We take the properties of water for granted. Yet scientists assure us that we have a lot to learn about our biologically essential old friend. Research supporting the counterintuitive claim that hot water freezes faster than cold illustrates this point.
The claim made news in 1969 when Erasto Mpemba, a Tanzanian schoolboy, said that his ice cream mixture froze faster when it started out hot than when it started out cold. Never mind that others have reported this strange behavior of water for centuries. Skeptics scoffed. The boy's teacher spoke derisively of "Mpemba's physics."
It's time to rethink the derision. Jonathan Katz at Washington University has studied the "Mpemba effect" and finds the claim valid. He explains that it has to do with the hardness of water. Dissolved bicarbonates of calcium and magnesium, which make water hard, also depress the water's freezing point and slow its rate of cooling compared to softer water that has less of the bicarbonates. Heating water causes the bicarbonates to precipitate out. This forms the deposits in hot water kettles, for example. Thus, preheating water (or Mpemba's ice cream mix) softens the water, allowing it to freeze faster and at a higher temperature than cold, hard water.
Dr. Katz speculates that the Mpemba effect doesn't always show up because experimenters are using soft water to begin with, according to a report in New Scientist. In that case, there would be no advantage to preheating the water.
Reviewing the physics of water in Science two years ago, Yan Zubavicus and Michael Grurze at the University of Heidelberg in Germany explained why "liquid water is one of the most mysterious substances in our world." Water can form 13 known forms of ice crystal "or probably [an] infinite number of its amorphous modifications," they wrote. Research at the molecular level is showing that the traditional image of water molecules as pyramid-like tetrahedrons holding hands with one another is grossly oversimplified.
For example, Greg Kimmel and colleagues at the Pacific Northwest National Laboratory in Richland, Wash., have made a film of water that repels other water like a water-repellent layer of paint. They spread a layer one molecule thick on a platinum surface. Instead of leaving attachment points free where other water molecules could bind, the molecules in the first layer fixed all their attachment points to the platinum. Any new incoming water was repelled as decisively as raindrops bouncing off a well-waxed automobile.
How about freezing water at room temperature? Heon Kang and colleagues at Korea's National University in Seoul have shown that strong electric fields can force water into a solid form. This is especially effective when water is confined in small channels such as those involved in biological processes at the cellular level. Meanwhile, Harald Reichert and others at the Max Planck Institute in Stuttgart, Germany, have developed methods to explore how water molecules rapidly rearrange their networks on microscopic scales. This can affect water's behavior in ways scientists have only begun to imagine. Water is one of the most important substances shaping our environment and sustaining organic life. Yet all we can say to this familiar substance is, "H2O, we hardly know you."