A peculiar, but surprisingly accessible, case is hydroxide ion acting as both an acid and a vase. In aqueous solution it would be only a base, but heating of dry hydroxides brings out the true amphoteric character. For instance, most metal hydroxides decompose into oxides plus water vapor on heating, and we can render this as an acid-base reaction between hydroxide ions:

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

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

$\ce{OH^- + H^+ -> H2O}$

This answer cites a reaction in which sodium hydroxide acts as an acid donating a proton to sodium amide:

$\ce{NaOH + NaNH2\overset{\Delta}{\->}Na2O + NH3}$

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

$\ce{NH2^- + H^+ -> NH3}$

A peculiar, but surprisingly accessible, case is hydroxide ion acting as both an acid and a base. In aqueous solution it would be only a base, but heating of dry hydroxides brings out the true amphoteric character. For instance, most metal hydroxides decompose into oxides plus water vapor on heating, and we can render this as an acid-base reaction between hydroxide ions:

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

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

$\ce{OH^- + H^+ -> H2O}$

This answer cites a reaction in which sodium hydroxide acts as an acid donating a proton to sodium amide:

$\ce{NaOH + NaNH2\overset{\Delta}{\->}Na2O + NH3}$

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

$\ce{NH2^- + H^+ -> NH3}$