Final answer:
If a Benedict's test is performed on an unknown substance and no color change occurs, it can be concluded that the substance does not contain any reducing sugars.
Step-by-step explanation:
The student performed a Benedict's test on an unknown substance and added the reagent, but nothing happened. From this, we can conclude that the unknown substance does not contain any reducing sugars. Benedict's reagent is used specifically to test for the presence of reducing sugars, such as glucose. When reducing sugars are present, a color change occurs, indicating a positive result.