Heavy water was discovered by harold Clayton Urey an American chemist in 1931. He was awarded the Nobel Prize in 1934 for this discovery. Naturally occurring hydrogen contains 99.985% of protium, about .015% deuterium and about 1 part in 1017 tritium. Tritium is radioactive in nature. When deuterium combines with oxygen it gives heavy water or deuterium oxide. Heavy water is prepared by electrolysis of water. If water is electrolysed the gas produced at cathode is mostly hydrogen and thus the residual water is enriched in deuterium oxide. Continued electrolysis of hundreds of liter of water yeild pure heavy water.
Ordinary water as obtained from most natural sources, contain about one part of heavy water for every 6760 parts of H2O. Ordinary water has a molecular weight of about 18 while heavy water has a molecular weight of 20. Density freezing point and boiling point of heavy water are higher than that of ordinary water. Heavy water is used as a moderator for slowing down the fast neutrons in nuclear reactors. In laboratories, it is used as an isotopic tracer in the studies of chemical and biological process. It is also used in the preparation of deuterium and its other compounds. It is also used as the coolant in nuclear reactors.