Final answer:
The reaction of iodine with starch, resulting in a color change to blue or blue-violet, serves as a visual indication of the presence of iodine or the progression of reactions involving iodine.
Step-by-step explanation:
The reaction of iodine with starch is used as a sensitive test to indicate the presence or absence of iodine. When iodine (I2) reacts with the amylose component in starch, a deep blue or blue-violet color is produced. This coloration is due to the formation of a complex between the iodine and the helical structure of amylose.
A color change from colorless to blue indicates the presence of excess iodine in a reaction (as a titrant), while a change from blue to colorless signifies the completion of a reaction consuming iodine (as the titrand).
For example, when observing a reaction between dichromate ions (Cr2O72-) and iodide ions (I-), iodine is produced, which can then react with starch to form that characteristic blue-violet complex.
Similarly, the detection of a deep blue color in a mixture of starch and iodide ion in water indicates the presence of trace amounts of iodine, showcasing how the color change serves as a visual clue to the reaction's progress or the analytes' presence.