Tests confirm Zn²⁺ cation via amphoteric behavior

Steps:

• White ppt with NaOH(aq) dissolving in excess indicates amphoteric hydroxide formation, like Zn(OH)₂ → [Zn(OH)₄]²⁻.
• White ppt with NH₃(aq) dissolving in excess indicates soluble ammine complex, like Zn(OH)₂ → [Zn(NH₃)₄]²⁺.
• This dual solubility rules out non-amphoteric cations like Al³⁺, which ppts but doesn't dissolve in NH₃.
• Anions do not form these ppts; observations point to specific cation reactivity.

Why C is correct:

• Zn²⁺ produces white Zn(OH)₂ ppt with both NaOH and NH₃, dissolving in excess via complex ions [Zn(OH)₄]²⁻ and [Zn(NH₃)₄]²⁺ as per qualitative analysis.

Why the others are wrong:

• A: Cl⁻ does not form ppts with NaOH or NH₃; these tests detect cations.
• B: NO₃⁻ is spectator ion, yielding no such ppts.
• D: NH₄⁺ + NaOH releases NH₃ gas, no ppt; no reaction with NH₃.

Final answer: C