In a laboratory, a student generates a gas by adding dilute sulfuric acid to sodium sulfite. The gas is bubbled into an aqueous solution of potassium dichromate acidified with dilute sulfuric acid. The solution changes from orange to green. Which gas is being produced?
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Correct answer
C. Sulfur dioxide
Principle or equation
Sulfite salts react with acids to produce sulfur dioxide gas. Sulfur dioxide is a reducing agent that can reduce dichromate ions (Cr₂O₇²⁻, orange) to chromium(III) ions (Cr³⁺, green) in acidic solution.
Why this answer is correct
The reaction of sodium sulfite with dilute sulfuric acid is: Na₂SO₃ + H₂SO₄ → Na₂SO₄ + H₂O + SO₂↑. Sulfur dioxide gas is released. When SO₂ is bubbled into acidified potassium dichromate solution, the dichromate ion is reduced to Cr³⁺, which is green, while SO₂ is oxidized to sulfate. This color change identifies the gas as sulfur dioxide.
Example
SO₂ reduces acidified K₂Cr₂O₇: Cr₂O₇²⁻ + 3SO₂ + 2H⁺ → 2Cr³⁺ + 3SO₄²⁻ + H₂O (orange to green).
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