Question

The iodine test is shown by: -
(a)- Glucose
(b)- Starch
(c)- Glycogen
(d)- Polypeptide

Answer 1

Hint: Iodine test is used to detect carbohydrates in the compound or material. This reaction gives an intense blue-black color. The triiodide produces an intense blue-black color.

Complete answer:
The presence of starch or iodine in the material is detected by the iodine-starch test. When starch and iodine are mixed it gives an intense blue-black color. Iodometry is the term used to define the interaction between starch and triiodide.
When the triiodide anion (${I_3}^{-}$ ) comes in contact with the starch it instantly produces an intense blue-black color. When the temperature is increased or the presence of water-miscible organic solvents such as ethanol decreases the intensity of the color. This test has to be done at a normal pH. At very low pH the starch undergoes hydrolysis and this test cannot be performed.
In the iodine test, the starch is used as a self indicator because of the formation of intense blue-black color with triiodide. Starch is also used as an indicator in many redox titrations where triiodide is present. If there is the formation of only iodine ion or only iodide ($I^{-}$ ) the complex will not be formed and the test will not be performed. The intensity of the color of the starch complex is so deep that even the concentration of iodine is as low as 0.00002 M can be detected by naked eyes.
So, the correct answer is “Option B”.

Note: Don’t get confused between glucose and starch. Concentrated iodine must be treated with some reactant like thiosulfate to remove most of the iodine before the addition of starch to the titration.