1. Common precipitation reactions with hydroxide ions. When sodium or ammonium hydroxide is added to a solution of metal ions an insoluble metal hydroxide is precipitated.

2. Reactions of M 2+ ions Transition metal ions are found as hydrated complexes, whose water ligands can be progressively replaced by hydroxide ions. [M(H 2 O) 6 ] 2+ [M(H 2 O) 5 OH] + M(H 2 O) 4 (OH) 2 M(H 2 O) 4 (OH) 2 is neutral and insoluble so precipitates out Basically giving M(OH) 2 (s)

3. Reaction of copper (ii) ions with sodium hydroxide. [Cu(H 2 O) 6 ] 2+ A blue precipitate of hydrated copper (ii) hydroxide forms. [Cu(H 2 O) 4 (OH) 2 ] (s) [Cu(H 2 O) 6 ] 2+ + 2OH - → [Cu(H 2 O) 4 (OH) 2 ] + 2H 2 O This precipitate is not soluble in excess sodium hydroxide.

4. Effect of heat [Cu(H 2 O) 4 (OH) 2 ] (s) On heating the precipitate turns black due to the formation of copper (ii) oxide. [Cu(H 2 O) 4 (OH) 2 ] (s) → CuO + 6H 2 O

5. Reaction of copper (ii) ions and ammonia. [Cu(H 2 O) 6 ] 2+ First a blue precipitate of hydrated copper (ii) hydroxide forms. This dissolves in excess ammonia to give a royal blue solution. Cu(H 2 O) 4 (OH) 2(s) [Cu(H 2 0) 2 (NH 3 ) 4 ] 2+ [Cu(H 2 O) 6 ] 2+ + 4NH 3 → [Cu(H 2 O) 2 (NH 3 ) 4 ] 2+ + 4H 2 O

6. Reaction of iron (ii) ions with sodium hydroxide [Fe(H 2 O) 6 ] 2+ A dark green gelatinous precipitate of hydrated iron (ii) hydroxide is formed Fe(OH) 2 (H 2 O) 4(s) This is insoluble in excess. [Fe(H 2 O) 6 ] 2+ + 2OH - → [Fe(H 2 O) 4 (OH) 2 ] (s) A similar reaction occurs with ammonia.

7. On standing the green precipitate of iron (ii) hydroxide slowly turns brown. Due to aerial oxidation forming iron (iii) hydroxide.

8. Reactions of M 3+ ions Transition metal ions are found as hydrated complexes, whose water ligands can be progressively replaced by hydroxide ions. [M(H 2 O) 6 ] 3+ [M(H 2 O) 5 OH] 2+ [M(H 2 O) 4 (OH) 2 ] + M(H 2 O) 3 (OH) 3 is neutral and insoluble so precipitates out. Basically giving M(OH) 3 (s) M(H 2 O) 3 (OH) 3 (s)

9. Reaction of iron (iii) ions with sodium hydroxide [Fe(H 2 O) 6 ] 3+ A rust brown gelatinous precipitate of iron (iii) hydroxide. Fe(OH) 3 (H 2 O) 3 (s) This is insoluble in excess, [Fe(H 2 O) 6 ] 3+ + 3OH - → [Fe(H 2 O) 3 (OH) 3 ] (s) + 3H 2 O A similar reaction occurs with ammonia.