The reagent IKI tests for the presence of which substance?

• A. Protein
• B. Glycogen
• C. Glucose
• D. Starch

Answer: (D)

The test with IKI detects starch because the iodine fits into the helical structure of starch molecules, especially the long, relatively unbranched amylose chains. When iodine binds inside these helices, it forms a stable polyiodide complex that yields a deep blue-black color. This color change is a clear indicator of starch presence.

Simple sugars like glucose don’t form those helices, so they don’t produce the blue-black color with IKI. Glycogen, with many branches, disrupts the regular helical regions, so the reaction is weak or negative. Other types of tests (for proteins or reducing sugars) rely on different chemical reactions, not the iodine–starch interaction, so they wouldn’t give the same result.