The H₂O critical point defines the parabolic vertex of the p(T) vaporization boundary and, as a geometric consequence, a positive vertical asymptote for first partial derivatives of the equation of state. Convergence of these derivatives, isothermal compressibility and isobaric expansivity, to the critical asymptote effectively controls thermodynamic, electrostatic, and transport properties of fluid H₂O and dependent transport and chemical processes in hydrothermal systems. The equation of state for fluid H₂O developed by Levelt Sengers et a1. (1983a) from modern theories of revised and extended scaling affords accurate prediction of state and thermodynamic properties in the critical region. This formulation has been used together with the virial equation of state proposed by Haar et a1. (1984) and predictive equations for the static dielectric constant (Uematsu and Franck, 1980), thermal conductivity (Sengers et a1., 1984), and dynamic viscosity (Sengers and Kamgar-Parsi, 1984) to present a comprehensive summary of fluid H₂O properties within and near the critical region. Specifically, predictive formulations and computed values for twenty-one properties are presented as a series of equations, three-dimensional P-T surfaces, isothermal and isobaric crosssections, and skeleton tables from 350°-475°C and 200-450 bar. The properties considered are density, isothermal compressibility, isobaric expansivity, Helmholtz and Gibbs free energies, internal energy, enthalpy, entropy, isochoric and isobaric heat capacities, the static dielectric constant, Z, Y, and Q Born functions (Helgeson and Kirkham, 1974a), dynamic and kinematic viscosity, thermal conductivity, thermal diffusivity, the Prandtl number, the isochoric expansivity-compressibility coefficient, and sound velocity. The equations and surfaces are analyzed with particular emphasis on functional form in the near-critical region and resultant extrema that persist well beyond the critical region. Such extrema in isobaric expansivity, isobaric heat capacity, and kinematic viscosity delineate state conditions that define local maxima in fluid and convective heat fluxes in hydrothermal systems; at the critical point, these fluxes are infinite in permeable media. Extrema in the Q and Y Born functions delineate state conditions that define local minima in the standard partial molal volumes and enthalpies of aqueous ions and complexes; at the critical point, these properties are negative infinite. Because these fluxes and thermodynamic properties converge to vertical asymptotes at the critical point, seemingly trivial variations in near-critical state conditions cause large variations in fluid mass and thermal energy transfer rates and in the state of chemical equilibrium.
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