What happens to the iodine solution if starch is present in a sample?

When starch is present in a solution, the iodine interacts with the starch molecules, resulting in a distinct color change. The iodine solution, originally browny-orange in color, will turn blue-black in the presence of starch. This reaction occurs because iodine molecules fit into the helical structure of the starch, forming a complex that reflects light in a way that produces the characteristic blue-black color. This test for starch is a standard procedure in biological experiments, demonstrating the presence of polysaccharides, and is frequently used in laboratory settings. The other options do not reflect the known chemistry of iodine with starch; for example, a clear solution would indicate the absence of starch, while yellow or browny-orange colors do not signify a positive starch test.