Larrisa McGarry, Caroline Voysey, Jordyn Neale and Alexandra Partridge, of Paraparaumu Beach School, asks :-

When we placed some iodine on pieces of banana, potato, apple, pear, onion, bread and potato crisps potato, bread and crisps went black but the others stayed the same. Why did this happen?

Richard Hartshorn, an organic chemist at the University of Canterbury, responded.

The black colour is seen when starch and iodine are mixed. Starch is usually found as granules in the roots, tubers, and seeds of plants, and in materials and foods made from them.

Starch consists of very large molecules that are made up of lots of glucose (sugar) units. Large molecules that are made up of lots of one kind of small one joined together are called polymers, and in starch there are two kinds of polymer, amylose and amylopectin. Amylose is made up of long linear chains, while amylopectin has lots of branches. It turns out that the amylose polymer chains like to fold up to form helical shapes, leaving a space in the middle that is just the right size for linear molecules like those of iodine (I2) to fit in to. In the presence of starch, a small amount of iodine reacts to form iodide ions, and then two iodine molecules and an iodide ion combine to make an I5- ion. It is this linear molecule bound in the amylose helix that gives the blue-black colour that you saw.

The colour is very intense, so iodine can be used as a very sensitive test for the presence of starch, or starch can be used as a test for the presence of iodine.