Question

Iodine test used to detect
A) Protein
B) Fat
C) Carbohydrate
D) Chitin

Answer 1

Hint:Iodine test is a test to detect the presence of starch, where the sample changes into blue-black colour after addition of a few drops of potassium iodide solution in the sample. it gets attached to the helical rings of starch.

Complete answer:
The iodine Test that is done to indicate the presence of Starch is used to indicate the presence of starch in biological materials. The reaction in iodine test is due to the formation of polyiodide chains from the reaction of starch and iodine. The amylose that is present in starch forms helices where iodine molecules assemble, forming a dark blue or black colour. When starch is hydrolysed into tiny carbohydrate units, the blue-black colour is not developed. Therefore, this test can also indicate completion of hydrolysis when a colour change does not occur.

Now let us gather information about given options:
A) Protein : protein can be indicated by the Biuret test. The biuret test is a blue solution that usually reacts with the protein present in the compound and results in the change of colour to pink-purple appearance.
B) Fat : the test for detection of fat is very simple. Just take a sample of food on a piece of paper. Then leave the sample to dry. If a translucent strain appears around the food when the piece of paper is held to light it shows a positive test for fat.
C) Carbohydrate : iodine test is used for carbohydrate detection as carbohydrate containing foods consists of starch. The iodine test is used to check the availability of starch like carbohydrates in a sample. Starch changes to a deep "blue-black" colour upon addition of aqueous solutions of the triiodide anion, because of the production of an intermolecular charge transfer component (complex).
D) Chitin : The chitin can be detected SensiNG, based on detection of the amount of chitin. The material is exposed to the antifungal drugs. Chitin is then stained with the calcofluor white. Then it is visible under a microscope.

Hence the correct answer is option ‘C’ Carbohydrate.

Note:In iodine-starch test, the triiodide anion quickly produces a deep blue-black colour upon contact with starch. The deepness of the colour reduces with rising the temperature and with the presence of water-soluble organic solvents such as ethanol. The test cannot be done at very low pH, because starch is hydrolysed in this condition.