Most of the hydrogen in the universe consists of a single proton orbited by a single electron and is also known as H-1 or protium. A small percentage of hydrogen atoms contain a neutron in the nucleus as well as the proton. This form of hydrogen is referred to as H-2, heavy hydrogen or deuterium. It is a stable atom like H-1. In the Earth's crust, for every six thousand four hundred and twenty H-1 atoms, there is a single H-2 atom.
Most of the water molecules in the universe contain two of these ordinary hydrogen atoms combined with an oxygen atom. A few of the water molecules will have H-2 atoms combined with oxygen. In all the water on Earth, in ten thousand water molecules there will be about 2 with H-2 atoms instead of H-1 atoms. Water with H-2 atoms is referred to as heavy water.
Just about all the deuterium in the universe is thought to have been created in the big bang. Heat can be used to separate the isotopes of hydrogen. The ratio of H1 to H-2 in gas giants and comets varies because of the effects of internal heat and solar heating. The fact that the ratio of H1 to H2 found in comets is close to that found in the oceans on Earth has been used to argue that the oceans were created by cometary impacts on the young Earth.
Deuterium was identified in the early 1930 soon after the discovery of the neutron. Harold Urey won a Nobel Prize in 1934 for discovering and naming deuterium. Since the discovery of deuterium, water containing deuterium in its molecules has been extracted from ordinary water through a steam distillation process. Canada used to be the leading world supplier of deuterium until its last heavy water production plant was closed in 1997.
The chemical and physical properties of compounds containing deuterium are similar to the behavior of the same compound without deuterium. However, there are still differences that are greater that those caused by any other change of particular isotopes in compounds. Heavy water is more viscous than ordinary water and ice created from heavy water will sink in ordinary water in contrast to ordinary ice which floats.
Heavy water is slightly toxic to multi-cellular creatures and single cell life forms whose cells contain a nucleus. More primitive single cell life that has no nucleus appear to not be harmed by it. A average person could consume five quarts of heavy water without serious injury but it half the water in the body was replace with heavy water, death would result.
Deuterium is used in experimental fusion reactors. When fusing hydrogen to helium, neutrons must be part of the mix because even though most hydrogen does not include neutrons, all helium nuclei do include neutrons. Heavy water is used as a moderator to slow neutrons in some nuclear reactor designs because it does not absorb neutrons like ordinary water. The Canada reactor design CANDU uses heavy water as a moderator. Deuterium is a useful tracer for chemistry and biochemistry because it is a non-radioactive and easily identified.