It's a conundrum that's baffled scientists since the time of Aristotle: Why do warmer liquids freeze faster than cooler ones? Researchers from Singapore's Nanyang Technological University have come up with an awesome new theory that may finally put the mystery to rest.
It’s called the Mpemba Effect, and scientists have tossed around a number of theories to explain it.
Some believe that the nucleation temperature of water and the specific impurities it contains determines whether or not the Mpemba Effect will occur. Alternate theories suggest that it may have something to do with certain elements that are in the water, like salt, carbon dioxide, or magnesium. These compounds form a briny muck that causes water to freeze lower and boil higher than it should. And because heating water will shake free some of these substances, the Mpemba Effect can be facilitated.
These aren’t great theories — but they’re the best we got. At least until now.
The Stretching of the Bonds
According to the new study, the Mpemba Effect is caused by a small amount of energy that’s stored in stretched hydrogen bonds.
So here’s the deal: Water molecules have one oxygen atom and two hydrogen atoms, which are held together by covalent bonds — chemical bonds which share a pair of electrons between atoms and a molecule. Then there's the hydrogen bond to consider; for water molecules, hydrogen atoms are likewise attracted to the oxygen atoms in other nearby water molecules, while water molecules repel each another.
O:H-O bond in water ice. Credit: Xi Zhang et al:
What the researchers discovered was that, as water gets warmer, distance increases between water molecules owing to the repellant force between them. This causes the hydrogen bonds to stretch, and a stretching bond means there’s energy being stored (heating stores energy into the hydrogen bond by shortening and stiffening it) — and this stored energy gets released as the water is cooled, allowing the molecules to get closer to one another. And when molecules get close enough to each other, we get that neat little effect we call cooling, and eventually, freezing.
Now, warm water has more of this hydrogen bond stretching action happening than in cool water. Warm water, therefore, stores more energy — and it has more to release when exposed to freezing temperatures. Which explains why warmer water freezes faster than cooler water.
It sounds like the researchers are really onto something, but it’s just conjecture at this point. Their paper, which appears at the pre-print archive arXiv, still needs to be scrutinized by their peers: “O:H-O Bond Anomalous Relaxation Resolving Mpemba Paradox.”