The flee energy of solvation of Cl-, OH-, HCl, and H2O and the relative acidity of HCl versus H2O are calculated with a continuum electrostatics model and compared with the molecular dynamics free energy perturbation (MD-FEP) simulation. The water densities span gas, liquid, and supercritical conditions. Compared with the simulation data, the simplest continuum model does not predict ion solvation accurately in supercritical water, as it neglects electrostriction, which is persistent to low densities. A generalization including the most basic element of electrostriction is reasonably accurate. However, for the free energy of dissociation of HCl relative to water. the errors are found to cancel between OH- and Cl- such that the simple continuum model is quite accurate. Because the reaction is isocoulombic and involves species of similar size, the effects of electrostriction cancel, corresponding entropic contributions are negligible, and the effect of density on the free energy change is small.