Question

why starch gives blue black when reacts with iodine but sugar do not
(in short)​

Answer 1

Hello there,

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, causes the dark blue/black color.

While sugar does not react with Iodine.

Hope you find my answer useful.

Harith

Answer 2

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. Amylose molecules are made up of single strands of glucose molecules shaped like springs.