Amphoteric hydroxide test for Al³⁺

Steps:

• Adding NaOH to the solution forms a white precipitate, indicating an insoluble metal hydroxide.
• The precipitate dissolves in excess NaOH, characteristic of amphoteric hydroxides.
• Aluminum ion (Al³⁺) produces Al(OH)₃, a white precipitate that dissolves as [Al(OH)₄]⁻ in excess base.
• Other ions do not match both precipitate color and solubility behavior.

Why A is correct:

• Al(OH)₃ is amphoteric, reacting with excess OH⁻ to form the soluble complex [Al(OH)₄]⁻ per the equation Al(OH)₃(s) + OH⁻(aq) → [Al(OH)₄]⁻(aq).

Why the others are wrong:

• B: Ca²⁺ forms white Ca(OH)₂ precipitate, but it remains insoluble in excess NaOH.
• C: Cu²⁺ forms blue Cu(OH)₂ precipitate, which does not dissolve in excess NaOH.
• D: Na⁺ does not form any precipitate with NaOH, as both are soluble.

Final answer: A