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Names | |||
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IUPAC name (O-2H1)Water | |||
Other names Deuterium hydrogen monoxide Deuterium hydrogen oxide, Water-d1, Water-d | |||
Identifiers | |||
3D model (JSmol) | |||
ChEBI | |||
ChemSpider | |||
115 | |||
PubChem CID | |||
CompTox Dashboard (EPA) | |||
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Properties | |||
H2HO or HDO | |||
Molar mass | 19.0214 g mol−1 | ||
Appearance | Very pale blue, transparent liquid, very similar to regular water | ||
Density | 1.054 g cm−3 | ||
Melting point | 3.81 °C (38.86 °F; 276.96 K) | ||
Boiling point | 100.74 °C (213.33 °F; 373.89 K) | ||
miscible | |||
log P | −0.65 | ||
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). |
Semiheavy water is the result of replacing one of the protium (normal hydrogen, 1H) in normal water with deuterium (2H; or less correctly, [1] D). [2] It exists whenever there is water with 1H and 2H in the mix. This is because hydrogen atoms (1,2H) are rapidly exchanged between water molecules. Water with 50% 1H and 50% 2H, is about 50% H2HO and 25% each of H2O and 2H2O, in dynamic equilibrium. [3] In normal water, about 1 molecule in 3,200 is HDO (1H2HO) (one hydrogen in 6,400 is 2H). By comparison, heavy water D2O or 2H2O [4] occurs at a proportion of about 1 molecule in 41 million (i.e., 1 in 6,4002). This makes semiheavy water far more common than "normal" heavy water.
The freezing point of semiheavy water is close to the freezing point of heavy water at 3.81°C compared to the 3.82°C of heavy water.
On Earth, semiheavy water occurs naturally in normal water at a proportion of about 1 molecule in 3,200; because 1 in 6,400 hydrogen atoms in water is deuterium, which is 1 part in 3,200 by weight. HDO may be separated from normal water by distillation or electrolysis, or by various chemical exchange processes, all of which exploit a kinetic isotope effect. Partial enrichment also occurs in natural bodies of water under certain evaporation conditions. [5] (For more information about the distribution of deuterium in water, see Vienna Standard Mean Ocean Water and Hydrogen isotope biogeochemistry.)