A Level Biology Practice Exam

What does a positive result with Benedict's reagent indicate?

1. The presence of proteins

2. The presence of reducing sugars

3. The presence of complex carbohydrates

4. The presence of lipids

The correct answer is: The presence of reducing sugars

A positive result with Benedict's reagent indeed indicates the presence of reducing sugars, which are sugars that can donate electrons to other molecules. This test is based on the ability of reducing sugars, such as glucose and fructose, to reduce copper(II) ions in the reagent to copper(I) oxide, which forms a brick-red precipitate when heated. The intensity of the color change in the solution can be indicative of the concentration of reducing sugars present. The other options do not correspond to the action of Benedict's reagent. Proteins would typically require a different test, like the Biuret test, to identify their presence. Complex carbohydrates, such as starch, are tested using iodine, and they do not yield a positive result with Benedict's. Lipids are assessed through tests like the emulsion test, which involves mixing the sample with ethanol and water, leading to a cloudy emulsion if lipids are present.