ground while boiling hot water turns into a snowy vapor when tossed into the air on below-freezing days.

The Royal Society of Chemistry even held a competition in 2012 to see if researchers could crack the case of the Mpemba effect (named for Erasto Mpemba, a Tanzanian schoolboy who famously froze boiling milk in the 1960s to make ice cream faster than his friends). The group received more than 22,000 submissions, and the answer was still something of a collective "We dunno?" due to a lack of consensus.

A study published in the Journal of Chemical Theory and Computation offers a new explanation for the phenomenon, one that works on a molecular level.

Water is made up of two hydrogen atoms and one oxygen atom, and the study proposes that hydrogen bonds, the links formed between hydrogen atoms and the oxygen atoms of neighboring water molecules, are the key to the Mpemba effect. In cold water, both strong and weak H-bonds are found, while hot water mostly contains only strong H-bonds, the weaker bonds having broken apart in the heating process.

The broken molecules can form into "hexagonal lattice of solid ice," according to researchers, and this is the key to the Mpemba effect, they argue. In cold water, the H-bonds still need to be broken down for the freezing process to begin, but with the bonds already broken in hot water, ice can form more quickly since the structures necessary for it already exist.

Obviously, replication studies need to be performed before we can settle on an explanation, but until then, we can marvel at boiling hot water turning into icy vapor.