Q.74 Chemistry · Section A ⭐ 360° Option Elimination Active
NEET 2024 Chemistry Question 74 Solution
Topic: Aldehydes, Ketones and Carboxylic Acids · Official NTA Booklet Code T1
Question
Fehling's solution 'A' is
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Verified Against NTA Final Answer Key
Option (3) : aqueous copper sulphate
📚 Aldehydes, Ketones and Carboxylic Acids Easy
📋 Given Parameters & Problem Setup
Reagent Fehling’s solution ‘A’
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⏱️ Save ~25 seconds The 30-Second Exam Hall Shortcut: ⚡ Fehling A & B Direct Definition
Fehling A = Aqueous Copper Sulphate (CuSO4)! Fehling B = Alkaline Sodium Potassium Tartrate (Rochelle’s salt)!
1 Fehling’s reagent is supplied as two separate stable stock solutions mixed in equal volumes prior to testing.
2 Fehling’s solution A is an aqueous solution of copper(II) sulphate (blue, CuSO4·5H2O).
3 Fehling’s solution B is an alkaline solution of sodium potassium tartrate (Rochelle’s salt + NaOH).
4 The question specifically asks for Fehling’s solution ‘A’ -> aqueous copper sulphate.
5 Option (3) in 2 seconds!
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Why You Can 100% Trust This Shortcut: NCERT Chemistry · Aldehydes, Ketones and Carboxylic Acids (Chemical tests to distinguish aldehydes: Fehling’s test).
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Step-by-Step Rigorous Workflow & Mathematical Proof
Step 1
Definition and Composition of Fehling’s Reagents
Fehling’s test is a standard diagnostic test used to detect aliphatic aldehydes and reducing sugars. The reagent comprises two separate solutions: - Fehling’s solution A: Aqueous copper(II) sulphate (CuSO4 · 5H2O), imparting a bright blue colour. - Fehling’s solution B: An alkaline solution containing sodium potassium tartrate (Rochelle’s salt, KOOC-CH(OH)-CH(OH)-COONa) and sodium hydroxide (NaOH).
Step 2
Role of Tartrate in Fehling’s Solution
When solutions A and B are mixed just prior to use, tartrate acts as a bidentate/chelating ligand for Cu²⁺ ions. This chelation prevents the premature precipitation of copper(II) hydroxide (Cu(OH)2) in strongly alkaline medium:
Step 3
Redox Reaction with Aldehydes
Upon heating with an aliphatic aldehyde, Cu²⁺ is reduced to Cu⁺, forming a characteristic red-brown precipitate of copper(I) oxide (Cu2O):
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4-Option Deep Scan
360° Option Elimination Engine Key Feature
Diagnose every choice: Know exactly why the right option works and why distractors fail.
1 (1)
✗ Incorrect Verdict & Scientific Reason:
This is the exact definition of Fehling’s solution B, not A.
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Student Fallacy (Why candidates pick this trap): Swapping solution A with solution B.
2 (2)
✗ Incorrect Verdict & Scientific Reason:
Citrate is the complexing agent in Benedict’s reagent, not Fehling’s.
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Student Fallacy (Why candidates pick this trap): Confusing Fehling’s reagent with Benedict’s reagent.
3 (3)
✓ 100% Correct Verdict & Scientific Reason:
Verbatim NCERT definition of Fehling’s solution A.
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Why this is airtight: None.
4 (4)
✗ Incorrect Verdict & Scientific Reason:
Copper sulphate alone in alkali precipitates immediately as Cu(OH)2; alkali is in solution B with tartrate.
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Student Fallacy (Why candidates pick this trap): Assuming copper sulphate is alkaline in bottle A.
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NTA Trap Alert: Swapping Fehling A and Fehling B
Option (1) describes Fehling B (Rochelle’s salt). Solution A is always the simple blue copper sulphate solution.
🛡️ How to avoid the -1 mark: A is Aqua/Blue (CuSO4); B is Basic/Buffer (Rochelle’s salt + Base).
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Core NEET Syllabus Standard
Chemistry Core Organic Chemistry · Aldehydes, Ketones and Carboxylic Acids Standard Benchmark
Core Scientific Principle Tested:
"Fehling’s solution A is aqueous copper sulphate; Fehling’s solution B is alkaline sodium potassium tartrate."
Syllabus Focus: Oxidation Reactions of Aldehydes: Fehling’s Test
✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗
A is Aqueous Copper blue; B is Basic Rochelle through!
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