Water doesn’t necessarily freeze at its freezing point. A skillful experimenter who maintains the liquid free of impurities can chill it well below 0 °C, or 273 K, in a metastable, supercooled state. In the 1970s chemists Austen Angell and Robin Speedy, then both at Purdue University, set out to determine just how cool water could go.1 What they observed remains one of the biggest mysteries in thermodynamics: As their sample dipped below 250 K, its isothermal compressibility and heat capacity began to soar, indications that its density and entropy were fluctuating wildly at the molecular scale. Water seemed on the verge of some never-before-seen transformation. But before the drama could play out, the sample froze.
Unable to usher the liquid below 247 K, the researchers did the next best thing: They extrapolated their data. And they concluded that whatever the mystery transformation was, it happened at 228 K,...
