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? A)From brown to blue–black B)From brown to orange–red C)From blue to pink D)From pink to blue–green
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.
