What color does iodine solution produce if starch is present?

• A. Orange-brown
• B. Blue-black
• C. Blue
• D. Yellow

Answer: (B)

Starch has a helical structure, especially in the amylose component, that can trap iodine molecules. When iodine solution meets starch, the iodine fits inside those helical cavities and forms a starch–iodine complex. This complex changes the way light is absorbed, producing a distinctive blue-black color. If starch isn’t present, the iodine remains its usual orange-brown color without turning blue-black. So the blue-black color indicates starch is present because it results from the specific interaction between iodine and the starch helix.