Editing Amphoterism (section)

==Etymology and terminology==
'''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 [[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>

===Amphiprotism===
Amphiprotism is exhibited by compounds with both Brønsted acidic and basic properties.<ref>{{cite book |chapter=Amphiprotic (solvent) |doi=10.1351/goldbook.A00304 |chapter-url=https://goldbook.iupac.org/terms/view/A00304 |title=The IUPAC Compendium of Chemical Terminology |date=2008 }}</ref>  A prime example is H<sub>2</sub>O.
Amphiprotic molecules can either [[deprotonation|donate]] or [[protonation|accept]] a [[proton]] ({{chem2|H+}}). [[Amino acid]]s (and [[protein]]s) are amphiprotic molecules because of their [[amine]] ({{chem2|\sNH2}}) and [[carboxylic acid]] ({{chem2|\sCOOH}}) groups. 

===Ampholytes===
'''Ampholytes''' are [[zwitterion]]s<ref>{{cite book |title=Ampholyte |chapter=Ampholytes |date=2014 |doi=10.1351/goldbook.A00305 |chapter-url=https://goldbook.iupac.org/terms/view/A00305}}</ref> ‒ molecules or ions that contain both acidic and basic [[functional group]]s. [[Amino acid]]s {{chem2|H2N\sRCH\sCO2H}} have both a basic group {{chem2|\sNH2}} and an acidic group {{chem2|\sCOOH}}.  Often such species exists as several structures in [[chemical equilibrium]]:
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:{{chem2 | H2N\sCRH\sCO2H + H2O <-> H2N\sCRH\sCOO- + H3O+ <-> H3N+\sCRH\sCOOH + HO- <-> H3N+\sCRH\sCOO- + H2O }}

In approximately neutral [[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 [[zwitterion]] {{chem2|H3N+\sRCH\sCOO-}}. The pH at which the average charge is zero is known as the molecule's ''[[isoelectric point]]''. Ampholytes are used to establish a stable pH gradient for use in [[isoelectric focusing]].

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