Amphoteric hydroxide formation and dissolution

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 that form soluble complex ions.
• Aluminum ions (Al³⁺) produce Al(OH)₃, a white precipitate that dissolves as [Al(OH)₄]⁻ in excess base.
• Other ions either do not precipitate or fail to dissolve in excess NaOH.

Why A is correct:

• Al³⁺ + 3OH⁻ → Al(OH)₃ (white ppt), then Al(OH)₃ + OH⁻ → [Al(OH)₄]⁻ (soluble), per amphoteric metal hydroxide behavior.

Why the others are wrong:

• B: Ca²⁺ forms Ca(OH)₂ white ppt, but it remains insoluble in excess NaOH.
• C: Cu²⁺ forms blue Cu(OH)₂ ppt, insoluble in excess NaOH.
• D: Na⁺ does not react with NaOH to form any precipitate.

Final answer: A