A positive reaction in an iodine test is indicated by a blue-black coloration of the sample being tested. This reaction typically occurs when starch is present.
The iodine test is commonly used to detect starch in various substances. When iodine solution, which is typically brown in color, comes into contact with starch, a complex is formed that results in the characteristic blue-black color. This change occurs because iodine molecules fit into the helical structure of the amylose in starch, leading to this distinctive coloration.
Conversely, if no starch is present, the solution will retain its brown color, indicating a negative reaction. Thus, the color change is a clear and simple way to test for the presence of starch in a sample.