8.3.2 Metal Hydroxides

Identifying Metal Ions with Sodium Hydroxide

Formation of Metal Hydroxide Precipitates

Identifying Specific Metal Ions

• Calcium (Ca²⁺):
  • Reaction: Forms calcium hydroxide $Ca(OH)₂)$, which appears as a white precipitate.
• Magnesium (Mg²⁺):
  • Reaction: Forms magnesium hydroxide $(Mg(OH)₂)$, which also appears as a white precipitate.
• Aluminium (Al³⁺):
  • Reaction: Forms aluminium hydroxide $(Al(OH)₃)$, a white precipitate.
  • Note: Aluminium hydroxide is unique because it dissolves in excess sodium hydroxide, forming a colourless solution. This behaviour distinguishes it from calcium and magnesium hydroxides, which remain insoluble.

Coloured Precipitates with Sodium Hydroxide

In addition to white precipitates, certain metal ions produce coloured precipitates when reacted with sodium hydroxide.

When copper and iron ions react with sodium hydroxide, they will produce coloured precipitates. Copper will form a blue precipitate of copper hydroxide and iron forms two precipitates, depending on the charge of the ion. Iron (II) $(Fe2+)$ forms a green precipitate and iron (III) $(Fe3+)$forms a brown precipitate.

Summary

Metal ions can be identified based on the colour of the precipitate they form when reacted with sodium hydroxide. The white precipitates of calcium, magnesium, and aluminium (which dissolve in excess NaOH) are typical, while copper and iron ions produce distinctive blue, green, and brown precipitates. These reactions are crucial in analytical chemistry for identifying specific metal ions in a solution.