Widom line

In the context of the pressure-temperature phase diagram of a substance and of the supercritical fluid state in particular, the Widom line is a line emanating from the critical point which in a way extends the liquid-vapor coexistence curve above the critical point.

It corresponds to the maxima or minima of certain physical properties of the supercritical fluid, such as the speed of sound, isothermal compressibility, isochoric and isobaric heat capacities.

A common criterion for locating the Widom line is indeed the maximum in the isobaric heat capacity.

[4] The concept of Widom line provides a useful framework for characterizing and predicting the properties of fluids, which are important for scientific research as well as various industrial processes.

Such a concept is indeed relevant to the physical properties of any single-component fluid at sufficiently high pressures and temperatures, and its study is an active research area.

Pressure–temperature phase diagram. The supercritical state is at pressures and temperatures above those of the critical point, but below the pressures required for transformation into a solid. The dashed green line shows the anomalous slope of the solid-liquid phase boundary exhibited by some substances, including water.