Aldehydes differ from ketones in their oxidation reactions. Aldehydes are easily oxidised to carboxylic acids on treatment with common oxidising agents like nitric acid, potassium permanganate, potassium dichromate, etc. Even mild oxidising agents, mainly Tollens’ reagent and Fehlings’ reagent also oxidise aldehydes. Ketones are generally oxidised under vigorous conditions, i.e., strong oxidising agents and at elevated temperatures. Their oxidation involves carbon-carbon bond cleavage to afford a mixture of carboxylic acids having lesser number of carbon atoms than the parent ketone.
-- NCERT Class 12 Chemistry, Ch. 8, p. 239Aldehyde Ketone Tests
8 MCQs9-step worked example
Source: NCERT Alcohols, Phenols, Ethers and Carbonyl CompoundsPYQ coverage: NEET 2024Official key: NTA-verifiedLast updated: 7 Oct 2026
Try this first
Fehling's solution 'A' is
- A.aqueous copper sulphate
- B.alkaline copper sulphate
- C.alkaline solution of sodium potassium tartrate (Rochelle's salt)
- D.aqueous sodium citrate
Tap to see the answer
Answer: A. Fehling reagent has two solutions; Fehling solution A is aqueous copper sulphate (NCERT Class 12 Chemistry, Chapter 8, page 239). This question appeared in NEET 2024.
B is wrong: B is wrong because the copper sulphate in solution A is aqueous; the alkali sits in solution B together with the tartrate.
C is wrong: C is wrong because the alkaline sodium potassium tartarate (Rochelle salt) is Fehling solution B, not A.
D is wrong: D is wrong because sodium citrate is not a component of Fehling's reagent as NCERT describes it.
Aldehyde Ketone Tests, explained for NEET
You have a colourless liquid that is either an aldehyde or a ketone, and one test tube of Tollens' reagent. Does the tube settle it?
It does, because aldehydes and ketones part ways on oxidation. Aldehydes are easily oxidised, even by mild oxidising agents, mainly Tollens' reagent and Fehling's reagent. Ketones are generally oxidised only under vigorous conditions, with strong oxidising agents at elevated temperature, and the oxidation breaks carbon-carbon bonds to give a mixture of carboxylic acids with fewer carbon atoms (NCERT Class 12 Chemistry, Chapter 8, page 239). The mild agents are the ones used to distinguish aldehydes from ketones.
Tollens' test: warm the compound with freshly prepared ammoniacal silver nitrate. An aldehyde gives a bright silver mirror, because silver metal forms and the aldehyde is oxidised to the carboxylate anion. The reaction occurs in alkaline medium (page 239).
Fehling's test: Fehling solution A is aqueous copper sulphate and solution B is alkaline sodium potassium tartarate (Rochelle salt). They are mixed in equal amounts just before the test. Heating an aldehyde with the mixture gives a reddish brown precipitate, and aromatic aldehydes do not respond (page 239).
Iodoform test (a different question): this does not separate aldehydes from ketones. Aldehydes and ketones with at least one methyl group on the carbonyl carbon are oxidised by sodium hypohalite to the sodium salt of an acid with one carbon atom less, and the methyl group becomes haloform. The iodoform reaction with sodium hypoiodite detects the CH3CO group, or the CH3CH(OH) group that gives CH3CO on oxidation (page 240).
NEET watch-out: keep the two questions apart. Silver mirror or red precipitate asks "aldehyde or not?". Iodoform asks "is there a methyl on the carbonyl carbon (or a CH3CH(OH) group)?". Fehling's solution A is the copper sulphate part; the alkaline tartrate is solution B.
How do you solve a Aldehyde Ketone Tests question? A worked example
- 1
Given
An organic compound A has molecular formula C8H8O. It forms an orange-red precipitate with 2,4-DNP reagent and a yellow precipitate on heating with iodine in the presence of sodium hydroxide. It neither reduces Tollens' nor Fehling's reagent, nor does it decolourise bromine water or Baeyer's reagent. Drastic oxidation with chromic acid gives a carboxylic acid B of formula C7H6O2.
- 2
Required
Identify A and B and explain the reactions.
- 3
Concept
This is NCERT Example 8.4 (NCERT Class 12 Chemistry, Chapter 8, page 240). A 2,4-DNP derivative means A is an aldehyde or a ketone. Tollens' and Fehling's tests separate the two, and the iodoform test checks for a methyl on the carbonyl carbon.
- 4
Formula
No numerical formula applies. The logic uses the tests: Tollens' or Fehling's positive means aldehyde, negative means ketone; iodoform positive means CH3CO group present; oxidation of a methyl ketone by hypohalite or chromic acid gives an acid with one carbon atom less.
- 5
Substitution
Apply each observation. 2,4-DNP positive: aldehyde or ketone. Tollens' and Fehling's negative: ketone. Iodoform positive: methyl ketone. No decolourisation of bromine water or Baeyer's reagent while C8H8O is highly unsaturated: the unsaturation is an aromatic ring, not a C=C bond.
- 6
Calculation
A is a methyl ketone with an aromatic ring. Build it from its pieces, C6H5 + CO + CH3: carbons 6 + 1 + 1 = 8, hydrogens 5 + 0 + 3 = 8, oxygen 1, which matches C8H8O. Oxidation removes one carbon atom: 8 minus 1 gives 7 carbons, so B is C6H5COOH (C7H6O2). No exact constants are involved in this identification.
- 7
Final answer
A is acetophenone (phenyl methyl ketone), C6H5COCH3, and B is benzoic acid, C6H5COOH.
- 8
Common trap
Treating "yellow precipitate with iodine and NaOH" as proof of an aldehyde. The iodoform test says only that a CH3CO (or CH3CH(OH)) group is present; the negative Tollens' and Fehling's tests are what show A is a ketone.
- 9
Similar NEET-style question
A liquid C3H6O gives a silver mirror with Tollens' reagent and no yellow precipitate with iodine and sodium hydroxide. Identify it. (Answer: the silver mirror shows an aldehyde, and the only aldehyde with formula C3H6O is propanal, CH3CH2CHO: 3 carbons, 6 hydrogens, 1 oxygen. Its carbonyl carbon carries no methyl group, which is consistent with the negative iodoform test; propanone is excluded because a ketone would not give a silver mirror.)
Can you answer these Aldehyde Ketone Tests MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
Fehling's solution 'A' is
Show answer and why every option is right or wrong
Answer: A. Fehling reagent has two solutions; Fehling solution A is aqueous copper sulphate (NCERT Class 12 Chemistry, Chapter 8, page 239). This question appeared in NEET 2024.
Why B is wrong: B is wrong because the copper sulphate in solution A is aqueous; the alkali sits in solution B together with the tartrate.
Why C is wrong: C is wrong because the alkaline sodium potassium tartarate (Rochelle salt) is Fehling solution B, not A.
Why D is wrong: D is wrong because sodium citrate is not a component of Fehling's reagent as NCERT describes it.
MCQ 2Easy RecallPractice
Which reagent produces a bright silver mirror on warming with an aldehyde?
Show answer and why every option is right or wrong
Answer: D. Warming an aldehyde with freshly prepared ammoniacal silver nitrate gives a bright silver mirror from silver metal (NCERT Class 12 Chemistry, Chapter 8, page 239).
Why A is wrong: A is wrong because Fehling's reagent gives a reddish brown precipitate with an aldehyde, not a silver mirror.
Why B is wrong: B is wrong because sodium hypoiodite is used to detect the CH3CO group through the iodoform reaction, not to give a mirror (page 240).
Why C is wrong: C is wrong because potassium dichromate oxidises aldehydes but is a common strong oxidising agent that also attacks ketones; no silver forms.
MCQ 3Direct ApplicationPractice
Which of these does NOT give a reddish brown precipitate when heated with Fehling's reagent?
Show answer and why every option is right or wrong
Answer: B. Aromatic aldehydes do not respond to Fehling's test, and benzaldehyde is the aromatic aldehyde here (NCERT Class 12 Chemistry, Chapter 8, page 239).
Why A is wrong: A is wrong because ethanal is an aliphatic aldehyde; heating an aldehyde with Fehling's reagent gives the reddish brown precipitate.
Why C is wrong: C is wrong because propanal is an aliphatic aldehyde and responds to the test.
Why D is wrong: D is wrong because butanal is an aliphatic aldehyde and responds to the test.
MCQ 4Easy RecallPractice
In a positive Tollens' test, what does the aldehyde become?
Show answer and why every option is right or wrong
Answer: C. The aldehyde is oxidised to the corresponding carboxylate anion while silver is deposited, in alkaline medium (NCERT Class 12 Chemistry, Chapter 8, page 239).
Why A is wrong: A is wrong because the aldehyde is oxidised, not reduced; an alcohol would be a reduction product.
Why B is wrong: B is wrong because oxidising an aldehyde does not give a ketone.
Why D is wrong: D is wrong because loss of a carbon atom belongs to ketone oxidation under vigorous conditions and to the haloform reaction, not to Tollens' test.
MCQ 5Direct ApplicationPractice
Which compound gives a positive iodoform reaction with sodium hypoiodite?
Show answer and why every option is right or wrong
Answer: D. The iodoform reaction detects the CH3CO group or the CH3CH(OH) group, which produces CH3CO on oxidation; propan-2-ol contains CH3CH(OH) (NCERT Class 12 Chemistry, Chapter 8, page 240).
Why A is wrong: A is wrong because propanal has no methyl group on the carbonyl carbon and no CH3CH(OH) group.
Why B is wrong: B is wrong because benzaldehyde has no CH3CO or CH3CH(OH) group.
Why C is wrong: C is wrong because both sides of the carbonyl in pentan-3-one are CH2 groups, so there is no methyl on the carbonyl carbon.
MCQ 6Concept TrapPractice
Ethanal and propanone are both available. Which single test separates them?
Show answer and why every option is right or wrong
Answer: B. Ethanal is an aldehyde and gives a silver mirror with Tollens' reagent, while propanone is a ketone and does not (NCERT Class 12 Chemistry, Chapter 8, page 239).
Why A is wrong: A is wrong because both ethanal and propanone have a methyl group on the carbonyl carbon, so both give the iodoform reaction (page 240).
Why C is wrong: C is wrong because the haloform reaction works on any carbonyl compound with a methyl on the carbonyl carbon, and both qualify (page 240).
Why D is wrong: D is wrong because both compounds contain the CH3CO group, so detecting it cannot separate them.
MCQ 7Concept TrapPractice
Why are Tollens' and Fehling's reagents chosen to distinguish aldehydes from ketones, rather than a strong oxidising agent?
Show answer and why every option is right or wrong
Answer: C. Aldehydes are easily oxidised even by mild agents, while ketones need strong oxidising agents and elevated temperature, so a mild agent discriminates (NCERT Class 12 Chemistry, Chapter 8, page 239).
Why A is wrong: A is wrong because nitric acid, potassium permanganate and potassium dichromate all oxidise aldehydes easily.
Why B is wrong: B is wrong because NCERT calls Tollens' and Fehling's mild oxidising agents, and potassium dichromate is listed among the common strong ones.
Why D is wrong: D is wrong because ketones are oxidised under vigorous conditions, with carbon-carbon bond cleavage.
MCQ 8Direct ApplicationPractice
A compound neither gives a silver mirror with Tollens' reagent nor a red precipitate with Fehling's reagent, but gives a yellow precipitate when heated with iodine and sodium hydroxide. Which could it be?
Show answer and why every option is right or wrong
Answer: A. No reaction with Tollens' or Fehling's reagent rules out an aldehyde, and the iodoform result shows a CH3CO group; propanone is a methyl ketone (NCERT Class 12 Chemistry, Chapter 8, pages 239-240).
Why B is wrong: B is wrong because propanal is an aldehyde and would give a silver mirror with Tollens' reagent.
Why C is wrong: C is wrong because benzaldehyde is an aldehyde, so it gives a silver mirror with Tollens' reagent.
Why D is wrong: D is wrong because pentan-3-one is a ketone but has no methyl group on the carbonyl carbon, so it gives no iodoform reaction.
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What to remember before solving Aldehyde Ketone Tests questions
7 NCERT lines
The mild oxidising agents given below are used to distinguish aldehydes from ketones:
-- NCERT Class 12 Chemistry, Ch. 8, p. 239(i) Tollens’ test: On warming an aldehyde with freshly prepared ammoniacal silver nitrate solution (Tollens’ reagent), a bright silver mirror is produced due to the formation of silver metal. The aldehydes are oxidised to corresponding carboxylate anion. The reaction occurs in alkaline medium.
-- NCERT Class 12 Chemistry, Ch. 8, p. 239These two solutions are mixed in equal amounts before test. On heating an aldehyde with Fehling’s reagent, a reddish brown precipitate is obtained. Aldehydes are oxidised to corresponding carboxylate anion. Aromatic aldehydes do not respond to this test.
-- NCERT Class 12 Chemistry, Ch. 8, p. 239(iii) Oxidation of methyl ketones by haloform reaction: Aldehydes and ketones having at least one methyl group linked to the carbonyl carbon atom (methyl ketones) are oxidised by sodium hypohalite to sodium salts of corresponding carboxylic acids having one carbon atom less than that of carbonyl compound. The methyl group is converted to haloform. This oxidation does not affect a carbon-carbon double bond, if present in the molecule.
-- NCERT Class 12 Chemistry, Ch. 8, p. 240Iodoform reaction with sodium hypoiodite is also used for detection of CH3CO group or CH3CH(OH) group which produces CH3CO group on oxidation.
-- NCERT Class 12 Chemistry, Ch. 8, p. 240Fehling reagent comprises two solutions, Fehling solution A and Fehling solution B. Fehling solution A is aqueous copper sulphate and Fehling solution B is alkaline sodium potassium tartarate (Rochelle salt).
-- NCERT Class 12 Chemistry, Ch. 8, p. 239Aldehyde Ketone Tests: NEET previous year questions (PYQs) with answers
1 question from NEET 2024, answers verified against NTA official keys
NEET 2024Revised key
1aqueous copper sulphate
2alkaline copper sulphate
3alkaline solution of sodium potassium tartrate (Rochelle’s salt)
4aqueous sodium citrate
NTA Answer: Option 1(revised_final)
Why the other options are wrong
- Option 2: Not alkaline. Fehling solution A is aqueous copper sulphate; the alkali is in solution B.
- Option 3: That is Fehling solution B: alkaline sodium potassium tartarate (Rochelle salt).
- Option 4: Sodium citrate is not part of Fehling reagent. Fehling solution A is aqueous copper sulphate.
All 23 past-paper questions from Alcohols, Phenols, Ethers and Carbonyl Compounds →
More in Alcohols, Phenols, Ethers and Carbonyl Compounds: 3 exam traps and mistakes · 2 formulas · 3 question patterns from its other lessons.
Sources
NCERT refs: Class 12 Chemistry Chapter 8, p.239
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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