Question

State your observation, when moist starch iodide paper is introduced into chlorine gas.

Answer 1

• When moist starch iodide paper is introduced into chlorine gas, the paper turns blue-black.
• Chlorine oxidizes iodide ions to iodine and starch reacts with it by producing a blue-black compound.

$\underset{\mathrm{Chlorine}\mathrm{gas}}{{\mathrm{Cl}}_2\left(\mathrm{aq}\right)}+\underset{\mathrm{iodide}\mathrm{gas}}{2{\mathrm{I}}^{-}\left(\mathrm{aq}\right)}\to \underset{\mathrm{iodine}}{{\mathrm{I}}_2\left(\mathrm{aq}\right)}+\underset{\mathrm{Chloride}\mathrm{ions}}{2{\mathrm{Cl}}^{-}\left(\mathrm{aq}\right)}$

• The ${\mathrm{I}}_2$ formed, gets absorbed in starch to give a blue-black colour.

${\mathrm{I}}_2\left(\mathrm{aq}\right)+\mathrm{starch}\to \mathrm{blue}\mathrm{black}\mathrm{colour}$

Therefore, when moist starch iodide paper is introduced into chlorine gas, the moist starch iodide paper turns blue-black.