Question

The reaction of the starch in the potato with the iodine solution gives:
A. Violet colour
B. Reddish colour
C. Bluish black colour
D. Yellowish colour

Answer 1

Think about the colour of the complex that is formed when the iodine solution is added to starch that may be present in potatoes or any other substance. Consider the part of starch that contributes to the formation of this complex.

Complete answer:
When iodine is added to any substance that contains starch, it has a linear triiodide complex. This complex has a deep blue colour that is imparted to the substance that contains the starch when the reaction is completed. Iodine is used as an indicator in many redox titrations that involve a starch. The triiodide complex immediately combines with the amylose part of the starch molecule to form a complex that will impart the deep blue colour to the substance.
This triiodide complex is what keeps the colour of the iodide indicator blue. This test does not show the presence of starch at extremely low pH levels since the amylose part of the starch will undergo hydrolysis and dissociate. So, the triiodide anion will not be able to form a complex with the amylose.
Here, the iodine indicator reacts with the starch in the potato which has a relatively neutral pH, so the amylose will not undergo hydrolysis and when the iodine reacts with the starch, it will give a deep blue or bluish black colour.

Hence the correct answer to this question is ‘C. Bluish black colour’.

Note:
When the reaction occurs, the linear triiodide molecule is encased in a helix which is made up of the amylose sugar of the starch molecule. It is this helical molecule that gives the bluish black colour to the substance.