Question

Explain why chlorine turns moist starch iodide paper blue -black

Answer 1

Chlorine turns moist starch iodide paper blue-black because the iodide ions on the paper are oxidised to iodine and the starch react to it to give blue black compound.
Cl₂ + 2I^- $\to$ I₂ + 2Cl^-
I₂ + starch $\to$ Blue Black compound.