A new calculation of the temperature at which water undergoes a unique phase change revises the point upward by about 100 °C, enabling researchers to test its existence for the first time (J. Chem. Phys. Lett. 2018, DOI: 10.1021/acs.jpclett.7b03068).
When pure liquid water is supercooled below –5 °C, it freezes in two distinct steps. At about –20 °C, a thin network of ice crystals rapidly forms. The energy released by that phase change prevents the remaining liquid from freezing until roughly –30 °C.
Those steps can happen as one in a process called hypercooling, when the liquid is cold enough that the excess energy from the first step is absorbed by the frozen solid. Sigurd Bauerecker of Braunschweig University of Technology says there has been almost no research on hypercooled water because a 1997 book calculated the hypercooling point of water to be –160 °C. That’s well below the point at which water will spontaneously nucleate and freeze, making it impossible to test the calculation.
Bauerecker and colleagues recalculated the hypercooling point using more accurate data about the temperature dependencies of water’s heat capacity and the enthalpy of freezing and found it to be –64 °C. Several researchers have already shown they can supercool water below that temperature, putting the hypercooling point within reach of experimentalists and possibly nature. “Up to now, hypercooling did not matter for water,” Bauerecker says. “Now it comes into play.”