Advanced #39

Liquid H2O and D2O differ considerably in their phase behavior

The phase diagrams of normal and heavy water show significant differences.

Scientific Explanation

The phase diagrams of H2O and D2O share the same basic topology but show significant quantitative differences. The triple point of D2O lies at 3.82 degrees Celsius and 661 pascals — noticeably higher than that of H2O at 0.01 degrees Celsius and 611 pascals. The critical point of D2O is found at approximately 370 degrees Celsius and 21.7 megapascals, while H2O has its critical point at 374 degrees Celsius and 22.1 megapascals.

Particularly noteworthy is that the phase boundaries between the various ice phases (ice I through ice XV and beyond) are systematically shifted to higher temperatures in D2O. This means D2O ice is thermally more stable than H2O ice. The transition pressures between ice phases also differ.

These shifts reflect the stronger effective hydrogen bonds in D2O. The lower zero-point energy of deuterium leads to a more stable crystalline ordering, which expands the stability range of solid phases. Simultaneously, the altered hydrogen bond dynamics also affect the liquid-gas boundary, though the differences there are less pronounced.

Phase Diagram Comparison H2O vs D2O Overlay of simplified phase diagrams for H2O and D2O showing that D2O has higher melting and boiling points, and the triple point and critical point differ significantly between the two isotopologues. Pressure high low Temperature 0.01 °C 374 °C 3.82 °C 370 °C Solid Liquid Gas H₂O D₂O Phase Diagrams
Phase diagrams of H2O and D2O compared: D2O phase boundaries are systematically shifted.

Everyday Relevance

The differences in phase behavior are important for neutron scattering research, where D2O is frequently used as a solvent. Researchers must consider that D2O may be in a different phase state than H2O at the same pressure and temperature conditions. In isotope geochemistry, the phase differences serve as a tool: the ratio of D2O to H2O in precipitation and ice cores enables conclusions about past climate conditions, since the different evaporation and condensation temperatures lead to natural isotope fractionation.