When starch reacts with iodine, why does it create the
characteristic blue-black colour?
Answer Posted / gurudut
Light is reflected or absorbed according to the shape and
form of the molecular complex it enters.
It so happens that the amylose helix with iodine ions in
the helical core is totally absorptive of all the longer
wavelengths of visible light. Colour is only reflection of
particular wavelengths.
The starch iodine test was selected for the entirely
fortuitous reason that it is so diagnostic for chance
reasons alone.
Evidence of the structural nature of the colour lies in
showing that a heated solution of starch-iodine complex
loses its colour at about 70?C and regains it again on
cooling i.e. the amylose denatures and renatures again.
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