Final answer:
When iodine is added to a food sample containing starch, it turns blue-black due to the formation of the amylose-iodine complex.
Step-by-step explanation:
When iodine is added to a food sample containing starch, the result is that it turns blue-black. This is due to the formation of a complex between starch and iodine called the amylose-iodine complex. Starch, a complex carbohydrate, is made up of two polymers called amylose and amylopectin. The characteristic blue-black color that appears when starch reacts with iodine is a result of the amylose-iodine complex forming.