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In chemistry, an amphoteric compound is a molecule or ion that can react as an acid as well as a base. The word is derived from the Greek word meaning "both". Many metals (such as copper, zinc, tin, lead, aluminium, and beryllium) form amphoteric oxides or hydroxides. Amphoterism depends on the oxidation state of the oxide. One type of amphoteric species are amphiprotic molecules, which can either donate or accept a proton (H+). Examples include amino acids and proteins, which have amine and carboxylic acid groups, and self-ionizable compounds such as water Ampholytes are amphoteric molecules that contain both acidic and basic groups and will exist mostly as zwitterions in a certain range of pH. 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. ==Amphoteric oxides and hydroxides== Zinc oxide (ZnO) reacts with both acids and with bases: * In acid: ZnO + 2H+ → Zn2+ + H2O * In base: ZnO + H2O + 2 OH− → ()2− This reactivity can be used to separate different cations, such as zinc(II), which dissolves in base, from manganese(II), which does not dissolve in base. Aluminium hydroxide is also amphoteric: * As a base (neutralizing an acid): Al(OH)3 + 3 HCl → AlCl3 + 3 H2O * As an acid (neutralizing a base): Al(OH)3 + NaOH → Na() Some other amphoteric compounds include: *Beryllium hydroxide * *with acid: Be(OH)2 + 2 HCl → BeCl2 + 2 H2O * *with base: Be(OH)2 + 2 NaOH → Na2() *Aluminium oxide * *with acid: Al2O3 + 3 H2O + 6 H3O+(aq) → 2 ()3+(aq) * *with base: Al2O3 + 3 H2O + 2 OH−(aq) → 2 ()−(aq) *Lead(II) oxide * *with acid: PbO + 2 HCl → PbCl2 + H2O * *with base: PbO + 2 NaOH + H2O → Na2() Some other elements which form amphoteric oxides are gallium, indium, scandium, titanium, zirconium, vanadium, chromium, iron, cobalt, copper, silver, gold, germanium, tin, antimony and bismuth, tellurium.〔(CHEMIX School & Lab - Software for Chemistry Learning, by Arne Standnes ) (program download required)〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Amphoterism」の詳細全文を読む スポンサード リンク
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