Ion identification via precipitation and gas tests

Steps:

• Green precipitate with NaOH indicates Cr³⁺, as Cr(OH)₃ is green (Fe²⁺ also possible but tested further).
• Ammonia gas (turns litmus blue) on heating excess NaOH confirms NH₄⁺, via NH₄⁺ + OH⁻ → NH₃ + H₂O.
• White precipitate with Ba(NO₃)₂ after acidification indicates SO₄²⁻, as BaSO₄ is insoluble white.
• Combination matches NH₄⁺, Cr³⁺, SO₄²⁻ in green crystals (e.g., (NH₄)₂Cr₂(SO₄)₃·12H₂O).

Why A is correct:

• Tests align with NH₄⁺ (ammonia evolution), Cr³⁺ (green Cr(OH)₃), and SO₄²⁻ (BaSO₄ formation per solubility rules).

Why the others are wrong:

• B: Fe²⁺ gives green Fe(OH)₂, SO₄²⁻ fits, but no NH₄⁺ for ammonia gas.
• C: Cr³⁺ fits green ppt, SO₄²⁻ fits BaSO₄, but CO₃²⁻ yields CO₂ (not ammonia) with acid/heat.
• D: Fe²⁺ fits green ppt, SO₄²⁻ fits, but NO₃⁻ produces no ammonia gas with NaOH.

Final answer: A