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Difficulty: MediumAluminium: Extraction, Amphoteric Properties, Alloys, and Compounds

When a few drops of aqueous sodium hydroxide are added to a solution of aluminium chloride, a white gelatinous precipitate forms. Upon adding excess sodium hydroxide, the precipitate dissolves to produce a clear, colorless solution. Which chemical species is present in the final clear solution?

  1. Tetrahydroxoaluminate(III) ion, [Al(OH)4]−[Al(OH)_4]^-Answer
  2. B
    Aluminium hydroxide, Al(OH)3Al(OH)_3
  3. C
    Aluminium oxide, Al2O3Al_2O_3
  4. D
    Hexaaquaaluminium(III) ion, [Al(H2O)6]3+[Al(H_2O)_6]^{3+}

Answer

The tetrahydroxoaluminate(III) ion, [Al(OH)4]−[Al(OH)_4]^-
Aluminium hydroxide, Al(OH)3Al(OH)_3, is an amphoteric hydroxide. It initially precipitates as a white gelatinous solid when alkali is added. When excess strong alkali like sodium hydroxide is added, it acts as an acid and reacts further with hydroxide ions to form the soluble complex ion, tetrahydroxoaluminate(III), [Al(OH)4]−[Al(OH)_4]^-, yielding a clear solution.

Step-by-Step Solution

1
Identify the initial precipitation reaction
Adding a few drops of OH−OH^- ions precipitates white gelatinous aluminium hydroxide: Al3+(aq)+3OH−(aq)→Al(OH)3(s)Al^{3+}(aq) + 3OH^-(aq) \rightarrow Al(OH)_3(s)
Aluminium ions react with hydroxide ions to form insoluble aluminium hydroxide.
2
Apply the amphoteric property of aluminium hydroxide with excess strong base
Adding excess sodium hydroxide causes Al(OH)3Al(OH)_3 to act as an acid, dissolving according to: Al(OH)3(s)+OH−(aq)→[Al(OH)4]−(aq)Al(OH)_3(s) + OH^-(aq) \rightarrow [Al(OH)_4]^-(aq)
Amphoteric hydroxides react with excess alkalis to form soluble complex aluminate ions.

Key Concept

Amphoteric nature of aluminium compounds
Estimated Time:1m 0s
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Topics

Aluminium: Extraction, Amphoteric Properties, Alloys, and Compounds

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