- water in which hydrogen atoms have been replaced by deuterium, used chiefly as a coolant in nuclear reactors.
- water that has been electrolytically decomposed to enrich it in the deuterium isotope in the form HDO or D 2 O
- Water in which deuterium, a heavy isotope of hydrogen, takes the place of hydrogen. Heavy water has physical and chemical properties that are like those of ordinary water, but heavy water is 10 percent heavier and has higher freezing and boiling points. Also called deuterium oxide. Chemical formula: D2O. ♦ Semiheavy water is similar to heavy water, but only one of the two hydrogen atoms in each molecule is replaced with deuterium. Chemical formula: DHO.
Water with a higher average molecular weight than ordinary water. In a molecule of heavy water, the hydrogen atoms are isotopes in which the nuclei each contain a proton and a neutron, and hence are twice as heavy as a normal hydrogen atom.