Water plus Benedict's reagent is a negative control for the sugar test. It demonstrates a negative test result (no sugar present).
Glucose plus Benedict's reagent is a positive control for the sugar test. It demonstrates what a strong positive result should look like. It also proves that our reagents haven't gone bad (they are capable of producing a positive result).
The point of controls is two fold. They give you standards to compare against, and they demonstrate that your reagents are working correctly. Benedict's solution is used to test for simple sugars, such as glucose. It is a clear blue solution of sodium and copper salts. In the presence of simple sugars, the blue solution changes color to green, yellow, and brick-red, depending on the amount of sugar. Benedict's reagent starts out aqua-blue. As it is heated in the presence of reducing sugars, it turns yellow to orange. The "hotter" the final color of the reagent, the higher the concentration of reducing sugar. In general, blue to blue-green or yellow-green is negative, yellowish to bright yellow is a moderate positive, and bright orange is a very strong positive.
