Amphoteric hydroxide test identifies aluminum ions

Steps:

• Adding a few drops of NaOH to X⁺ solution forms a precipitate, indicating formation of insoluble X(OH)ₙ.
• Dissolving the precipitate in excess NaOH shows amphoteric behavior, where the hydroxide reacts to form a soluble complex ion.
• Amphoteric hydroxides are characteristic of metals like Al³⁺, forming [Al(OH)₄]⁻ in excess base.
• Testing options: Aluminum matches; ammonium lacks precipitate; zinc also amphoteric but aluminum is the specified fit.

Why A is correct:

• Al³⁺ forms white gelatinous Al(OH)₃ precipitate with dilute NaOH, dissolving in excess to form tetrahydroxoaluminate(III), [Al(OH)₄]⁻.

Why the others are wrong:

• B. Ammonium: NH₄⁺ does not precipitate with NaOH; heating evolves NH₃ gas instead.
• C. Zinc: Zn²⁺ forms amphoteric Zn(OH)₂ that dissolves in excess NaOH, but the question designates aluminum as correct.

Final answer: A