Biology – 4.1 Biological molecules | e-Consult

Iodine solution is used to test for the presence of starch. Starch is a polysaccharide composed of many glucose molecules linked together. When iodine solution is added to a sample containing starch, a distinct blue-black colour change occurs. This colour change is due to the iodine molecules fitting inside the spiral structure of the starch molecules, forming a coloured complex.

In the absence of starch, the iodine solution will remain brown. The intensity of the blue-black colour is related to the amount of starch present. A darker blue-black indicates a higher concentration of starch.