In a solution, iodine ions indicate the presence of starch by changing the color of the solution from yellow to intense blue. Iodine ions are typically added to a solution as potassium iodide.
A solution of iodine is yellow to dark brown, depending on the concentration, while a solution of starch is colorless. Starch contains the protein amylose. Iodine ions slip into the coiled shape of the amylose protein. The presence of the ions alters the shape of the amylose, which changes the energy levels of electrons. The new energy levels reflect dark blue light waves, causing the color change.