Heavy water

Heavy water, formally called deuterium oxide or ²H₂O or D₂O, is a form of water that contains the hydrogen isotope deuterium (D), instead the common  protium (H) isotope.

Heavy water is not radioactive. It has a density about 11% greater than water, but otherwise, is physically and chemically similar.

This difference in density (especially affecting the biological properties) is larger than in all other isotope-substituted compounds because deuterium is unique among heavy stable isotopes in being twice as heavy as the lightest isotope.

This difference increases the strength of water's hydrogen-oxygen bonds, and this in turn is enough to cause differences that are important to some biochemical reactions. The human body naturally contains deuterium equivalent to five grams of heavy water, which is harmless. When a large fraction of water (> 50%) in higher organisms is replaced by heavy water, the result is cell dysfunction and death.

The scientists from the Institute for the Biology of Ageing (Moscow) conducted an experiment to determine the effect of heavy water on longevity using fruit flies. They found that while large amounts were deadly, smaller quantities increased their life spans by up to 30%.

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.

Heavy-oxygen water - Water enriched in the heavier oxygen isotopes 17o and 18o (1h217o or 1h218o). It is used in scientific investigations as a non-radioactive isotopic tracer.

It is "heavy water" as it is denser than normal water (H₂¹⁸O is approximately as dense as D₂O, H₂¹⁷O is about halfway between H₂O and D₂O), but is rarely called heavy water, since it does not contain the deuterium which gives D₂O its unusual nuclear and biological properties. It is more expensive than D₂O due to the more difficult separation of ¹⁷O and ¹⁸O.

Some properties of different forms of water

Property	D2O (Heavy water)	HDO (Semiheavy water)	H2O (Light water)
Freezing point (°C)	3.82		0.0
Boiling point (°C)	101.4	100.7	100.0
Density at STP (g/mL)	1.1056	1.054	0.9982
Temp. of maximum density (°C)	11.6		4.0
Dynamic viscosity  (at 20°C, mPa·s)	1.2467	1.1248	1.0016
Surface tension  (at 25 °C, μJ)	7.187	7.193	7.198
Heat of fusion  (cal/mol)	1,515		1,436
Heat of vaporisation (cal/mol)	10,864	10,757	10,515
pH (at 25°C)	7.43	7.266	6.9996
Refractive index  (at 20°C, 0.5893 μm)	1.32844		1.33335