Question

Which of the following carbohydrates develop blue colour on treatment with iodine solution?

• A. Glucose
• B. Sorbose
• C. Starch
• D. Fructose

Answer 1

The correct option is C Starch

Starch develops blue colour on treatment with iodine solution due to the formation of the starch-iodine complex. Iodine (in the form of $I_5^{-}$ ions) gets stuck in the coils of beta amylose molecules (beta amylose is a soluble starch).

The starch forces the iodine atoms into a linear arrangement in the central groove of the amylose coil. There is some transfer of charge between the starch and the iodine that changes the way electrons are confined and so, changes the spacing of the energy levels.

The iodine-starch complex has energy level spacing that is suitable for absorbing visible light giving the complex its intense blue color.