Question

46-) When amylose is suspended in water in the presence of I₂, a blue color results due to the ability of iodine to occupy the interior of the helix. A drop of yellow iodine solution on a potato slice turns blue, but when a drop of iodine is placed on an apple slice, the color of the solution remains yellow. Explain why.

Answer 1

Starch in natural substances can be categorised as amylose and amylopectin. Amylose can form a colloidal suspension in hot water whereas amylopectin cannot dissolve in water. When a solution of iodine in potassium iodide reacts with a substance containing amylose, the triiodide ion formed due to reaction between iodine and potassium iodide, enters the helix of the amylose resulting in a dark blue-black complex. A drop of the yellow iodine solution in potassium iodide reacts with amylose present in potato and gives the dark blue-black colour. On the other hand apple doesn't have amylose, so it doesn't give the dark blue-black complex with iodine solution and the colour of the iodine solution remains yellow.