To obtain a positive test for proteins, the student must add a few drops of biuret reagent to the food they would like to test. If the food turns purple, there is protein present. The independent variable is the food you are testing; the control variable is the amount of biuret, the temperature and the time; and the dependent variable is what colour the food turns out to be. The student should repeat this test at least once more and keep the variables the same for every time they perform this experiment so they can achieve a reliable and accurate result. To obtain a positive test for starch, the student must add a few drops of iodine solution to the food they would like to test. If the food turns black, there is starch present. The independent variable is the food you are testing; the control variable is the amount of iodine, the temperature and the time; and the dependent variable is what colour the food turns out to be. The student should repeat this test at least once more and keep the variables the same for every time they perform this experiment so they can achieve a reliable and accurate result. To obtain a positive test for simple sugars, the student must add a few drops of Benedict's solution to the food they would like to test. If the food turns to green, yellow or red, there are varying amounts of simple sugars present. The independent variable is the food you are testing; the control variable is the amount of Benedict's, the temperature and the time; and the dependent variable is what colour the food turns out to be. The student should repeat this test at least once more and keep the variables the same for every time they perform this experiment so they can achieve a reliable and accurate result.