aluminium ion: Al 3+ (aq) + 3OH - (aq) ==> Al(OH) 3 (s) gives a white precipitate of aluminum hydroxide, which is not soluble in excess of the weak alkali ammonia, but dissolves in the stronger base/alkali sodium hydroxide (amphoteric) to give a clear colourless solution.

Al(OH) 3(s) + 3OH - (aq) ==> [Al(OH) 6] 3- (aq) (amphoteric behaviour because it dissolves in acids too)

calcium ion: Ca 2+ (aq) + 2OH - (aq) ==> Ca(OH) 2 (s) gives a white precipitate

of calcium hydroxide with sodium hydroxide IF the concentration of calcium ion is high. It is not soluble in excess of NaOH. No precipitate is formed with ammonia solution.

magnesium ion: Mg 2+ (aq) + 2OH - (aq) ==> Mg(OH) 2 (s) gives a white precipitate

of magnesium hydroxide, which is not soluble in excess of either NH 3 or NaOH. You could distinguish Mg from Ca with a flame test or ammonia test above.

copper(II) ion: Cu 2+ (aq) + 2OH - (aq) ==> Cu(OH) 2(s) gives a blue/turquoise ppt. of copper(II) hydroxide, which dissolves in excess ammonia to give a deep blue solution of an ammine complex, but copper(II) hydroxide is NOT soluble in excess NaOH.

Cu(OH) 2(s) + 4NH 3(aq) ==> [Cu(NH 3) 4] 2+ (aq) + 2OH - (aq)

iron(II) ion: Fe 2+ (aq) + 2OH - (aq) ==> Fe(OH) 2 (s) gives a dark green precipitate

of iron(II) hydroxide, which is not soluble in excess of NH 3 or NaOH. Darkens in air due to oxidation to Fe(OH) 3.

iron(III) ion: Fe 3+ (aq) + 3OH - (aq) ==> Fe(OH) 3 (s) forms a brown precipitate of iron(III) hydroxide, which is not soluble in excess of NH 3 or NaOH.

Another test for iron(III) ions is to add a few drops of potassium/ammonium thiocyanate solution and a blood-red coloured compound is formed.

zinc ion: Zn 2+ (aq) + 2OH - (aq) ==> Zn(OH) 2 (s) a white precipitate formed of zinc hydroxide, which dissolves in both excess (i) sodium hydroxide or (ii) ammonia to give a clear colourless solution: (i) Zn(OH) 2(s) + 2OH - (aq) ==> [Zn(OH) 4] 2- (aq)  (amphoteric behaviour because zinc hydroxide dissolves in acids too).

(ii) Zn(OH) 2(s) + 4NH 3(aq) ==> [Zn(NH 3) 4] 2+ (aq)  + 2OH - (aq) (soluble complex ion formation)

chromium (III) ion: Cr 3+ (aq) + 3OH - (aq) ==> Cr(OH) 3 (s) a grey-green precipitate forms of chromium(III) hydroxide, which is soluble in excess of NaOH (amphoteric, dissolves in acids too) but not soluble in excess ammonia NH 3. With sodium hydroxide a dark green soluble hexahydroxo-complex ion is formed.

Cr(OH) 3(s) + 3NaOH (aq) ==> [Cr(OH) 6] 3- (aq)

manganese(II) ion: Mn 2+ (aq) + 2OH - (aq) ==> Mn(OH) 2 (s) produces an off-white precipitate

of manganese(II) hydroxide, which is NOT soluble in excess of NH 3 or NaOH and rapidly turns brown=>black in air due to oxidation to manganese(III) oxide Mn 2O 3 and then manganese(IV) oxide, MnO 2.

lead(II) ion: Pb 2+ (aq) + 2OH - (aq) ==> Pb(OH) 2 (s) a white precipitate forms of lead(II) hydroxide, which dissolves in excess sodium hydroxide (amphoteric) to give a clear colourless solution but does not dissolve in excess ammonia solution.

Pb(OH) 2(s) + 2OH - (aq) ==> [Pb(OH) 4] 2- (aq)  (amphoteric behaviour with NaOH as with zinc hydroxide which also dissolves in acids too)

The barium ion, Ba 2+ (aq) does not give a hydroxide precipitate because barium hydroxide, Ba(OH) 2, is too soluble.