Question

Which chemical is used to detect the presence of starch in a food product?
A.Hydrochloric acid
B.Iodine solution
C.Copper sulphate solution
D.None of the above

Answer 1

Hint:The most common carbohydrate included in human diet is starch. In green plants, the excess glucose produced by the plant is converted in the form of starch and is stored in different parts of a plant and serves as a reserve food supply. Starch is a polymer formed by linkage of glucose monomers.

Complete answer:
Starch is a polymeric form of glucose units formed by glycosidic linkage. They exist as polysaccharides or poly carbohydrates which is produced by most of the green plants as energy storage. The excess food produced during photosynthesis is stored in this form in various plant parts. It is the most common type of carbohydrates included in our diets, especially corn, potatoes, rice, wheat etc.
A very common chemical test to detect the presence of starch is called Iodine test. A triiodide solution which is formed by mixing iodine and iodide (mostly, potassium iodide) solution is used to test for starch, as it forms an intense blue-black coloured starch-iodine complex. In the absence of starch, the brown colour of the triiodide solution remains as such.
$I_2+I^{-}\rightleftharpoons I_3^{-}$
This complex formation upon addition of triiodide anion ( $I_3^{-}$ ) solution to starch is due to the formation of an intermolecular charge transfer complex. This interaction of starch with iodide forms the basis for iodometry.

Hence, option (B) iodine solution is the correct answer.

Note:
Starch forms intense blue-black coloured complex with triiodide solution formed by mixing of iodine and iodide solutions. It is very important to note that this complex formation does not occur if only iodine or only iodide is present in the solution.