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Chemistry Question 55 – NEET-UG 2026

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The amino acid that gives a red-blood colour on treating its sodium fusion extract with sodium nitroprusside is

This question tests your knowledge of qualitative tests for elements present in organic compounds.

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Ninja Strategy•Sulfur Content Recognition

Quickly identify the amino acid among the options that contains sulfur, as the sodium nitroprusside test is specific for sulfur.

Step 1: Understand the Sodium Nitroprusside Test✦ Active

The sodium nitroprusside test is a qualitative test used to detect the presence of sulfur in an organic compound. In this test, the organic compound is first fused with sodium metal to convert any sulfur present into sodium sulfide (Na2S). The sodium sulfide then reacts with sodium nitroprusside (Na2[Fe(CN)5NO]) to form a violet or red-blood coloured complex, Na4[Fe(CN)5NOS].

Step 2: Identify Sulfur-containing Amino Acids○ Expand

To give a positive sodium nitroprusside test, the amino acid must contain sulfur. Let's examine the given options:

1. Leucine: Does not contain sulfur.

2. Threonine: Does not contain sulfur.

3. Methionine: Contains sulfur (specifically, a thioether group, −CH2CH2SCH3 in its side chain).

4. Serine: Does not contain sulfur.

💡 Teacher's Secret Hint

Recall the structures of common amino acids to identify those with sulfur.

Step 3: Conclude the Result○ Expand

Since methionine is the only amino acid among the choices that contains sulfur, its sodium fusion extract will form sodium sulfide, which will then react with sodium nitroprusside to give a red-blood colour.

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