05/11/2013
Scientists Claim to Have Solved the Mysterious Mpemba Effect
Did you know that warm water freezes faster than cold water? Well, yes, and it has been a mystery since Aristotle.
This was named the Mpemba effect after the Tanzanian student Mpemba who asked a professor at the end of a lecture "If you take two similar containers with equal volumes of water, one at 35 °C (95 °F) and the other at 100 °C, and put them into a freezer, the one that started at 100 °C freezes first. Why?"
He was ridiculed, but later the professor tried it experimentally and found that Mpemba was correct. And this is pretty mysterious.
Scientists have always tried to explain this, but the explanations are not convincing. A new research from scientists from Singapore claim a new explanation.
As we all know, water molecules consist of 1 oxygen atom and 2 hydrogen atoms—all held together by covalent bonds. Plus, hydrogen atoms are attracted to the oxygen atoms in other nearby water molecules by hydrogen bonds.
The researchers has noted that the warmer water gets, the more distance there is between the water molecules due to the repellant force between them. That they say, forces the hydrogen bonds to become stretched out, and stretching out a bond means that there is energy being stored.
That energy, the researchers suggest, is released as the water is cooled allowing the molecules to become closer to one another, and (as every chemistry student knows) giving up energy means cooling.
Warm water has more hydrogen bond stretching going on than cool water, thus it stores more energy, and has more to release when exposed to freezing temperatures. That is why, the researchers say, it freezes faster than cool water.