Question

Which of the following gives a positive test with ninhydrin ?

• A. Cellulose
• B. Starch
• C. Polyvinyl chloride
• D. Egg albumin

Answer 1

The Correct Option is D

Ninhydrin is a test for amino acids. Egg albumin, which contains proteins (polymers of amino acids), gives a positive test with ninhydrin due to the reaction with amino groups.

Answer 2

Step 1: Recall the principle of the ninhydrin test
The ninhydrin test is a chemical test used to detect compounds containing α-amino groups (–NH₂) linked to the α-carbon of amino acids. When such compounds react with ninhydrin, they undergo oxidative deamination to produce an intense blue or purple color (Ruhemann’s purple). This reaction is characteristic of amino acids and proteins that contain free amino groups.

Step 2: Evaluate the given options conceptually
Substances that can give a positive ninhydrin test must contain:

• Free α-amino groups, or
• Peptide bonds that can release amino groups upon hydrolysis.

Step 3: Apply the concept to Egg albumin
Egg albumin is a protein, composed of many amino acids linked together by peptide bonds. When treated with ninhydrin, the peptide bonds or terminal amino groups react, liberating amino acids that produce the characteristic blue or purple coloration.

Hence, egg albumin gives a positive ninhydrin test because of the presence of amino acids and peptide linkages that contain –NH₂ groups.

Step 4: Explanation for why others (if present) may not respond
Other organic compounds without α-amino groups (like carbohydrates, fats, or hydrocarbons) do not react with ninhydrin and hence give no color change.

Final answer

Egg albumin