Question

A few drops of iodine solution were added to rice water. The solution turned blue-black. What does this indicate?

Answer 1

Hint: These are substances which will change the colour when it is added to some solution. This phenomenon is based on the change in pH. Indicators are acid-base substances. They are weak bases or acids. Their conjugate acid or base will have different colours because of the differences in their absorption spectra.

Complete answer: The iodine-starch test was first described by J. J. Colin and H. F. Gaultier de Claubry, and independently by F. Stromeyer in 1814. Starch has amylose which is responsible for the formation of deep blue-black colour when iodine is present. In the amylose coil, the iodine molecule will slip in. Iodine is not soluble in water, so the iodine reagent has to be made by dissolving iodine in water containing potassium iodide. Now, a linear triiodide ion complex and a very dark blue-black colour are formed. In some of the inorganic compounds, iodine can be used as an indicator that will lead to a change of iodide ion to iodide and iodine element. When the starch solution is added only with the iodine element, in the presence of iodide ion, it will give us the characteristic blue-black colour.

Note: All plants store glucose as the polysaccharide; All cereal grains such as wheat, rice, oats, corn, etc and as well as tubers like potatoes are very much rich in starch. Starch is made up of two units- amylopectin and amylose. Naturally occurring starches are a mixture of amylose (10 to 20 per cent) and amylopectin (80-90 per cent).