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Semiheavy water
Spacefill model of water
Names
	IUPAC name (O-2H1)Water
	Other names Deuterium hydrogen monoxide; Deuterium hydrogen oxide, Water-d1, Water-d
Identifiers
CAS Number	14940-63-7;
3D model (JSmol)	Interactive image;
ChEBI	CHEBI:33806;
ChemSpider	123344;
Gmelin Reference	115
PubChem CID	139859;
CompTox Dashboard (EPA)	DTXSID50164262 ;
	InChI InChI=1S/H2O/h1H2/i/hD Key: XLYOFNOQVPJJNP-DYCDLGHISA-N;
	SMILES [2H]O;
Properties
Chemical formula	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)
Solubility in water	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). verify (what is ?) Infobox references

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 3.8°C compared to the 3.82°C of heavy water.

Production

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.)

References

^ "Provisional Recommendations". Nomenclature of Inorganic Chemistry. Chemical Nomenclature and Structure Representation Division. IUPAC. § IR-3.3.2. Archived from the original on 27 October 2006. Retrieved 2024-12-30.
^ Tashakor S (2016-09-28). "Neutronic Investigation of Semi-Heavy Water Application in Hplwr New Flow Pattern". CNL Nuclear Review: 1–5. doi:10.12943/CNR.2016.00019.
^ Goncharuk VV, Kavitskaya AA, Romanyukina IY, Loboda OA (June 2013). "Revealing water's secrets: deuterium depleted water". Chemistry Central Journal. 7 (1): 103. doi:10.1186/1752-153X-7-103. PMC 3703265. PMID 23773696.
^ "Heavy water | chemical compound". Encyclopedia Britannica. Retrieved 2019-04-24.
^ Craig, H.; Gordon, L. I.; Horibe, Y. (1963). "Isotopic exchange effects in the evaporation of water: 1. Low-temperature experimental results". Journal of Geophysical Research. 68 (17): 5079–5087. Bibcode:1963JGR....68.5079C. doi:10.1029/JZ068i017p05079.