(210a) Puzzles of Cold Water

Liquid water is still a puzzle. This is probably the most studied and yet least understood state of matter. Something dramatic happens with liquid water below the “biological” temperature (30-40 ^oC). Unlike ordinary substances, one can regard water near the triple point and in the supercooled region, on the one side, and water near the vapor-liquid critical point, on the other side, as “the same substance – two different liquids”. Highly-compressible, low-dielectric-constant near-critical water is commonly used as a supercritical-fluid solvent. On the low-temperature side of the phase diagram, water is an almost incompressible, high-dielectric constant solvent with mysterious thermodynamic anomalies. These anomalies become especially pronounced in the supercooled water that exists between the melting line and the line of homogeneous ice formation. A popular hypothesis that explains the anomalies of cold and supercooled water is the existence of a metastable liquid-liquid transition hidden just below the line of homogeneous ice nucleation. We have developed an equation of state for the thermodynamic properties of cold and supercooled water, in which water is considered as a non-ideal “mixture” of two alternative structures. The equation is valid for temperatures from the homogeneous ice nucleation temperature up to 300 K and for pressures up to 400 MPa, and can be extrapolated up to 1000 MPa. This equation has been adopted by the International Association for the Properties of Water and Steam (IAPWS) as an official guideline for scientific and industrial use.