The starch-iodine test is a standard test for starch - often used by Biologists and Biochemists. It is also used by chemists testing for or titrating iodine.
In fact the iodine used is these tests is a solution of iodine in potassium iodide - sometimes written as I/KI. Under these circumstances, iodine forms a triiodide ion I
3-.
The element iodine is actually a silvery solid, which produces an impressive purple vapour when heated.
This model shows a section of amylose, the name given to linear sections of the starch molecule, consisting of α (alpha) 1,4 linked glucose units forming a single helical structure.
This has a cylindrical shape, actually somewhat tightly coiled compared with other examples on this site, There is a central space into which iodine migrates.
This amylose-iodide complex has a distinct blue-black coloration, as opposed to the orange-red colour of iodine solution (I/KI).
Within the centre of the cylinder can be seen (in purple) five copies of the iodine or triiodide ion I
3-.