Why does starch turn blue black with iodine?

Hint: On interaction with starch, the triiodide anion immediately creates a vivid blue-black hue. With rising temperature and with the existence of water-miscible organic solvents i.e. ethanol, the strength of the colour reduces.

Complete Answer:
- Plants reserve glucose as the starch of the polysaccharide; starch is also rich in cereal grains and also tubers like potatoes. It is possible to divide starch into two fractions — amylopectin and amylose.
- Blends of amylose (10-20 percent) and amylopectin (80-90 percent) are regular starches. A very small part of the amylose chain reveals amylose. Most monomer units are alpha-D-glucose, and C1 of one glucose and C4 of the next glucose are linked by all the alpha acetal connexions. In fact, amylose creates a spiral much like a coiled spring because of the bond angles in the alpha acetal bond.
- In the presence of iodine, amylose present in starch is responsible for producing a deep blue tint. Within the amylose coil, the iodine molecule slips within. Iodine-KI Reagent: Iodine does not completely soluble in liquids, so the iodine reagent is formulated in the presence of potassium iodide by breaking down iodine in liquids. This creates the complex of a linear triiodide ion soluble, which slides into the starch coil, creating a vivid blue-black appearance.

Note: In hot water, amylose develops a colloidal dispersion, while amylopectin is totally insoluble. The amylose structure involves a long glucose unit polymer chain bound via an alpha acetal linkage.