d) Starch


  • A solution of iodine and potassium is added to a starch-containing substance instead of molecular iodine because molecular iodine is not soluble in water.
  • Starch is a polymer of amylose and amylopectin, of which amylose is responsible for the blue colour of the reaction.
  • The reaction proceeds as follows:-

I2 + I→ I3– + starch (dark blue colour)

  • In the absence of starch solution, remains as light brown colour which is the colour of iodine solution.

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