Editing Hydrogen sulfide (section)

===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>