From quantitative P, T, V, x data on the system NaCl☒H2O in the supercritical region it has been possible to calculate apparent molal volumes, partial molal volumes, and compressibilities with accuracies of about 10 percent, 25 percent, and 15 percent, respectively.
The apparent molal and partial molal volumes of NaCl show similar, remarkable, and quite anomalous behavior. Above the critical temperature of the particular solution studied and at low NaCl compositions, the molal volumes are very large and negative, being of the order of magnitude of liters/mole. As the densities decrease, the values become more negative, pass through a minimum, and then rise again.
These minima decrease in absolute magnitude with increasing temperature, but seem to occur (for 0.2 percent solutions) at densities not very different from the critical density of water. When the partial molal volumes at constant temperature are plotted against partial densities of water, the results seem to fall on a smooth curve independent of the concentration of NaCl.
The compressibilities show a behavior which resembles that of the pure solvent and whose significance has heretofore escaped any detailed attention. They go through maxima at densities near the density at the critical temperature.
These observations can be accounted for qualitatively if it is assumed that above the critical temperature all fluids are capable of undergoing continuous, short‐range condensation over a narrow range of densities near the critical density of the fluid.
The particular behavior of the NaCl☒H2O systems emphasizes quantitatively the enormous effect of small amounts of solutes (capable of associating strongly with the solvent) on the behavior of the solvent near its critical temperature.