We know that ordinary water is a compound of hydrogen and oxygen. It has two atoms of hydrogen and one atom of oxygen and is represented by the chemical formula H2O. Heavy water is a compound of deuterium (an isotope of hydrogen) and oxygen and is represented by the chemical formula D2O. In fact, hydrogen has three isotopes — protium (ordinary hydrogen), deuterium (heavy hydrogen) and tritium. Protium nucleus contains only one proton, while deuterium nucleus contains one proton and one neutron and the tritium nucleus contains one proton and two neutrons. Naturally occurring hydrogen contains 99.985% of protium, about 0.15% 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. Ordinary water as obtained from most natural sources, contains about 1 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 those of ordinary water. Heavy water was discovered by Harold Clayton Urey, an American chemist in 1931. He was awarded the Nobel Prize in 1934 for this discovery. In 1933, Lewis and Donald were able to prepare a few millilitres of pure heavy water by long continued electrolysis of water. 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 litres of water yields pure heavy water. The operation of electrolysis is carried out on industrial scale. Bhabha Atomic Research Centre (BARC), Trombay has a big plant for the production of heavy water. It is also produced at Vadodara, Kota, Talcher, Hazira Manuguru, Thal and Tuticorin. 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 processes. It is also used in the preparation of deuterium and its other compounds. It is also used as the coolant in nuclear reactors.