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Why is the hydroxide ion not considered to be amphoteric.

Why is $$\ce{2OH- <<=> H2O + O^{2-} }$$ not considered ?? Even though the equilibrium would mostly lie towards the left but still the hydroxide ion can accept a hydrogen ion and donate hydrogen ion , but still it fits the definition for an Amphoteric substance - A substance that can both donate and accept an $\ce{H+}$ ion.

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    $\begingroup$ Hydroxide ion isn't a substance... $\endgroup$
    – Mithoron
    Commented Jul 23, 2019 at 16:36
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    $\begingroup$ The hydroxide ion can act as an acid, though it is a tremendously weak one, and in almost every case its basic character dominates its behaviour. Ultimately most chemical species are technically amphoteric, if you expose them to strong enough acids and strong enough bases in sufficiently forceful conditions. However, amphotericity is a term most often used in a practical sense. $\endgroup$
    – Nicolau Saker Neto
    Commented Jul 23, 2019 at 23:41

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The $\ce{O^{2-}}$ dianion is very unstable, especially in water. There is too much electronic density for the nuclear charge of oxygen to hold. We generally do not consider $\ce{OH-}$ to be amphoteric because, except in very extreme conditions, it will not give its proton away.

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  • $\begingroup$ Beside content (+1): For future reference, ChemSE benefits from mhchem's markup which eases input and display of chemistry-related content in questions, answers and comments -- just have a look at chemistry.meta.stackexchange.com/questions/86/…. $\endgroup$
    – Buttonwood
    Commented Jul 23, 2019 at 20:28
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How about heating a metal hydroxide such as magnesium hydroxide? At sufficiently high temperature the hydroxide decomposes according to the reaction

$\ce{Mg(OH)2 -> MgO + H2O}$

which is most simply interpreted as a proton transfer from one hydroxide ion to another:

$\ce{2 OH^- -> O^{2-} + H2O}$

This "disproportionation" would not take place at ambient temperature, but upon heating it's driven by the entropy of water vapor and the superior lattice energy of the solid oxide product.

Magnesium is far from unique here. Most metal hydroxides do the same thing, although the simple proton transfer concept is compromised if the compound has a high degree of covalent character as the oxides would with some transition metals.

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