Heavy Water

Heavy water (D₂O) is the oxide of "heavy hydrogen" -- deuterium. In ordinary water, only about one part in 5000 is naturally heavy water. Heavy water is mainly obtained as a by-product of the electrolysis of ordinary water: because D₂O does not undergo electrolysis as readily as H₂O, prolonged electrolysis of water progressively enriches the remaining liquid in D₂O (as already covered in Section 4.2.3), and this enriched fraction is the practical source of heavy water. …