Metal ion identification via selective precipitation and solubility tests

Steps:

• NaOH test shows white precipitate dissolving in excess to colorless solution, indicating amphoteric metal hydroxide forming a soluble complex like [M(OH)₄]⁻.
• NH₃ test produces white precipitate unchanged by excess, ruling out metals forming soluble ammine complexes.
• Al³⁺ matches: forms Al(OH)₃ (white ppt) soluble in excess NaOH as [Al(OH)₄]⁻, but insoluble in excess NH₃.
• Other ions fail one or both tests based on their hydroxide/ammonia behaviors.

Why A is correct:

• Al³⁺ hydroxide is amphoteric, dissolving in excess NaOH per the reaction Al(OH)₃ + OH⁻ → [Al(OH)₄]⁻, but remains insoluble in NH₃.

Why the others are wrong:

• B. Ca²⁺: Forms white Ca(OH)₂ ppt insoluble in excess NaOH or NH₃.
• C. Fe²⁺: Forms green gelatinous ppt with NaOH, insoluble in excess; dirty green ppt with NH₃, insoluble.
• D. Zn²⁺: White ppt with NaOH dissolves in excess; white ppt with NH₃ dissolves to form [Zn(NH₃)₄]²⁺.

Final answer: A