Question

Which of the following gives a positive Fehling's solution test?

• A. Glucose
• B. Sucrose
• C. Fat
• D. Protein

Hint:

Remember: All monosaccharides like glucose and fructose (except those without free carbonyl groups) give a positive Fehling's test, while disaccharides like sucrose usually do not.

Answer 1

The Correct Option is A

Step 1: Recall what Fehling's test detects.
Fehling's solution test is used to detect reducing sugars. Reducing sugars have a free aldehyde group (\(-CHO\)) or a free ketone group (\(-C=O\)) that can be oxidized. On heating with Fehling's solution, such sugars reduce Cu$^{2+}$ ions to Cu$_2$O, producing a red precipitate.

Step 2: Analyze each option.
- \( \text{(A) Glucose:} \) Glucose is an aldohexose and a reducing sugar. It contains a free aldehyde group that can reduce Fehling's solution, giving a positive test (red precipitate of Cu$_2$O).
- \( \text{(B) Sucrose:} \) Sucrose is a non-reducing sugar because the glycosidic bond involves the reducing groups of both glucose and fructose units, so it does not give a positive Fehling's test.
- \( \text{(C) Fat:} \) Fats are esters of glycerol and fatty acids; they do not have free aldehyde or ketone groups, so they do not respond to Fehling's test.
- \( \text{(D) Protein:} \) Proteins contain amino acids linked by peptide bonds; they do not act as reducing sugars, hence no reaction with Fehling's solution.

Step 3: Final Answer.
Only glucose gives a positive Fehling's solution test.
\[ \boxed{\text{Correct Answer: Glucose (A)}} \]