NaOH and NH3

The precipitates of amphoteric hydroxides can be re-dissolved in excess sodium hydroxide. This is because these insoluble hydroxides can react with excess hydroxide ions to form anions, and the sodium salts of these anions are soluble in water. Common amphoteric hydroxides include aluminium hydroxide, zinc hydroxide and lead(II) hydroxide.

Al(OH)3(s) + NaOH(aq) → Na[Al(OH)4](aq)
or: Al(OH)3(s) + OH^-(aq) → [Al(OH)4]^-(aq)

Zn(OH)2(s) + 2NaOH(aq) → Na2[Zn(OH)4](aq)
or: Zn(OH)2(s) + 2OH^-(aq) → [Zn(OH)4]^2-(aq)

Pb(OH)2(s) + 2NaOH(aq) → Na2[Pb(OH)4](aq)
or: Pb(OH)2(s) + 2OH^-(aq) → [Pb(OH)4]^2-(aq)


The precipitates of some insoluble hydroxides or oxides can be re-dissolved in excess ammonia solution if these precipitates can form soluble complexes in excess ammonia solution. Such precipitates commonly include zinc hydroxide, copper(II) hydroxide and silver oxide.

Zn(OH)2(s) + 4NH3(aq) → [Zn(NH3)4]^2+(aq) + 2OH^-(aq)
or: Zn(OH)2(s) + 4NH3(aq) → [Zn(NH3)4](OH)2(aq)

Cu(OH)2(s) + 4NH3(aq) → [Cu(NH3)4]^2+(aq) + 2OH^-(aq)
or: Cu(OH)2(s) + 4NH3(aq) → [Cu(NH3)4](OH)2(aq)

Ag2O(s) + H2O(l) + 4NH3(aq) → 2[Ag(NH3)2]^+(aq) + 2OH^-(aq)
or: Ag2O(s) + H2O(l) + 4NH3(aq) → 2[Ag(NH3)2]OH(aq)