These decompose in water and liberates hydrogen gas. The most common compound is deuterium oxide, D 2 O, or heavy water . This name is appropriate because the deuterium atom is twice as heavy as 1 H. Since deuterium occurs naturally, in every 7000 molecules of ordinary H 2 O, there is one D 2 O. Heavy water can be prepared by prolonged electrolysis of ordinary water. Approximately 100,000 gallons of water have to be carefully electrolyzed to produce a single gallon of pure heavy water. Considering the cost of the electrical energy involved in such a process, heavy water is generally regarded as a scarce commodity.
Heavy water is a suitable and convenient moderator in nuclear reactors.