- 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.