Chemical Tests for Ions and Gases: Question 4
Syllabus 12.5
During a stock check, a technician notices a slow gas leak from a cracked valve on a cylinder in an industrial storeroom. The leak is too faint to see any colour, but the escaping gas has a sharp, choking smell, and could be either chlorine gas or sulfur dioxide gas, since cylinders of both are kept in that part of the store. The emergency response is different for the two gases, so staff need a single quick chemical test that would confirm the leaking gas is sulfur dioxide and not chlorine.
Which test and result would confirm this?
Show worked solution Hide worked solution
Worked solution
Step 1: Recall the specified test for each gas
Each gas in this scenario has one specific confirmatory test:
| Gas | Test and result |
|---|---|
| chlorine, | bleaches damp litmus paper |
| sulfur dioxide, | turns acidified aqueous potassium manganate(VII) from purple to colourless |
Step 2: Match the required test to sulfur dioxide
Sulfur dioxide is a reducing agent, so it decolourises acidified aqueous potassium manganate(VII), turning the solution from purple to colourless. Chlorine does not give this result. Chlorine’s own specified test is bleaching damp litmus paper, not decolourising potassium manganate(VII).
Since the staff need a test result that would occur for sulfur dioxide but not for chlorine, decolourising acidified aqueous potassium manganate(VII) is the test that confirms the leak is sulfur dioxide.
Why the other options are wrong
- A: bleaching damp litmus paper is the specified test result for chlorine, not sulfur dioxide. Choosing this would wrongly confirm the opposite gas.
- B: damp red litmus paper turning blue is the test for ammonia, a gas that is not part of this scenario at all.
- D: relighting a glowing splint is the specified test for oxygen, which is unrelated to either chlorine or sulfur dioxide.
Final answer
The correct test is : the gas turns acidified aqueous potassium manganate(VII) from purple to colourless, confirming sulfur dioxide.