What is a test for iodine on starch?

Answer 1

The Iodine test is used to test for the presence of starch. Iodine solution — iodine dissolved in an aqueous solution of potassium iodide — reacts with starch producing a deep blue-black color.

This reaction is the result of the formation of polyiodide chains from the reaction of starch and iodine. The amylose, or straight chain portion of starch, forms helices where iodine molecules assemble, forming a dark blue/black color.