Q. Which of the following carbohydrates gives a dark blue colour with iodine?

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Solution:

The iodine test is used for the detection of carbohydrates in the sample solution.
Around two drops of iodine solution is added to 2ml of the carbohydrate-containing test solution and observed for the color change.
The blue-black color is due to the formation of a starch-iodine complex. Starch is a polymer of α-amylose and amylopectin that forms a complex with iodine to produce a blue-black color. On the addition of iodine to amylose, the helical shape of the unbranched polysaccharide traps iodine to produce a deep blue-black complex.
Cellulose, amylopectin and glycogen react with iodine to produce red-brown color. Glycogen produces a reddish-purple color.
Monosaccharides are too small to trap iodine molecules and do not form dark colors with iodine.