O Levels Chemistry (5070)•5070/12/M/J/18
Explanation
Tests for Alkaline Ammonia and Acidic/Reducing SO2
Steps:
- Identify ammonia (NH3) test: alkaline gas turns damp red litmus paper blue.
- Identify sulfur dioxide (SO2) test: acidic reducing gas decolorizes acidified potassium manganate(VII) from purple to colorless.
- Compare options: seek row pairing red litmus for NH3 and manganate(VII) for SO2.
- Select matching row as correct.
Why C is correct:
- Damp red litmus detects NH3's alkalinity (pH > 7, turns blue); acidified KMnO4 confirms SO2 via reduction (MnO4- + SO2 → Mn2+ + SO42-, decolorizes).
Why the others are wrong:
- A: Damp litmus (unspecified color) fails to specify red for NH3; no clear SO2 pairing.
- B: Both litmus types target pH but miss SO2's reducing test.
- D: Damp blue litmus redundant for NH3; no SO2 test included.
Final answer: C
Topic: Identification of ions and gases
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