Which of the following gives a positive test with ninhydrin ?

Solution

Chemistry ma’am · 3 steps

Ninhydrin looks for free amino groups, so ask which of these four is a protein.

1. What ninhydrin detects. Ninhydrin reacts with the free -NH₂ group of an α-amino acid. When warmed, it gives a deep blue-violet colour, called Ruhemann's purple. Proteins and peptides give the test too, because their end amino groups and side-chain amino groups react the same way.

   Substance	What it is made of	Free amino group?
   Polyvinyl chloride	Synthetic polymer of CH₂=CHCl, only C, H and Cl	No
   Cellulose	Polysaccharide of β-D-glucose	No
   Egg albumin	Protein, a chain of amino acids	Yes
   Starch	Polysaccharide of α-D-glucose	No

2. No nitrogen, no colour. Cellulose and starch contain only C, H and O. PVC contains only C, H and Cl. None of them has an amino group, so ninhydrin has nothing to react with.
3. So. Only egg albumin, a protein, turns purple with ninhydrin. The answer is (c).

The idea to keep. Ninhydrin is the test for amino acids and proteins: a free amino group gives purple. Carbohydrates and nitrogen-free polymers never give it.

Answer from the NTA final answer key. Solution by PrepWiser.