For years we’ve known, or thought we’d known, that hot water tends to freeze a bit faster than cold water, the so-called Mpemba Effect. However, no explanation, one way or another, seemed to make any sense until now:
Each molecule of water is made up of two hydrogen atoms bonded covalently to a single atom of oxygen. Those bonds, which involve atoms sharing electrons, are well understood. But the separate water molecules are bound together, too, by weaker forces generated by hydrogen bonds. They occur when a hydrogen atom from one molecule of water sits close to an oxygen atom from another—and they give rise to many of water’s interesting properties, like its strangely high boiling point.
Now, Xi Zhang [Nanyang Technological University, Singapore] is suggesting that those same bonds cause the Mpemba effect. The idea is pretty simple: bring water molecules into close contact, and a natural repulsion between the molecules causes the covalent bonds to stretch and store energy. As the the liquid warms up, the hydrogen bonds stretch as the water gets less dense and the molecules move further apart.
That extra stretch in hydrogen bonds allows the covalent bonds to relax and shrink a little, giving up their energy. The process of covalent bonds giving up energy is equivalent to cooling, so warm water should in theory cool faster than cold.
Seems plausible enough to me, with my, um, smattering of chemistry.
Cite: arXiv:1310.6514 [physics.chem-ph]
(Via Daily Pundit.)