What color change indicates a positive test for starch using iodine?

A positive test for starch using iodine is indicated by a color change from blue to black. This reaction occurs because iodine interacts with starch molecules, particularly amylose, forming a complex that reflects a specific color. The blue-black color arises due to the helical structure of the amylose molecules, which trap the iodine molecules and give rise to this characteristic color change.

When starch is present, the iodine solution darkens as it binds to the starch, confirming its presence in the solution or on the surface being tested. This specific color change is a widely accepted indicator in laboratory settings for determining starch content in various substances, such as food items or biological samples.