JAMB Chemistry 2015 Paper

Concept:Chlorine is an oxidising agent that liberates iodine from potassium iodide, and iodine forms a blue-black complex with starch.Explanation:Damp starch-iodide paper is impregnated with potassium iodide ($\mathrm{KI}$) and starch.When chlorine gas ($\mathrm{Cl}_2$) comes into contact with the damp paper, it oxidises iodide ions ($\mathrm{I}^-$) to iodine ($\mathrm{I}_2$).The reaction is: $\mathrm{Cl}_2 + 2\mathrm{KI} \rightarrow 2\mathrm{KCl} + \mathrm{I}_2$.The liberated iodine reacts with starch to give an intense dark blue or blue-black colour.This test is used to detect oxidising gases like chlorine.Therefore, damp starch-iodide paper turns dark blue in chlorine gas.Answer:D. dark blue