Amphoterism

Amphoterism is the ability of a substance to act as either an acid or a base, depending on its reaction partner or the conditions.

Brønsted–Lowry acid–base theory: A theory in which acids donate protons and bases accept them. It explains amphoterism through a substance’s ability to donate or accept a proton.
Acid: A substance that donates protons or accepts electron pairs, depending on the acid–base theory used. An amphoteric substance must be able to take on acid behavior in at least one context.
Water: A polar molecule composed of two hydrogen atoms bonded to one oxygen atom. Water can donate a proton to form hydroxide or accept one to form hydronium.
Buffer solution: A solution that resists changes in pH when modest amounts of acid or base are added. Amphoteric species such as bicarbonate can participate in both sides of a buffer system.
Lewis acid–base theory: A theory in which acids accept electron pairs and bases donate them. It extends amphoterism beyond proton transfer to electron-pair donation and acceptance.
Base: A substance that accepts protons or donates electron pairs, depending on the acid–base theory used. An amphoteric substance must also be able to take on base behavior.
Bicarbonate: The hydrogen carbonate ion, HCO₃⁻, formed by partial deprotonation of carbonic acid. Bicarbonate can donate a proton to become carbonate or accept one to become carbonic acid.
Isoelectric point: The pH at which a molecule has zero net electrical charge on average. For amino acids and proteins, amphoteric ionization changes with pH and determines this point.
Conjugate acid–base pair: Two species related by the gain or loss of a proton. An amphoteric species can belong to different conjugate pairs in different reactions.
pH: A logarithmic measure of hydrogen-ion activity in aqueous solution. Solution pH can influence which acid–base role an amphoteric species takes.

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