A colourless gas with a pungent smell is evolved when dilute hydrochloric acid is added to a sample of a salt. The gas evolved could turn

Analyze the question: A salt reacts with dilute HCl to give a colourless gas with a pungent smell. The most likely gas is SO₂, which comes from a sulphite (SO₃²⁻ + 2H⁺ → SO₂ + H₂O). The other candidates are H₂S (rotten-egg smell, so not "pungent" in the usual textbook sense) and CO₂ (odourless).

Concept: SO₂ is a reducing agent. Its reactions with oxidising agents are:

• Acidified K₂Cr₂O₇: orange → green (Cr³⁺). It does not go colourless.
• Acidified KMnO₄: purple → colourless (Mn²⁺).
• Pb(NO₃)₂ paper turns black only with H₂S (forming PbS). SO₂ does not do this.
• Fe(NO₃)₂ is already pale green, and SO₂ would not turn it green.

Check each option:

• A. Wrong. Dichromate turns green with SO₂, not colourless.
• B. Correct. SO₂ decolourises acidified permanganate: 5SO₂ + 2MnO₄⁻ + 2H₂O → 5SO₄²⁻ + 2Mn²⁺ + 4H⁺
• C. Wrong. Fe²⁺ is already green, and SO₂ would not produce this change.
• D. Wrong. This is the test for H₂S, which has a rotten-egg smell.

Tip: Remember the SO₂ tests as "dichromate goes green, permanganate goes colourless." Remember H₂S as "lead(II) ethanoate or nitrate paper goes black."