What happens to iodine when starch is present?

When starch is present, iodine interacts with the helical structure of amylose, one of the components of starch, creating a complex that is visibly blue-black in color. This reaction is a classic biochemical test for the presence of starch, often used in laboratory settings to indicate the presence of this polysaccharide. The iodine molecule fits into the helical structure of the starch, which alters the way light is absorbed and reflects off the molecule, resulting in the characteristic blue-black color change. This distinctive color change serves as a reliable indicator for students and scientists alike, confirming the presence of starch in a sample.