Heavy water contains heavy hydrogen, or deuterium, instead of normal hydrogen. Deuterium is the stable, nonradioactive isotope of hydrogen with atomic weight 2.01363 and symbol D, or 2H. It is commonly called heavy hydrogen because its atomic weight is approximately double that of ordinary hydrogen, but it has identical chemical properties. Deuterium has about twice the atomic weight of normal hydrogen because its nucleus contains a proton and a neutron, instead of just a proton. Hydrogen as it occurs in nature contains approximately 0.02 percent of deuterium. The boiling point of deuterium is -249.49° C (-417.08° F), or 3.28° C (5.90° F) higher than that of ordinary hydrogen. Heavy water (deuterium oxide, D2O) boils at 101.42° C (214.56° F) as compared to 100° C (212° F), the boiling point of ordinary water. It freezes at 3.81° C (38.86° F) as compared to 0° C (32° F) for ordinary water. Its density at room temperature is 10.79 percent greater than that of ordinary water.
Deuterium, which was discovered by the American chemist Harold Urey and his associates in 1932, was the first isotope to be separated in a pure form from an element. Several methods have been used to separate the isotope from natural hydrogen. The two processes that have been most successful have been fractional distillation of water and a catalytic exchange process between hydrogen and water. In the latter system, when water and hydrogen are brought together in the presence of a suitable catalyst, about three times as much deuterium appears in the water as in hydrogen. Deuterium has also been concentrated by electrolysis, centrifuging, and fractional distillation of liquid hydrogen.