Heavy water (deuterium oxide is a form of water that contains a larger than normal amount of the hydrogen isotopedeuterium rather than the common hydrogen-1 isotope that makes up most of the hydrogen in normal water.
A molecule of heavy water has two deuterium atoms in place of the two protium atoms of ordinary "light" water. The weight of a heavy water molecule, however, is not substantially different from that of a normal water molecule, because about 89% of the molecular weight of water comes from the single oxygen atom rather than the two hydrogen atoms.
The colloquial term heavy water refers to a highly enriched water mixture that contains mostly deuterium oxide D2O, but also some hydrogen-deuterium oxide (HDO) and a smaller number of ordinary hydrogen oxide H2O molecules.
For instance, the heavy water used in CANDU reactors is 99.75% enriched by hydrogen atom-fraction—meaning that 99.75% of the hydrogen atoms are of the heavy type. For comparison, ordinary water (the "ordinary water" used for a deuterium standard) contains only about 156 deuterium atoms per million hydrogen atoms.
Types of Heavy Water
Semiheavy water, HDO, exists whenever there is water with light hydrogen (protium, 1H) and deuterium (D or 2H) in the mix. This is because hydrogen atoms (hydrogen-1 and deuterium) are rapidly exchanged between water molecules. Water containing 50% H and 50% D in its hydrogen actually contains about 50% HDO and 25% each of H2O and D2O, in dynamic equilibrium. In normal water, about 1 molecule in 3,200 is HDO (one hydrogen in 6,400 is in the form of D), and heavy water molecules (D2O) only occur in a proportion of about 1 molecule in 41 million. Thus semiheavy water molecules are far more common than pure heavy water molecules.
Water enriched in the heavier oxygen isotopes 17 O and 18 O is also commercially available, e.g., for use as a non-radioactive isotopic tracer. It is "heavy water" as it is denser than normal water (H180−2O is approximately as dense as D2O, H170−2O is about halfway between H2O and D2O)—but is rarely called heavy water, since it does not contain the deuterium that gives D2O its unusual nuclear and biological properties. It is more expensive than D2O due to the more difficult separation of 17O and 18O.
Tritiated water contains tritium (3H) in place of protium (1H) or deuterium (2H).