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A potato tuber has been cut into two halves. A few drops of iodine solution are placed on the cut surface of one of the halves. What colour change will be noticed?
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- From brown to blue–black
- From brown to orange–red
- From blue to pink
- From pink to blue–green
- From brown to blue–black
Correct Option: A
Any form of starch (carbohydrate) turns blue-black when iodine solution is applied to it. This is because starch is composed of polymers of glucose. Long linear chains are amylose. Amylopectin is similar but contains a branch point about every 25th glucose or so. Amylose coils into a helical secondary structure resembling a tube with a hollow core. Certain molecules including fatty acids and iodine can lodge inside the core as already mentioned. The complex of iodine stuck inside the amylose coil produces a characteristic blue-black colour.
