heavywater

Heavy water, or deuterium oxide, is a chemical compound with the formula D2O. It consists of two deuterium atoms, a stable isotope of hydrogen, bonded to oxygen. Deuterium is heavier than protium, the most common hydrogen isotope, so heavy water has different physical properties but behaves similarly chemically.

Natural occurrence: Deuterium occurs in the oceans at trace levels. As a result, heavy water is present only in trace amounts in ordinary water; molecules containing deuterium (such as HDO and D2O) occur at very low abundances compared with H2O.

Physical properties: D2O has a higher melting point (about 3.8 °C) and boiling point (about 101.4 °C) than H2O, and it is denser, with a density around 1.105 g/mL at 25 °C. Isotopic substitution also affects reaction rates and hydrogen-bonding properties, leading to subtle differences in solvent behavior and spectroscopy.

Production: Industrial production relies on isotopic separation, often by fractional distillation due to the small but exploitable differences in physical properties, or by chemical exchange and electrolysis-based processes. The Girdler-Sulfide process is a well-known method that concentrates deuterium through exchange with hydrogen sulfide, typically in combination with distillation steps.

Uses: The main use is as a neutron moderator and coolant in certain nuclear reactors, notably heavy-water reactors such as the CANDU design. It is also employed in scientific research, isotopic labeling, spectroscopy, and various cryogenic or analytical applications that require a stable, non-radioactive isotope.

Safety: Heavy water is not radioactive and is only mildly toxic at very high concentrations, since prolonged replacement of body water with D2O can disrupt biological processes. It is not a hazard at trace levels but is generally avoided in large ingestions.

History: Heavy water was identified and isolated in the early 1930s by Harold Urey and colleagues, an achievement that spurred developments in isotopic separation and heavy-water reactors.