The Pressure-Volume-Temperature Relations of the Liquid, and the Phase Diagram of Heavy Water

The pressure-volume-temperature relations of both liquid D2O and H2O are measured between —20° and 95°C and up to 12,000 kg/cm2, and the transition parameters of the liquid and solid modifications of D2O in the range between —60° and +20°C and up to about 9000 kg/cm2. An unstable modification of solid D2O, for which the designation IV is proposed, is found in the field of stability of V. Reference to the original work on H2O shows that the corresponding modification of H2O also exists. In general the properties of H2O and D2O covered by these measurements are very much alike, and differ in the direction suggested by the greater zero-point energy of H2O: the molar volume of D2O is always greater than that of H2O at the same pressure and temperature, and the transition lines of D2O always run at higher temperatures. In finer detail, however, the differences between the two waters do not vary regularly, and probably other considerations than of zero-point energy alone are necessary for a complete explanation.