Question

which of the following foods turn blue-black with iodine solution?
A.Milk
B.Groundnut
C.Tomato
D.Raw Potato

Answer 1

Hint:Any substance that contains starch turns blue-black in presence of iodine solution. Iodine solution is also used as a confirmatory test in the detection of starch.

Complete step by step answer:
Starch is a type of carbohydrate found mainly in plant and plant products. A solution of iodine and potassium iodide which is pale yellow in colour is added to a starch containing substance and hence the substance turns deep blue. The reaction proceeds as follows:
\[{{\text{I}}_{\text{2}}}\text{ + }{{\text{I}}^{\text{-}}}\to {{\text{I}}_{\text{3}}}^{\text{-}}\text{ + starch}\to \text{Dark - Blue Complex}\]
Among the above foods, milk does not contain starch, it is mostly water, protein, lactose, and fats, so definitely it will not turn starch blue-black. Coming to groundnuts, it is mainly proteins and fats, and very less carbohydrates, so no chance of blue-black colour formation. Coming to tomatoes, it has mostly water, and different vitamins but no starch so no blue black colour. Lastly, we have raw potato which contains starch and hence will react with the iodine solution turning blue-black itself.

So the correct answer is option D, raw potato.

Note:
The other food items that are laden with starch include rice, cassava, wheat, tapioca, corn, etc. these foods will also react with the iodine solution to give a blue-black precipitate of amylose-iodine complex. The Amylose is the molecule present in Starch is the main reason behind this reaction. This amylose entraps the iodide complex in its structure and thus the colour is caused.