Editing Amphoterism (section)

==Amphiprotic molecules==
According to the [[Brønsted–Lowry acid–base theory|Brønsted-Lowry theory of acids and bases]], acids are proton donors and bases are proton acceptors.<ref>{{cite book |last1=Petrucci |first1=Ralph H. |last2=Harwood |first2=William S. |last3=Herring |first3=F. Geoffrey |date=2002 |title=General chemistry: principles and modern applications |url=https://archive.org/details/generalchemistry00hill |url-access=registration |edition=8th |location=Upper Saddle River, NJ |publisher=Prentice Hall |isbn=978-0-13-014329-7 |lccn=2001032331 |oclc=46872308 |page=[https://archive.org/details/generalchemistry00hill/page/669 669]}}</ref> An amphiprotic molecule (or ion) can either donate or accept a [[proton]], thus acting either as an [[acid]] or a [[base (chemistry)|base]]. [[Water]], [[amino acid]]s, [[bicarbonate|hydrogencarbonate]] ion (or bicarbonate ion) {{chem2|HCO3-}}, [[dihydrogen phosphate]] ion {{chem2|H2PO4-}}, and [[hydrogensulfate]] ion (or bisulfate ion) {{chem2|HSO4-}} are common examples of amphiprotic species. Since they can donate a proton, all amphiprotic substances contain a hydrogen atom. Also, since they can act like an acid or a base, they are amphoteric.

===Examples===
The water molecule is amphoteric in aqueous solution. It can either gain a proton to form a [[hydronium]] ion {{chem2|H3O+}}, or else lose a proton to form a [[hydroxide]] ion {{chem2|OH-}}.<ref>{{cite book |url=https://www.worldcat.org/title/824171785 |title=Fundamentals of analytical chemistry |last1=Skoog |first1=Douglas A. |last2=West |first2=Donald M. |last3=Holler |first3=F. James |last4=Crouch |first4=Stanley R. |year=2014 |isbn=978-0-495-55828-6 |edition=Ninth |location=Belmont, CA |pages=200 |oclc=824171785}}</ref>

Another possibility is the [[molecular autoionization]] reaction between two water molecules, in which one water molecule acts as an acid and another as a base.
:{{chem2 | H2O + H2O <-> H3O+ + HO- }}

The [[bicarbonate]] ion, {{chem2|HCO3-}}, is amphoteric as it can act as either an acid or a base:
:As an acid, losing a proton: {{chem2 | HCO3- + OH- <-> CO3(2-) + H2O }}
:As a base, accepting a proton: {{chem2 | HCO3- + H+ <-> H2CO3 }}
Note: in dilute aqueous solution the formation of the [[hydronium ion]], {{chem2|H3O+(aq)}}, is effectively complete, so that hydration of the proton can be ignored in relation to the equilibria.

Other examples of inorganic polyprotic acids include anions of [[sulfuric acid]], [[phosphoric acid]] and [[hydrogen sulfide]] that have lost one or more protons. In organic chemistry and biochemistry, important examples include [[amino acid]]s and derivatives of [[citric acid]].

Although an amphiprotic species must be amphoteric, the converse is not true. For example, a metal oxide such as [[zinc oxide]], ZnO, contains no hydrogen and so cannot donate a proton. Nevertheless, it can act as an acid by reacting with the hydroxide ion, a base:
:{{chem2|ZnO + 2 OH- + H2O -> [Zn(OH)4](2-)}}
Zinc oxide can also act as a base:
:{{chem2|ZnO + 2H+ +  5 H2O -> [Zn(H2O)6](2+)}}