Here's why it redissolves in excess sodium hydroxide:
1. Formation of Tetrahydroxoaluminate(III) Ion:
* When aluminium hydroxide is added to a solution of sodium hydroxide, it initially reacts to form a white gelatinous precipitate of aluminium hydroxide:
Al(OH)₃(s) + 3NaOH(aq) → Al(OH)₃(s) + 3Na⁺(aq) + 3OH⁻(aq)
* However, in the presence of excess hydroxide ions (from the NaOH), the aluminium hydroxide reacts further. The hydroxide ions attack the aluminium hydroxide, forming the soluble tetrahydroxoaluminate(III) ion [Al(OH)₄]⁻:
Al(OH)₃(s) + OH⁻(aq) → [Al(OH)₄]⁻(aq)
2. Equilibrium Shift:
* The reaction between aluminium hydroxide and hydroxide ions is an equilibrium reaction. Adding excess hydroxide ions shifts the equilibrium to the right, favoring the formation of the tetrahydroxoaluminate(III) ion, and thus causing the aluminium hydroxide precipitate to dissolve.
Overall:
The reaction can be summarized as:
Al(OH)₃(s) + 4OH⁻(aq) ⇌ [Al(OH)₄]⁻(aq) + 3H₂O(l)
Note: This is a classic example of how amphoteric substances behave in different environments. The ability of aluminium hydroxide to dissolve in excess base highlights its dual acidic and basic nature.
