Editing Hydrogen sulfide (section)

==Properties==
Hydrogen sulfide is slightly denser than air. A mixture of {{chem2|H2S}} and air can be explosive.

===Oxidation===
In general, hydrogen sulfide acts as a [[reducing agent]], as indicated by its ability to reduce sulfur dioxide in the [[Claus process]].  Hydrogen sulfide burns in oxygen with a blue flame to form [[sulfur dioxide]] ({{chem2|SO2}}) and [[water (molecule)|water]]:
:{{chem2|2 H2S + 3 O2 → 2 SO2 + 2 H2O}}

If an excess of oxygen is present, [[sulfur trioxide]] ({{chem2|SO3}}) is formed, which quickly hydrates to [[sulfuric acid]]:

:{{chem2|H2S + 2 O2 → H2SO4}}

===Acid-base properties===
It is slightly soluble in water and acts as a [[weak acid]] ([[Acid dissociation constant|p''K''<sub>a</sub>]]&nbsp;=&nbsp;6.9 in 0.01–0.1&nbsp;mol/litre solutions at 18&nbsp;°C), giving the [[hydrosulfide]] ion {{chem2|HS-}}. Hydrogen sulfide and its solutions are colorless. When exposed to air, it slowly oxidizes to form elemental sulfur, which is not soluble in water. The [[sulfide]] anion {{chem2|S(2-)}} is not formed in aqueous solution.<ref>{{cite journal|last1=May|first1=P.M.|last2=Batka|first2=D.|last3=Hefter|first3=G.|last4=Könignberger|first4=E.|last5=Rowland|first5=D.|title=Goodbye to S2-|journal=Chem. Comm.|volume=54|issue=16|pages=1980–1983|date=2018|doi=10.1039/c8cc00187a|pmid=29404555}}</ref>

{{chem2|H2S and H2O}} exchange protons rapidly.  This behavior is the basis of technologies for the purification of deuterium oxide ("heavy water" or {{chem2|D2O}}), which exploits the easy distillation of these compounds.<ref>{{cite book |doi=10.1002/0471238961.0919152008011212.a01 |chapter=Isotope Separation |title=Kirk-Othmer Encyclopedia of Chemical Technology |date=2000 |last1=von Halle |first1=Edward |isbn=978-0-471-48494-3 }}</ref>

===Extreme temperatures and pressures===
At pressures above 90&nbsp;GPa ([[gigapascal]]), hydrogen sulfide becomes a metallic conductor of electricity. When cooled below a [[Superconductivity#Superconducting phase transition|critical temperature]] this high-pressure phase exhibits [[superconductivity]]. The critical temperature increases with pressure, ranging from 23&nbsp;K at 100&nbsp;GPa to 150&nbsp;K at 200&nbsp;GPa.<ref name=Drozdov2014>{{Cite arXiv|last1= Drozdov|first1=A.|last2=Eremets|first2=M. I.|last3=Troyan|first3=I. A.|title=Conventional superconductivity at 190 K at high pressures|eprint=1412.0460|year=2014|class=cond-mat.supr-con}}</ref> If hydrogen sulfide is pressurized at higher temperatures, then cooled, the critical temperature reaches {{convert|203|K|°C}}, which was the highest accepted superconducting critical temperature until the discovery of [[Lanthanum decahydride]] in 2019. By substituting a small part of sulfur with phosphorus and using even higher pressures, it has been predicted that it may be possible to raise the critical temperature to above {{convert|0|°C|K}} and achieve [[Room-temperature superconductor|room-temperature superconductivity]].<ref name=cart15>{{cite journal |last1=Cartlidge |first1=Edwin |title=Superconductivity record sparks wave of follow-up physics |journal=Nature |date=August 2015 |volume=524 |issue=7565 |pages=277 |doi=10.1038/nature.2015.18191 |pmid=26289188 |bibcode=2015Natur.524..277C |doi-access=free }}</ref>

Hydrogen sulfide decomposes without a presence of a catalyst under atmospheric pressure around 1200&nbsp;°C into hydrogen and sulfur.<ref>{{cite journal |title=The direct conversion of hydrogen sulfide to hydrogen and sulfur |journal=International Journal of Hydrogen Energy |date=1998 |volume=23 |issue=6 |pages=451–456 |doi=10.1016/S0360-3199(97)00099-2|last1=Faraji |first1=F. |bibcode=1998IJHE...23..451F }}</ref>

===Reactions with metals===
Hydrogen sulfide reacts with metal ions to form metal sulfides, which are insoluble, often dark colored solids. This behavior is the basis of the use of hydrogen sulfide as a reagent in the [[qualitative inorganic analysis]] of metal ions. In these analyses, heavy metal (and [[Nonmetal (chemistry)|nonmetal]]) ions (e.g., Pb(II), Cu(II), Hg(II), As(III)) are precipitated from solution upon exposure to {{chem2|H2S}}. The components of the resulting solid are then identified by their reactivity. [[Lead(II) acetate]] paper is used to detect hydrogen sulfide because it readily converts to [[lead(II) sulfide]], which is black.<ref name=King>{{cite book|first1=Edward J.|last1=King|first2=Larkin H.|last2=Farinholt|url=https://books.google.com/books?id=zikLAAAAIAAJ|title=Qualitative Analysis and Electrolytic Solutions|year=1959|publisher=Harcourt, Brace|oclc=594863676|location=New York}}</ref><ref>{{cite book|last1=Vogel|first1=A. I.|last2=Svehla|first2=G.|url=https://books.google.com/books?id=MpPenWMDPd0C|title=Vogel's Qualitative Inorganic Analysis|date=1996|publisher=Longman|location=Harlow, England (1996); New Delhi, India (2008)|isbn=9788177582321|oclc=792729931}}</ref>

Hydrogen sulfide is also responsible for [[tarnish]]ing on various metals including [[copper]] and [[silver]]; the chemical responsible for black [[Toning (coin)|toning]] found on silver coins is [[silver sulfide]] ({{chem2|Ag2S}}), which is produced when the silver on the surface of the coin reacts with atmospheric hydrogen sulfide.<ref>{{Cite journal |last=JCE staff|date=March 2000 |title=Silver to Black - and Back|url=https://pubs.acs.org/doi/pdf/10.1021/ed077p328A|journal=Journal of Chemical Education|volume=77|issue=3|pages=328A|doi=10.1021/ed077p328a |bibcode=2000JChEd..77R.328J |issn=0021-9584}}</ref> Coins that have been subject to toning by hydrogen sulfide and other sulfur-containing compounds may have the toning add to the numismatic value of a coin based on aesthetics, as the toning may produce [[thin-film interference]], resulting in the coin taking on an attractive coloration.<ref>{{Cite web |title=What causes coins to tone – ICCS |url=https://iccscoin.ca/what-causes-coins-to-tone/ |access-date=2024-02-11 |website=iccscoin.ca|date=23 September 2021 }}</ref> Coins can also be intentionally treated with hydrogen sulfide to induce toning, though artificial toning can be distinguished from natural toning, and is generally criticised among collectors.<ref>{{Cite web|title=Coin Toning 101: The Differences between Naturally and Artificially Toned Coins|url=https://www.originalskincoins.com/blogs/news/coin-toning-101-the-differences-between-naturally-and-artificially-toned-coins|access-date=2021-10-15 |website=Original Skin Coins|date=26 June 2016 }}</ref>