The principle of detecting protein is applied to prove that a substance can produce a polypeptide, and biuret reagent is used. When we test protein, the peptide bond in protein will react with biuret reagent to produce purple, so biuret reagent can also be used to prove polypeptide. The specific design of the experiment is as follows:

1. Take the same amount of solution to be tested in two test tubes, numbered AB;

2. Add a proper amount of biuret reagent solution A to test tube A, and shake well, and add an equal amount of distilled water to test tube B, and shake well;

3. Add a proper amount of biuret reagent B into test tube A and shake well. Also, add the same amount of distilled water to test tube B and shake well;

4. Observe and record the color changes of the two test tubes.