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. NaNO${}_3$

Answer 1

The correct option is A NaI

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

$I_2$ 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.