Question

A substance $X$ when heated with conc. $H_{2}S{O}_4$ liberates a gas which turns starch paper blue. The substance $X$ is

• A. $NaI$
• B. $NaBr$
• C. $NaCl$
• D. $NaN{O}_3$

Answer 1

Answer: (A)

$8NaI+9H_{2}S{O}_4\to 4I_2+H_{2}S+8NaHS{O}_4+4H_{2}O$
$I_2$ 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.