K₂Cr₂O₇ paper acidified with dilute H₂SO₄ turns green when exposed to :

Solution

Chemistry ma’am · 2 steps

Green means Cr³⁺, so look for the gas that can reduce orange dichromate.

1. What green means. In dichromate chromium is +6, and the Cr₂O₇²⁻ ion is orange. When it is reduced to Cr³⁺ (+3), the colour turns green. So the gas must be a reducing agent.

   Gas	Oxidation state of S or C	Can it reduce dichromate?
   SO₂	S is +4, can rise to +6	Yes: the paper turns green, with no other change
   SO₃	S is +6, already its highest	No
   H₂S	S is -2	Yes, but it also deposits sulphur. Its standard test is lead acetate paper turning black
   CO₂	C is +4, already its highest	No

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

2. So. Acidified dichromate paper turning green is the standard lab test for sulphur dioxide, which is option (a).

The idea to keep. Orange to green in dichromate always means Cr⁶⁺ was reduced to Cr³⁺. A gas whose central atom is already in its highest oxidation state cannot do it.

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