Heavy water

Heavy water, known scientifically as deuterium oxide (D2O), is a distinct form of water. It is defined by the presence of deuterium, which is a heavy isotope of hydrogen. In most ordinary water, hydrogen atoms consist of a single proton called protium. However, a deuterium nucleus contains both a proton and a neutron. This extra neutron makes a deuterium atom approximately twice as heavy as a protium atom. While a heavy water molecule is heavier, the total mass change is not massive. This is because about 89% of a water molecule's mass comes from the single oxygen atom.

The physical properties of heavy water differ significantly from the light water we use daily. Heavy water is about 10.6% denser than regular water. You can observe this by dropping heavy ice into normal water; the ice will sink. Heavy water also has a higher melting point of 3.7 °C. It lacks the very faint blue color seen in regular water. This occurs because the molecular vibrations shift the absorption of red light into the infrared spectrum. Some humans may even notice a slightly sweet taste due to specific taste receptors.

There are different variations of water containing heavy isotopes. Semiheavy water, or HDO, occurs when both protium and deuterium are mixed. Because hydrogen atoms exchange rapidly between molecules, HDO is much more common than pure D2O. In normal water, about one in 3,200 molecules is semiheavy water. Another type is heavy-oxygen water, which is enriched with heavier oxygen isotopes. This type is denser but lacks the unique nuclear properties of deuterium-based water. Finally, tritiated water contains tritium, which makes the water radioactive.

The history of heavy water is tied to major scientific breakthroughs. Harold Urey, a Nobel laureate, discovered the deuterium isotope in 1931. Shortly after, his mentor Gilbert Newton Lewis isolated the first pure sample using electrolysis in 1933. In 1934, George de Hevesy and Erich Hofer used heavy water for early biological tracer experiments. They used it to estimate how quickly water turns over in the human body. After the discovery of nuclear fission in 1938, heavy water became vital for nuclear energy research.

Heavy water is essential in the field of nuclear engineering. It is used as a neutron moderator, which is a substance that slows down neutrons. Some reactors, such as the CANDU design, use highly enriched heavy water. This mixture is 99.75% enriched by hydrogen atom-fraction. These reactors are useful because they can run on natural uranium. This avoids the need for graphite moderators, which can pose explosion hazards during decommissioning. Heavy water also helps in the production of radioactive materials like plutonium and tritium.

Beyond energy, heavy water has many specialized industrial uses. It is used in nuclear magnetic resonance and infrared spectroscopy. It also plays a role in neutrino detection and neutron capture therapy. Scientists use it for metabolic rate testing and in the production of radiopharmaceuticals. The most cost-effective way to produce it is through the Girdler sulfide process. It is sold in various grades of purity depending on the specific scientific or industrial need.

Heavy water has complex effects on biological systems. It can alter enzymes, hydrogen bonds, and the way cells divide in eukaryotes. For multicellular organisms, replacing more than 50% of their water with heavy water is lethal. This is because the heavier atoms change important biochemical reactions. However, humans are naturally safe around small amounts. The human body already contains about five grams of heavy water. While it can be toxic in large amounts, it is not a constant threat to life.