Editing Heavy water (section)

==Physical properties==
{| class="wikitable"
|+Physical properties of isotopologues of water<ref>{{cite web|url=http://www1.lsbu.ac.uk/water/water_properties.html |archiveurl=https://web.archive.org/web/20141007210436/http://www.lsbu.ac.uk/water/water_properties.html |archivedate=7 October 2014 |title=Water Properties (including isotopologues) |website=lsbu.ac.uk |author= Martin Chaplin |access-date=4 December 2017}}</ref>
|-
! Property 
!T{{sub|2}}O (Tritiated water)|| D{{sub|2}}O (Heavy water) || HDO (Semiheavy water) || H{{sub|2}}O (Light water)
|-
| [[Melting point]] ([[Standard atmosphere (unit)|standard pressure]]) 
|{{convert|4.49|C|F K}}|| style="text-align:center;" | {{convert|3.82|C|F K}} || style="text-align:center;"| {{convert|2.04|C|F K}} || style="text-align:center;"| {{convert|0.0|C|F K}}
|-
| [[Boiling point]] 
|{{convert|101.5|C|F K}}|| style="text-align:center;" | {{convert|101.4|C|F K}} || style="text-align:center;"| {{convert|100.7|C|F K}} || style="text-align:center;"| {{convert|100.0|C|F K}}
|-
| [[Density]] at [[Standard temperature and pressure|STP]] (g/[[millilitre|mL]]) 
|1.2133|| style="text-align:center;" | 1.1056 || style="text-align:center;"| 1.054 || style="text-align:center;"| 0.9982
|-
| Temp. of maximum density 
|{{convert|13.4|C|F}}|| style="text-align:center;" | {{convert|11.6|C|F}}|| style="text-align:center;"|Unverified|| style="text-align:center;" | {{convert|3.98|C|F}}<ref>{{cite book |title=Chemistry and Chemical Reactivity, Volume 1 |edition=7th |first1=John |last1=Kotz |first2=Paul |last2=Teichel |first3=John |last3=Townsend |publisher=Cengage Learning |date=2008 |isbn=978-0-495-38711-4 |page=15 |url=https://books.google.com/books?id=a9emw3FvCrAC}} [https://books.google.com/books?id=a9emw3FvCrAC&pg=PT43 Extract of page 15]</ref>
|-
| [[Dynamic viscosity]] (at 20&nbsp;°C, [[millipascal|mPa]]·[[second|s]]) 
|1.40 (estimated)|| style="text-align:center;" | 1.2467 || style="text-align:center;"| 1.1248 || style="text-align:center;"| 1.0016
|-
| [[Surface tension]] (at 25&nbsp;°C, [[Newton (unit)|N]]/[[Metre|m]])<!--- 1 dyn-cm = 0.1 μJ = 100 nJ ---> 
|Unverified|| style="text-align:center;" | 0.07187 || style="text-align:center;"| 0.07193 || style="text-align:center;"| 0.07198
|-
| [[Heat of fusion]] ([[joule|kJ]]/[[mole (unit)|mol]]) 
|Unverified|| style="text-align:center;" | 6.132 || style="text-align:center;"| 6.227 || style="text-align:center;"| 6.00678
|-
| [[Enthalpy of vaporization|Heat of vaporisation]] (kJ/mol) 
|Unverified|| style="text-align:center;" | 41.521 || style="text-align:center;"|Unverified
| style="text-align:center;" | 40.657
|-
| [[pH]] (at 25&nbsp;°C)<ref name="discussion of pD">[http://www.nuceng.ca/candu/pdf/15%20-%20Chemistry%20in%20CANDU%20Process%20Systems.pdf discussion of pD],</ref> 
|Unverified|| style="text-align:center;" | 7.44 ("pD") || style="text-align:center;"| 7.266 ("pHD") || style="text-align:center;"| 7.0
|-
| [[Base dissociation constant|p''K''<sub>b</sub>]] (at 25&nbsp;°C)<ref name="discussion of pD"/> 
|Unverified|| style="text-align:center;" | 7.44 ("p''K''<sub>b</sub> D<sub>2</sub>O") || style="text-align:center;"|Unverified|| style="text-align:center;" | 7.0
|-
| [[Refractive index]] (at 20&nbsp;°C, 0.5893 [[micrometre|μm]])<ref>{{cite web |url=http://refractiveindex.info/?group=LIQUIDS&material=Heavy_water|title=RefractiveIndex.INFO |access-date=21 January 2010}}</ref> 
|Unverified|| style="text-align:center;" | 1.32844 || style="text-align:center;"|Unverified || style="text-align:center;" |1.33335
|}
The physical properties of water and heavy water differ in several respects. Heavy water is less dissociated than light water at given temperature, and the true concentration of D{{sup|+}} ions is less than {{H+}} ions would be for light water at the same temperature. The same is true of OD{{sup|−}} vs. {{OH-}} ions. For heavy water Kw D{{sub|2}}O (25.0&nbsp;°C) = 1.35 × 10{{sup|−15}}, and [D{{sup|+}}{{hsp}}] must equal [OD{{sup|−}}{{hsp}}] for neutral water. Thus pKw D{{sub|2}}O = p[OD{{sup|−}}] + p[D{{sup|+}}] = 7.44 + 7.44 = 14.87 (25.0&nbsp;°C), and the p[D{{sup|+}}] of neutral heavy water at 25.0&nbsp;°C is 7.44.

The pD of heavy water is generally measured using pH electrodes giving a pH (apparent) value, or pHa, and at various temperatures a true acidic pD can be estimated from the directly pH meter measured pHa, such that pD+ = pHa (apparent reading from pH meter) + 0.41. The electrode correction for alkaline conditions is 0.456 for heavy water. The alkaline correction is then pD+ = pH{{sub|a}}(apparent reading from pH meter) + 0.456. These corrections are slightly different from the differences in p[D+] and p[OD-] of 0.44 from the corresponding ones in heavy water.<ref>[http://www.nuceng.ca/candu/pdf/15%20-%20Chemistry%20in%20CANDU%20Process%20Systems.pdf discussion of pD+],</ref>

Heavy water is 10.6% denser than ordinary water, and heavy water's physically different properties can be seen without equipment if a frozen sample is dropped into normal water, as it will sink. If the water is ice-cold the higher melting temperature of heavy ice can also be observed: it melts at 3.7&nbsp;°C, and thus does not melt in ice-cold normal water.<ref>{{cite magazine |url=http://www.popsci.com/popsci/how20/a07160a72252c010vgnvcm1000004eecbccdrcrd.html |title=How 2.0 |access-date=21 January 2008 |last=Gray |first=Theodore |date=2007 |magazine=Popular Science |url-status=dead |archive-url=https://web.archive.org/web/20071216010940/http://www.popsci.com/popsci/how20/a07160a72252c010vgnvcm1000004eecbccdrcrd.html |archive-date=16 December 2007 }}</ref>

A 1935 experiment reported not the "slightest difference" in taste between ordinary and heavy water.<ref>{{cite journal|doi=10.1126/science.81.2098.273-a  |date=15 March 1935 |volume=81 |issue= 2098|journal=Science |page=273|bibcode = 1935Sci....81..273U |pmid=17811065 | last1 = Urey | first1 = HC |title=Concerning the Taste of Heavy Water | last2 = Failla | first2 = G}}</ref> However, a more recent study confirmed anecdotal observation that heavy water tastes slightly sweet to humans, with the effect mediated by the [[TAS1R2]]/[[TAS1R3]] taste receptor.<ref>{{cite journal|doi=10.1038/s42003-021-01964-y |date=6 April 2021 |volume=4 |issue=1 |journal=Communications Biology |first1=Natalie| last1=Ben Abu| first2=Philip E.| last2=Mason |title=Sweet taste of heavy water|page=440 |pmid=33824405 |pmc=8024362 |doi-access=free }}</ref> Rats given a choice between distilled normal water and heavy water were able to avoid the heavy water based on smell, and it may have a different taste.<ref>{{cite journal|doi=10.1016/0031-9384(79)90124-0 |pmid= 515218 |title= Taste responses to deuterium oxide | volume=23 |issue= 1 |journal=Physiology |pages=69–74 |year=1979 | last1 = Miller | first1 = Inglis J. | last2 = Mooser | first2 = Gregory|s2cid= 39474797 }}</ref> Some people report that minerals in water affect taste, e.g. potassium lending a sweet taste to hard water, but there are many factors of a perceived taste in water besides mineral contents.<ref>{{cite web | title=Is there really a north-south water taste divide? |url=https://www.bbc.com/news/magazine-22247668 | date=29 April 2013 | access-date=12 October 2020 |last=Westcott | first=Kathryn | publisher=BBC News Magazine}}</ref>

Heavy water lacks the [[Color of water|characteristic blue color]] of light water; this is because the [[molecular vibration]] harmonics, which in light water cause weak absorption in the red part of the visible spectrum, are shifted into the [[infrared]] and thus heavy water does not absorb red light.<ref name="Webexhibits">{{cite web |url=http://www.webexhibits.org/causesofcolour/5B.html |title=Colours from Vibration |author=WebExhibits |work=Causes of Colour |publisher=[[WebExhibits]] |url-status=live |archive-url=https://web.archive.org/web/20170223212434/http://www.webexhibits.org/causesofcolour/5B.html |archive-date=23 February 2017 |access-date=21 October 2017 |quote=Heavy water is colourless because all of its corresponding vibrational transitions are shifted to lower energy (higher wavelength) by the increase in isotope mass. }}</ref>

No physical properties are listed for "pure" semi-heavy water because it is unstable as a bulk liquid. In the liquid state, a few water molecules are always in an [[Self-ionization of water|ionized state]], which means the hydrogen atoms can exchange among different oxygen atoms. Semi-heavy water could, in theory, be created via a chemical method,{{explain|date=June 2021}} but it would rapidly transform into a dynamic mixture of 25% light water, 25% heavy water, and 50% semi-heavy. However, if it were made in the gas phase and directly [[Deposition (phase transition)|deposited]] into a solid, semi-heavy water in the form of ice could be stable. This is due to collisions between water vapor molecules being almost completely negligible in the gas phase at standard temperatures, and once crystallized, collisions between the molecules cease altogether due to the rigid lattice structure of solid ice.{{citation needed|date=February 2017}}

Heavy water exchanges with atmospheric water until it reaches the usual hydrogen-isotopic ratio.