8.3.2 Metal Hydroxides

Sodium hydroxide (NaOH) solution can be used as a reagent to identify certain metal ions, specifically metal hydroxides, through the formation of precipitates. The reaction involves the interaction between the metal ions and hydroxide ions (OH^-) in the sodium hydroxide solution.

When sodium hydroxide solution is added to solutions containing aluminium (Al), calcium (Ca), and magnesium (Mg) ions, white precipitates are formed. However, only the aluminium hydroxide (Al(OH)₃) precipitate is soluble in excess sodium hydroxide solution. This means that when more sodium hydroxide is added to the mixture containing aluminium hydroxide, the precipitate dissolves, forming a clear solution.

On the other hand, solutions of copper(II) (Cu²⁺), iron(II) (Fe²⁺), and iron(III) (Fe³⁺) ions form coloured precipitates when sodium hydroxide solution is added. Copper(II) ions form a blue precipitate of copper(II) hydroxide (Cu(OH)₂), iron(II) ions form a green precipitate of iron(II) hydroxide (Fe(OH)₂), and iron(III) ions form a brown precipitate of iron(III) hydroxide (Fe(OH)₃).

These observations of precipitates can serve as a qualitative test to identify the presence of specific metal ions in a solution. By adding sodium hydroxide solution to an unknown solution and observing the resulting precipitate colour, we can infer the presence of aluminium, calcium, magnesium, copper(II), iron(II), or iron(III) ions based on their characteristic colours.

The identification of metal ions through precipitationWhere a solid forms from a liquid solution, often by the addition of another substance. reactions with sodium hydroxide is based on the solubility properties of the resulting hydroxide compounds. The solubility behaviour varies for different metal hydroxides, allowing us to distinguish between them.

Aluminium hydroxide

The reaction between aluminium ions (Al³⁺) and hydroxide ions (OH^-) forms aluminium hydroxide, which is initially a white precipitate. However, aluminium hydroxide dissolves in excess sodium hydroxide, forming a colourless solution called sodium aluminate. The balanced equation for the formation of aluminium hydroxide is:

Al³⁺ (aq) + 3OH^- (aq) → Al(OH)₃ (s)

Calcium hydroxide

Calcium ions (Ca²⁺) react with hydroxide ions to produce calcium hydroxide, which appears as a white precipitate. The balanced equation is:

Ca²⁺ (aq) + 2OH^- (aq) → Ca(OH)₂ (s)

Magnesium hydroxide

Magnesium ions (Mg²⁺) react with hydroxide ions to form magnesium hydroxide, which is also a white precipitate. The balanced equation is:

Mg²⁺ (aq) + 2OH^- (aq) → Mg(OH)₂ (s)

Copper(II) hydroxide

Copper(II) ions (Cu2+) combine with hydroxide ions to produce copper(II) hydroxide, which appears as a blue precipitate. The balanced equation is:

Cu²⁺ (aq) + 2OH^- (aq) → Cu(OH)₂ (s)

Iron(II) hydroxide

Iron(II) ions (Fe²⁺) react with hydroxide ions to form iron(II) hydroxide, which is a green precipitate. The balanced equation is:

Fe²⁺ (aq) + 2OH^- (aq) → Fe(OH)₂ (s)

Iron(III) hydroxide

Iron(III) ions (Fe³⁺) combine with hydroxide ions to produce iron(III) hydroxide, which appears as a brown precipitate. The balanced equation is:

Fe³⁺ (aq) + 3OH^- (aq) → Fe(OH)₃ (s)