of water, during which hydrogen is evolved preferentially
to deuterium at the cathode. Consequently the residual water is
enriched in deuterium oxide, D2O, ('heavy water*). The D2O finally
obtained has a b.p. 374.2 K and a density at 293 K of 1.106 gcm"3
(water, 0.998 g cm ~3); electrolysis of D2O gives deuterium which
again has physical properties slightly different from those of hydrogen
(for example b.p. 24 K). Ordinary hydrogen contains about 1
part in 6000 of deuterium.