A sample of a gas may be identified as chlorine if it turns

Analyze the question: You are asked which test identifies chlorine gas, Cl₂.

Concept: Chlorine is an oxidizing agent and a bleaching agent. Its identifying tests are:

• It bleaches damp litmus paper. Blue litmus first turns red (because chlorine in water forms HCl and HOCl) and is then bleached white.
• It turns damp starch-iodide paper blue-black, because it displaces iodine from iodide.

Step-by-step check of the options:

A. Damp blue litmus paper red. Any acidic gas (CO₂, SO₂, HCl) does this. Chlorine does turn it red briefly, but then bleaches it white. This test is not specific to chlorine.

B. Lime water milky. This is the test for carbon dioxide, forming insoluble CaCO₃: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O So this is not chlorine.

C. Lead ethanoate paper black. This is the test for hydrogen sulphide, forming black lead(II) sulphide (PbS). So this is not chlorine.

D. Starch-iodide paper blue-black. Chlorine is a stronger oxidizing agent than iodine, so it displaces iodine from iodide: Cl₂ + 2I⁻ → 2Cl⁻ + I₂ The liberated iodine forms a blue-black complex with starch. This is a good test for chlorine, since it works through oxidation.

Answer: D.

Tip: Match each gas to its test:

• CO₂ → lime water turns milky
• H₂S → lead ethanoate paper turns black
• Cl₂ → bleaches litmus and turns starch-iodide paper blue-black
• NH₃ → turns damp red litmus blue