Question

Iodine gives blue black colour with?
a. Protein
b. Oil
c. Starch
d. Sucrose

Answer 1

Hint: The reason for color formation is due to so-called charge transfer (CT) complexes. Molecular iodine $(I^2)$ is feebly soluble in water, due to which potassium iodide is added. Jointly, they form polyiodide ions of the many types, for example, $I^{3-}$, $I^{5-}$, or $I^{7-}$. The negatively charged iodide in these compounds behaves as a charge donor, the uncharged iodine as a charge acceptor.

Complete answer:
Electrons in such charge-transfer complexes are easy to excite to a better energy state by light. The light is absorbed in the process and its complementary color is observed by the human eye. Electrons in such charge-transfer complexes are easy to excite to a better energy state by light. The light is absorbed in the process and its complementary color is observed by the human eye.

The iodine test is used for the testing of the presence of starch in a sample. Molecular iodine is not soluble in water and for this reason potassium iodide is added to it. Together they form a polyiodide complex which upon the accumulation of starch turns a strong blue-black color.

Starch is a polymer of amylose and amylopectin, of which amylose is accountable for the blue color of the reaction. The nonappearance of starch solution stays as light brown color which is the color of iodine solution.

Hence, the correct answer is option (C).

Note: Proteins get react with iodine under certain conditions like a much lower pH, 3.2-3.4. Adding up iodine to the fat or oil facilitates food chemists to identify which fatty acids are present. Iodine has a dark color. This vanishes when it reacts with oil. Using fatty acids, iodine atoms adjoin to any carbon-carbon double bonds, consequently, the color disappears.