A Level Biology Practice Exam

What is indicated by a brick-red precipitate when using Benedict's reagent?

1. The absence of reducing sugars

2. A low pH in the solution

3. High concentration of reducing sugars

4. The presence of starch

The correct answer is: High concentration of reducing sugars

A brick-red precipitate formed when using Benedict's reagent is indicative of a high concentration of reducing sugars present in the solution. Benedict's reagent is a qualitative test used to determine the presence of reducing sugars, which are sugars capable of donating electrons to reduce copper (II) ions in the reagent to copper (I) oxide. When reducing sugars are present, the solution undergoes a color change, moving from blue (the color of the original reagent) to green, yellow, orange, and finally brick-red as the concentration of reducing sugars increases. The brick-red precipitate, in particular, suggests a significant amount of these sugars, thus reflecting a high concentration. This reaction is absent in substances like starch, which does not react with Benedict's reagent in the same manner, as starch is a polysaccharide and does not have the same reducing properties as monosaccharides or some disaccharides. The presence of a low pH is not directly indicated by the color change in the Benedict's test, nor is it linked to the absence of reducing sugars.