It’s easy to think of water as a typical liquid, but many aspects of its behavior are highly atypical. For example, its freezing and boiling points are high compared with those of liquids with similarly-sized molecules, and it becomes less dense as it freezes. The unusual behavior is presumed to have something to do with the hydrogen bonds that form between the hydrogen and oxygen atoms of different water molecules, but the details of the connection are not fully known.
To better understand water’s anomalous nature, and to test theoretical models that connect its microscopic and macroscopic properties, researchers can study its behavior in metastable liquid phases. Supercooled water is metastable (see the article by Pablo Debenedetti and Eugene Stanley, PHYSICS TODAY, June 2003, page 40), as is water under tension, or negative pressure (see the article by Humphrey Maris and Sébastien Balibar, PHYSICS TODAY, February 2000, page 29...
