A singular thermodynamically consistent temperature at the origin of the anomalous behavior of liquid water

The density maximum of water dominates the thermodynamics of the system under ambient conditions, is strongly P-dependent, and disappears at a crossover pressure P(cross) ~ 1.8 kbar. We study this variable across a wide area of the T–P phase diagram. We consider old and new data of both the isothermal compressibility K(T)(T, P) and the coefficient of thermal expansion α(P)(T, P). We observe that K(T)(T) shows a minimum at T* ~ 315±5 K for all the studied pressures. We find the behavior of α(P) to also be surprising: all the α(P)(T) curves measured at different P cross at T*. The experimental data show a “singular and universal expansivity point” at T* ~ 315 K and α(P)(T*) ≃ 0.44 10(−3) K(−1). Unlike other water singularities, we find this temperature to be thermodynamically consistent in the relationship connecting the two response functions.