Which of the following gives positive Fehling’s solution test?

• (E) Sucrose
• (F) Glucose
• (G) Fats
• (H) Protein

To determine which of the substances gives a positive Fehling’s solution test, we first need to understand what Fehling’s solution is and how it works. Fehling’s solution is a chemical reagent used primarily to test for the presence of reducing sugars, which can donate electrons to other molecules. This test is particularly useful in identifying monosaccharides and some disaccharides.

Understanding Fehling's Solution

Fehling’s solution consists of two components: Fehling's A, which contains copper(II) sulfate, and Fehling's B, which contains a mixture of sodium potassium tartrate and sodium hydroxide. When mixed, the solution forms a deep blue color due to the presence of copper(II) ions.

How the Test Works

When a reducing sugar is present and heated with Fehling’s solution, it reduces the copper(II) ions to copper(I) oxide, which precipitates out of the solution as a red or brick-red solid. This color change is a clear indicator of a positive test.

Analyzing the Options

• Sucrose: This is a disaccharide composed of glucose and fructose. However, sucrose is a non-reducing sugar because it does not have a free aldehyde or ketone group available to participate in the redox reaction. Therefore, it will not give a positive Fehling’s test.
• Glucose: This is a monosaccharide and a well-known reducing sugar. It has a free aldehyde group that can easily reduce copper(II) ions in Fehling’s solution, resulting in a positive test. Thus, glucose will yield a red precipitate.
• Fats: These are lipids and do not contain the functional groups necessary to act as reducing agents. Therefore, fats will not give a positive Fehling’s test.
• Protein: Proteins are made up of amino acids and do not typically participate in the redox reactions that are characteristic of reducing sugars. As such, proteins will also not yield a positive result in this test.

Conclusion

From the analysis above, the only substance that gives a positive Fehling’s solution test is glucose. It’s important to remember that the test is specific for reducing sugars, which include certain monosaccharides and some disaccharides, but not all carbohydrates or biomolecules.