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Q.
A substance $X$ when heated with conc. $ H_{2}SO_{4} $ liberates a gas which turns starch paper blue. The substance $X$ is
Uttarkhand PMTUttarkhand PMT 2011
Solution:
$8 NaI +9 H _{2} SO _{4} \rightarrow 4 I _{2}+ H _{2} S +8 NaHSO _{4}+4 H _{2} O$
$I _{2}$ reacts with starch and gives a blue colour.
The colors are caused by so-called charge transfer (CT) complexes. Molecular iodine is not easily soluble in water, which is why potassium iodide is added. Together, they form poly iodide ions of the type $I_{n}^{-}$, for example, $I_{3}^{-}, I_{5}^{-}$or $I_{7}^{-}$.
The negatively charged iodide in these compounds acts as charge donor, the neutral iodine as a charge acceptor. Electrons in such charge-transfer complexes are easy to excite to a higher energy level by light. The light is absorbed in the process and its complementary color is observed by the human eye.