The test for starch uses iodine dissolved in potassium iodide solution, which is orange-brown.A few drops of the iodine solution are added to the sample. Solid samples, such as a slice of potato, can be tested directly, or the sample can be ground up with water first.If starch is present the colour changes from orange-brown to blue-black. If there is no starch the solution stays orange-brown.The blue-black colour forms because iodine (as triiodide ions) fits inside the helical coils of amylose, forming a coloured complex.The test is qualitative: it shows whether starch is present but not how much. It can be used to follow the digestion of starch by amylase, because the blue-black colour no longer appears once all the starch has been hydrolysed to maltose.