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{{Short description|Chemical property of reacting with either an acid or base}}
{{Acids and bases}}

In [[Chemistry:Chemistry|chemistry]], an '''amphoteric''' [[Chemistry:Chemical compound|compound]] ({{etymology|el|amphi-|both}}) is a [[Physics:Molecule|molecule]] or [[Physics:Ion|ion]] that can react both as an [[Chemistry:Acid|acid]] and as a [[Chemistry:Base|base]].<ref>{{GoldBookRef |title=amphoteric |file=A00306}}</ref> What exactly this can mean depends on [[Chemistry:Acid–base reaction|which definitions of acids and bases]] are being used.

One type of amphoteric species are '''amphiprotic''' molecules, which can either [[Chemistry:Deprotonation|donate]] or [[Chemistry:Protonation|accept]] a [[Physics:Proton|proton]] ({{chem2|H+}}). This is what "amphoteric" means in [[Chemistry:Brønsted–Lowry acid–base theory|Brønsted–Lowry acid–base theory]]. For example, [[Chemistry:Amino acid|amino acid]]s and [[Biology:Protein|protein]]s are amphiprotic molecules because of their [[Chemistry:Amine|amine]] ({{chem2|\sNH2}}) and [[Chemistry:Carboxylic acid|carboxylic acid]] ({{chem2|\sCOOH}}) groups. [[Physics:Molecular autoionization|Self-ionizable compounds]] like [[Chemistry:Water|water]] are also amphiprotic.

'''Ampholytes''' are amphoteric molecules that contain both acidic and basic [[Chemistry:Functional group|functional group]]s. For example, an amino acid {{chem2|H2N\sRCH\sCO2H}} has both a basic group {{chem2|\sNH2}} and an acidic group {{chem2|\sCOOH}}, and exists as several structures in [[Chemistry:Chemical equilibrium|chemical equilibrium]]:
<!--splits added to allow text wrapping for mobile-->
:<chem>H2N-RCH-CO2H + H2O</chem><chem><=> H2N-RCH-COO- + H3O+</chem><chem><=> H3N+-RCH-COOH + OH-</chem><chem><=> H3N+-RCH-COO- + H2O</chem>

In approximately neutral [[Chemistry:Aqueous solution|aqueous solution]] (pH ≅ 7), the basic amino group is mostly protonated and the carboxylic acid is mostly deprotonated, so that the predominant species is the [[Physics:Zwitterion|zwitterion]] {{chem2|H3N+\sRCH\sCOO-}}. The pH at which the average charge is zero is known as the molecule's ''[[Biology:Isoelectric point|isoelectric point]]''. Ampholytes are used to establish a stable pH gradient for use in [[Biology:Isoelectric focusing|isoelectric focusing]].

Metal oxides which react with both acids as well as bases to produce salts and water are known as amphoteric oxides. Many metals (such as [[Chemistry:Zinc|zinc]], [[Chemistry:Tin|tin]], [[Chemistry:Lead|lead]], [[Chemistry:Aluminium|aluminium]], and [[Chemistry:Beryllium|beryllium]]) form amphoteric oxides or hydroxides. [[Chemistry:Aluminium oxide|Aluminium oxide]] ({{chem2|Al2O3}}) is an example of an amphoteric oxide. Amphoterism depends on the oxidation states of the oxide. Amphoteric oxides include [[Chemistry:Lead(II) oxide|lead(II) oxide]] and [[Chemistry:Zinc oxide|zinc oxide]], among many others.<ref>{{Housecroft2nd|pages=173–4}}</ref>

==Etymology==
'''Amphoteric''' is derived from the Greek word {{lang|el-Latn|amphoteroi}} ({{lang|el|ἀμφότεροι}}) meaning "both".  Related words in acid-base chemistry are '''amphichromatic''' and '''amphichroic''', both describing substances such as [[Chemistry:PH indicator|acid-base indicators]] which give one colour on reaction with an acid and another colour on reaction with a base.<ref>Penguin Science Dictionary 1994, Penguin Books</ref>

==Amphiprotic molecules==
According to the [[Chemistry: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 [[Physics:Proton|proton]], thus acting either as an [[Chemistry:Acid|acid]] or a [[Chemistry:Base|base]]. [[Chemistry:Water|Water]], [[Chemistry:Amino acid|amino acid]]s, [[Chemistry:Bicarbonate|hydrogencarbonate]] ion (or bicarbonate ion) {{chem2|HCO3-}}, [[Chemistry:Dihydrogen phosphate|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 [[Chemistry:Hydronium|hydronium]] ion {{chem2|H3O+}}, or else lose a proton to form a [[Chemistry:Hydroxide|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 [[Physics:Molecular autoionization|molecular autoionization]] reaction between two water molecules, in which one water molecule acts as an acid and another as a base.
:<chem>H2O + H2O <=> H3O+ + OH-</chem>

The [[Chemistry:Bicarbonate|bicarbonate]] ion, {{chem2|HCO3-}}, is amphoteric as it can act as either an acid or a base:
:As an acid, losing a proton: <chem>HCO3- + OH- <=> CO3^2- + H2O</chem>
:As a base, accepting a proton: <chem>HCO3- + H+ <=> H2CO3 </chem>
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 [[Chemistry:Sulfuric acid|sulfuric acid]], [[Chemistry:Phosphoric acid|phosphoric acid]] and [[Chemistry:Hydrogen sulfide|hydrogen sulfide]] that have lost one or more protons. In organic chemistry and biochemistry, important examples include [[Chemistry:Amino acid|amino acid]]s and derivatives of [[Chemistry:Citric acid|citric acid]].

Although an amphiprotic species must be amphoteric, the converse is not true. For example, a metal oxide such as [[Chemistry:Zinc oxide|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:
:<chem>ZnO_{(s)}{} + 2OH- + H2O -> Zn(OH)_{4(aq)}^2-</chem>
This reaction is not covered by the [[Chemistry:Brønsted–Lowry acid–base theory|Brønsted–Lowry acid–base theory]]. Because zinc oxide can also act as a base:
:<chem>ZnO_{(s)}{} + 2H+ -> Zn^2+_{(aq)}{} + H2O</chem>,
it is classified as amphoteric rather than amphiprotic.

==Oxides==
[[Chemistry:Zinc oxide|Zinc oxide]] (ZnO) reacts with both acids and with bases:
*<chem>ZnO + \overset{acid}{H2SO4} -> ZnSO4 + H2O</chem>
*<chem>ZnO + \overset{base}{2 NaOH} + H2O -> Na2[Zn(OH)4]</chem>
This reactivity can be used to separate different cations, for instance zinc(II), which dissolves in base, from manganese(II), which does not dissolve in base.

[[Chemistry:Lead(II) oxide|Lead oxide]] (PbO):
* <chem>PbO + \overset{acid}{2 HCl} -> PbCl2 + H2O</chem>
* <chem>PbO + \overset{base}{2 NaOH} + H2O -> Na2[Pb(OH)4]</chem>

[[Chemistry:Aluminium oxide|Aluminium oxide]] ({{chem2|Al2O3}}):
* <chem>Al2O3 + \overset{acid}{6 HCl} -> 2 AlCl3 + 3 H2O</chem>
* <chem>Al2O3 + \overset{base}{2 NaOH} + 3 H2O -> 2 Na[Al(OH)4]</chem> (hydrated [[Chemistry:Sodium aluminate|sodium aluminate]])

Stannous oxide (SnO):
* <chem>SnO + \overset{acid}{2 HCl} <=> SnCl2 + H2O</chem>
* <chem>SnO + \overset{base}{4 NaOH} + H2O <=> Na4[Sn(OH)6]</chem>

[[Chemistry:Vanadium(IV) oxide|Vanadium dioxide]] ({{chem2|VO2}}):
*<chem>VO2 + \overset{acid}{2 HCl} -> VOCl2 + H2O</chem>
*<chem>4 VO2 + \overset{base}{2 NaOH} -> Na2V4O9 + H2O</chem>

Some other elements which form amphoteric oxides are [[Chemistry:Gallium|gallium]], [[Chemistry:Indium|indium]], [[Chemistry:Scandium|scandium]], [[Chemistry:Titanium|titanium]], [[Chemistry:Zirconium|zirconium]], [[Chemistry:Chromium|chromium]], [[Chemistry:Iron|iron]], [[Chemistry:Cobalt|cobalt]], [[Chemistry:Copper|copper]], [[Chemistry:Silver|silver]], [[Chemistry:Gold|gold]], [[Chemistry:Germanium|germanium]], [[Chemistry:Antimony|antimony]], [[Chemistry:Bismuth|bismuth]], [[Chemistry:Beryllium|beryllium]], and [[Chemistry:Tellurium|tellurium]].

==Hydroxides==
[[Chemistry:Aluminium hydroxide|Aluminium hydroxide]] is also amphoteric:<!--simplified reactions that are correct -->
*<chem>Al(OH)3 + \overset{acid}{3 HCl} -> AlCl3 + 3 H2O</chem>
*<chem>Al(OH)3 + \overset{base}{NaOH} -> Na[Al(OH)4]</chem>

[[Chemistry:Beryllium hydroxide|Beryllium hydroxide]]:
*<chem>Be(OH)2 + \overset{acid}{2 HCl} -> BeCl2 + 2 H2O</chem>
*<chem>Be(OH)2 + \overset{base}{2 NaOH} -> Na2[Be(OH)4]</chem><ref>[http://webarchive.loc.gov/all/20020808162614/http://home.c2i.net/astandne/ CHEMIX School & Lab - Software for Chemistry Learning, by Arne Standnes] (program download required)</ref>

[[Chemistry:Chromium(III) hydroxide|Chromium hydroxide]]:
*<chem>Cr(OH)3 + \overset{acid}{3 HCl} -> CrCl3 + 3H2O</chem>
*<chem>Cr(OH)3 + \overset{base}{NaOH} -> Na[Cr(OH)4]</chem>

==See also==
* [[Chemistry:Ate complex|Ate complex]]
* [[Biology:Isoelectric point|Isoelectric point]]
* [[Physics:Zwitterion|Zwitterion]]

==References==
{{Reflist}}

[[Category:Amphoteric compounds| ]]
[[Category:Acid–base chemistry]]
[[Category:Chemical properties]]
[[Category:General chemistry]]

{{Sourceattribution|Amphoterism}}