During the qualitative analysis of SO₃²⁻ using dilute H₂ SO₄, SO₂ gas is evolved which turns K₂ Cr₂ O₇ solution (acidified with dilute H₂ SO₄) :

Solution

Chemistry ma’am · 3 steps

SO₂ is a reducing agent: it pulls orange Cr(+6) down to green Cr(+3).

1. Who does what. In acid, K₂Cr₂O₇ is a strong oxidising agent, and SO₂ is a reducing agent. So SO₂ is oxidised to sulphate, and dichromate is reduced.

   Cr₂O₇²⁻ + 3SO₂ + 2H⁺ → 2Cr³⁺ + 3SO₄²⁻ + H₂O

2. Track the colours. Chromium goes from +6 in the orange Cr₂O₇²⁻ ion to +3 in the green Cr³⁺ ion. Sulphur goes from +4 in SO₂ to +6 in SO₄²⁻.
3. So. The acidified dichromate solution turns from orange to green, which is option (d). This colour change confirms SO₂, and so confirms SO₃²⁻ in the salt.

The idea to keep. Acidified dichromate is a reducing-gas detector: orange Cr₂O₇²⁻ turning green means Cr(+6) became Cr³⁺. SO₂ and H₂S both do this.

Answer from the NTA final answer key. Solution by PrepWiser.