Question

Why $KI-starch$ solution become blue on adding water to chlorine?

Answer 1

In halide displacement reaction, more electronegative (EN) halogen $(Cl>Br>I)$ tends to displace a lesser EN halide from its solution. Therefore, more EN halogen acts as oxidising agent and forms halide molecule. In case of $KI$-starch solution, $C{l}_2$ oxidises $I^{-}$ and liberates $I_2$ which imparts blue colour to the solution.