Water in which hydrogen atoms, 1H, are replaced by the heavier isotope deuterium, 2H (symbol D). It is a colourless liquid, which forms hexagonal crystals on freezing. Its physical properties differ from those of ‘normal’ water; r.d. 1.105; m.p. 3.8°C; b.p. 101.4°C. Deuterium oxide, D2O, occurs to a small extent (about 0.003% by weight) in natural water, from which it can be separated by fractional distillation or by electrolysis. It is useful in the nuclear industry because of its ability to reduce the energies of fast neutrons to thermal energies and because its absorption cross-section is lower than that of hydrogen and consequently it does not appreciably reduce the neutron flux. In the laboratory it is used for labelling other molecules for studies of reaction mechanisms. Water also contains the compound HDO.