How are the reagents useful in determining which macromolecules are present?

There are reagents that are specific for each macro molecules in foods. When these reagent are applied to the the food molecules, color changes occur which allow us to know the exact identification the the food macro molecule present in the food.
For instance, iodine is a reagent, that will give blue-black color when applied to starch.

Generally travels west to east but often shifts to north and south

Wikipedia aswell can be used to solve the following questions