heavy water

heavy water (D₂O), water composed of two atoms of deuterium, the hydrogen isotope with a mass double that of ordinary hydrogen, and one atom of oxygen. (Ordinary water has a composition represented by H₂O.) Thus, heavy water has a molecular weight of about 20 (the sum of twice the atomic weight of deuterium, which is 2, plus the atomic weight of oxygen, which is 16), whereas ordinary water has a molecular weight of about 18 (twice the atomic weight of ordinary hydrogen, which is 1, plus oxygen, which is 16).

As obtained from most natural sources, ordinary water contains about one deuterium atom for every 6,760 ordinary hydrogen atoms. Continued electrolysis of hundreds of liters of water until only a few milliliters remain yields practically pure deuterium oxide. This operation, until 1943 the only large-scale method used, has been superseded by less expensive processes, such as the Girdler sulfide process (deuterium is exchanged between hydrogen sulfide [H₂S] and water) and fractional distillation (D₂O becomes concentrated in the liquid residue because it is less volatile than H₂O). The heavy water produced is used as a moderator of neutrons in nuclear power plants. In the laboratory heavy water is employed as an isotopic tracer in studies of chemical and biochemical processes.