The oxides and hydroxides of elements can be classified on the basis of their acid-base properties as either acidic, basic or amphoteric.
The oxide and hydroxide of an element in a particular oxidation state are equivalent for this purpose because they are related by addition of water. This is shown in the reactions below where only the stoichiometric amount of water is present. Na2O(s) + H2O 
2NaOH(s)
Al2O3(s) + 3H2O 2Al(OH)3(s)
Basic oxides/hydroxides either dissolve in excess water to give hydroxide ion, or if insoluble in water, react with aqueous acid to dissolve.
Na2O(s) + H2O 2Na+(aq) + 2OH–(aq)
Mg(OH)2(s) + 2H+(aq) Mg2+(aq) + 2H2O
Amphoteric oxides/hydroxides are insoluble but react to dissolve in either aqueous acid or base.
Zn(OH)2(s) + 2OH–(aq) Zn(OH)42–(aq)
Zn(OH)2(s) + 2H+(aq) Zn2+(aq) + 2H2O
The oxide and hydroxide of an element in a particular oxidation state are equivalent for this purpose because they are related by addition of water. This is shown in the reactions below where only the stoichiometric amount of water is present.
2NaOH(s)Al2O3(s) + 3H2O
2Al(OH)3(s)Balancing equation tip 1: Balance the metal ion first, then oxygen, then hydrogen.
Basic oxides/hydroxides either dissolve in excess water to give hydroxide ion, or if insoluble in water, react with aqueous acid to dissolve.
Na2O(s) + H2O
2Na+(aq) + 2OH–(aq)Mg(OH)2(s) + 2H+(aq)
Mg2+(aq) + 2H2OBalancing equation tip 2. For all balanced equations, the total charge at the right and left are equal!!
Amphoteric oxides/hydroxides are insoluble but react to dissolve in either aqueous acid or base.
Zn(OH)2(s) + 2OH–(aq)
Zn(OH)42–(aq)Product formula writing tip 1. The metal-containing product of dissolving in excess aqueous hydroxide ion is an anion with the metal in the same oxidation state as in the same oxidation state as in the reactant metal hydroxide.
Zn(OH)2(s) + 2H+(aq)
Zn2+(aq) + 2H2OProduct formula writing tip 2: The metal-containing product of dissolving a metal hydroxide in acid is a cation containing the metal in the same oxidation state as the reactant hydroxide.
In the reaction with OH–(aq), Zn(OH)2 is acting as a Lewis acid
Lewis acids react with electron pair donors (hydroxide ion in this case) to form covalent bonds.
In the reaction with H+(aq), Zn(OH)2 is acting as a Bronsted base (proton acceptor).
In the reaction with OH–(aq), Zn(OH)2 is acting as a Lewis acid
Lewis acids react with electron pair donors (hydroxide ion in this case) to form covalent bonds.
In the reaction with H+(aq), Zn(OH)2 is acting as a Bronsted base (proton acceptor).