[Chem] chemical equation

1.
NH3 is a weak alkali which partly ionized in water to give hydroxide ions.
NH3(aq) + H2O(l) → NH4+(aq) + OH-(aq)
Aluminium ions and hydroxide ions react to give a white jelly-like precipitate of aluminium hydroxide.
Al3+(aq) + 3OH-(aq) → Al(OH)3­(s)
The precipitate does NOT dissolve in excess NH3(aq).

2.
When a small amount of sodium hydroxide is added to zinc sulphate solution, a white precipitate of zinc hydroxide is formed.
Zn2+(aq) + 2OH-(aq) → Zn(OH)2(s)
The white precipitate dissolves in excess sodium hydroxide solution to give a colourless solution of sodium zincate.
Zn(OH)2(s) + 2OH-(aq) → [Zn(OH)4]2-(aq)

2007-03-23 14:19:48 補充：
1. Al(OH)3 does NOT dissolove in excess NH3(aq). It only dissolves in excess NaOH(aq).2. Zn2 (aq) will NOT give [Zn(OH)4]^2-(aq) directly. Instead, it will give Zn(OH)2 precipitate first, and then dissolves in excess NaOH solution to give [Zn(OH)4]^2-.

4OH- + Al3+ --> [Al(OH)4]-
4OH- + Zn2+ --> [Zn(OH)4]2-

2007-03-23 21:33:37 補充：
to michael:doesn't NH3(aq) dissociate into NH4+ andOH- into water?and OH- derived then form complex with Al3+?