Concept:Chlorine is a strong oxidizing agent. It liberates iodine from iodide salts, and the free iodine gives a blue-black colour with starch.
Explanation:Damp starch iodide paper contains potassium iodide and starch.
When chlorine gas
Cl2 comes into contact with the paper, it oxidizes iodide ions
I− to iodine
I2.
The liberated iodine forms a deep blue-black complex with starch.
Thus, a white starch iodide paper turning blue-black is a reliable test for chlorine.
The other tests point to different gases.
Damp blue litmus turning red indicates an acidic gas, not chlorine.
Lime water turning milky indicates carbon dioxide.
Lead ethanoate paper turning black indicates hydrogen sulphide.
Answer:The correct option is D: starch iodide paper blue-black.