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 presence of carbohydrates in a solution can easily be identified performing certain tests and iodine tests (in which iodine solution is used) is shown by a specific type of carbohydrate. For example, when iodine solution is added to a raw potato, it turns blue-black colour due to reaction between the polysaccharide present in potato and iodine.

Complete step by step answer:
As per the given question, when the following carbohydrates react with iodine solution it will result to,
- With glucose, iodine will not react because glucose is a monosaccharide and iodine tend to react with only polysaccharides having long chains of monosaccharides.
$\text{Glucose}+{{I}_{3}}^{-}\to No Reaction$

- Similarly, with sorbose iodine will not show any reaction because it is a monosaccharide.
$\text{Sorbose}+{{I}_{3}}^{-}\to No Reaction$

- Whereas with starch, starch develops complex blue colour on treatment with iodine solution due to formation of the starch-iodine complex. During the reaction, the iodine (in the form of ${{I}_{3}}^{-}$ ions) gets stuck in the coils of beta amylose molecules (beta amylose is a soluble starch having a long chain of monosaccharides). So, the starch directs the iodine atoms into a linear arrangement in the central groove of the coil forming an iodine-starch complex giving blue colour during the reaction. On heating, the blue colour disappears while on cooling the blue colour reappears.
$\text{Starch}+{{I}_{3}}^{-}\to \text{Starch-iodine complex(Blue)}+{{I}^{-}}$

- Same with fructose, as it is a monosaccharide iodine will not show any reaction.
$\text{Fructose}+{{I}_{3}}^{-}\to No Reaction$

So, the correct answer is “Option C”.

Additional Information:
The following are the test which can be used to identify the presence of carbohydrates:
- Molisch’s test: This test is given by almost all of the carbohydrates and it says whether carbohydrate is present or not.
- Fehling’s test: This test is only given by reducing carbohydrates.
- Benedict’s test: This test is also given by only reducing carbohydrates.
- Iodine test: This test is only given by starch which is a polysaccharide.

Note: The appearance of blue colour solution confirms the presence of starch.