Why Water Has Freezing Leeway

A computational study shows that water molecules freeze not only as a result of lowering temperatures but also because of spontaneous crystal rearrangements. The research helps explain how water remains liquid below its freezing point and indicates that –48 °C is the coldest at which this phenomenon can occur (Nature, DOI: 10.1038/nature10586). Usually, an impurity or small ice crystal “seeds” ice formation. But in the case of ultrapure, supercooled water, the spontaneous molecular restructuring that sets off the crystallization cascade happens too rapidly for experimental observation. Valeria Molinero and Emily B. Moore of the University of Utah developed a computer model showing that as the temperature nears –48 °C, water molecules with increasing rapidity begin forming tetrahedrons by attaching to four other water molecules, spurring crystallization.