Question

how can iodine change Color of starch in bulish black

Answer 1

when iodine reacts with starch in food it converts it into Violet or black.
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Answer 2

Starch is a polysaccharide consisting of two kind of molecules : amylose and amylopectin. Amylose in starch is responsible for the formation of a deep blue color in the presence of iodine.

In the Iodine test , Iodine dissolved in aqueous solution of Potassium Iodide (KI) reacts with starch producing a bluish colour.

Charge transfer between Iodine and Starch changes the energy level spacings in the Iodine/Starch complex that are just so for absorbing the visible light giving the complex its intense blue colour