At first, it sounds crazy wouldn't hot water take longer to freeze? Weirdly enough, the opposite is sometimes true. Join Lauren to learn more about a mystery that's puzzled humanity for centuries in this episode of BrainStuff.

p.s. If you wanna conduct this experiment yourself, you have to control your variables really carefully! Here's a good guide: http://www.sciencebuddies.org/science...

Whether the topic is popcorn or particle physics, you can count on the HowStuffWorks team to explore and explain the everyday science in the world around us on BrainStuff.

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Transcript:
Hi, I’m Lauren Vogelbaum, and this is BrainStuff. Water is one of the most important substances on Earth – as far as life goes, at least. And it can do some weird stuff. For example, it reaches a maximum density at around 4 degrees Celsius, and then becomes less dense before it freezes. And here’s another strange thing: hot water can freeze faster than cold water. But why?

OK, this is a very old question – Rene Descartes thought about it, and Francis Bacon before him. Aristotle pondered the mystery, too. Today, we call this phenomenon the Mpemba effect, after a guy named Erasto Mpemba, who first observed the effect on ice cream when he was just 13 years old. His teacher, for the record, laughed at him. Well, who’s laughing now?

Turns out Erasto was onto something big, and teams of experts would spend the next few decades trying to explain why water behaves this way. Could evaporation be the key, for example? What about gasses dissolving from hot water? Scientists around the world cooked up dozens of theories, and none were able to completely explain the process. But, as of 2013, that may have changed.

A research assistant at the University of Zagreb in Croatia claims that convection currents in warm water cause it to cool more rapidly, and the Royal Society of Chemistry agrees with him – they actually gave the guy a thousandpound prize for his experiments. But he’s not the only contender for a solution.

A team of physicists at the Nanyang Technological University in Singapore took the question to a much smaller level. They claim that the answer lies in the bonding – not, you know, hanging out with friendstype bonding, but molecular bonding. Two kinds, actually.

You see, every water molecule is made of two hydrogen atoms bonded to a single atom of oxygen. These are covalent bonds, which means that they share electrons between one another. This is a chemical bond. Separate water molecules are bound to their neighbors by weaker hydrogen bonds, which occur when a hydrogen atom from one water molecule is close to the oxygen atom of another water molecule. This is an electromagnetic attraction, and it’s what gives water some of its strange abilities, like its unusually high boiling point of 100° Celsius – that’s 212 Fahrenheit.
So, these scientists in Singapore argue that the Mpemba effect comes from an interaction between the covalent bonds – the inner water molecule bonds – and the hydrogen bonds between different molecules.

Here’s how they think it works: When water molecules are close together, their natural repulsion causes the covalent bonds to stretch and store energy. As the water warms up, it becomes less dense. The hydrogen bonds stretch as the molecules move further apart. When these hydrogen bonds stretch, they allow the covalent bonds to shrink and release their energy.

This is equivalent to cooling – so, according to the Nanyang theory, hot water is essentially able to release energy faster, leading to a quicker freeze. It’s important to note that at this point that the work hasn’t been peerreviewed, so it’s not a sure thing yet. But even if it’s just a contributing factor, it’s a promising lead to a question that’s puzzled humanity for some 2500 years.

Research Sources:
How Water Works: http://science.howstuffworks.com/envi...
Full Paper: “O:HO Bond Anomalous Relaxation Resolving Mpemba Paradox” http://arxiv.org/pdf/1310.6514v2.pdf
Covalent Bonds: http://www.chemguide.co.uk/atoms/bond...
Have scientists worked out why hot water freezes faster than cold water?
http://www.telegraph.co.uk/science/sc...
Royal Chemistry Society Winner Interview: http://www.rsc.org/mpembacompetition...