Research shows why water acts weird

Although ubiquitous on Earth and central to the chemistry of life, water remains mysterious on the molecular level. Its elusive molecular behavior is all the more frustrating to chemists because it’s unlike that of other liquids–expanding when frozen, for example, instead of contracting.

Now, however, the mystery is unraveling. Reporting in the Jan. 18 Nature, researchers have found a precise relationship between water’s structure and its unusual physical properties.

The link showed up clearly when Jeffrey R. Errington and Pablo G. Debenedetti of Princeton University combined two methods of measuring the arrangements of molecules in water. The technique put hard numbers to both the distances between and the orientations of water molecules.

Unlike most liquids’ constituents, which tend to form dense clusters with their neighbors, molecules of water generally arrange themselves with four neighbors in a predictable pattern orchestrated by hydrogen bonds. Computer simulations revealed that the distances and orientations of the water molecules are tightly dependent on each other over a particular range of densities and temperatures, the Princeton researchers report.

Two of water’s quirks occur within this range. Its density peaks at 4C, before it freezes, and its molecular motion becomes faster when the density increases. These properties thus seem linked to water’s structural order, Debenedetti says.

The group plans to look for similar structural changes in other substances, such as silica and more complicated liquids, including sugar solutions used for storing pharmaceuticals.

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