Why does iodine solution turn blue-black?

Iodine solution turns blue-black because iodine molecules fit into the spiral structure of amylose, one of the main components of starch. This interaction forms a starch-iodine complex that produces the characteristic blue-black colour.

The colour change occurs only when starch is present, making it a reliable indicator in the iodine test. If a food sample does not contain starch, the iodine solution remains brown or yellow-brown, showing that no starch-iodine complex has formed.