On this page
"Which of these gives a positive Tollens' test?" Students usually learn "aldehydes yes, ketones no" and stop there. The questions that cost marks sit at the edges: benzaldehyde with Fehling's, fructose, formic acid, sucrose. This post sorts the common compounds into pass and fail, gives the equations and uses them for a quantity calculation.
Where is this in the syllabus?
The NTA Syllabus for JEE (Main) 2026 lists "chemical tests to distinguish between aldehydes and ketones" and, under practical chemistry, detection of the carbonyl group. The NEET (UG) 2026 syllabus has the same tests. The NCERT Class 12 chapter Aldehydes, Ketones and Carboxylic Acids (Reprint 2026-27) describes both tests, and the Biomolecules chapter uses them to define reducing sugars. Our post on aldehyde and ketone reactions covers the wider chapter.
What do the two reagents do?
From the NCERT chapter:
Tollens' reagent is freshly prepared ammoniacal silver nitrate. Warmed with an aldehyde, it gives "a bright silver mirror ... due to the formation of silver metal."
Fehling's reagent is a mix of equal amounts of Fehling A (aqueous copper sulphate) and Fehling B (alkaline sodium potassium tartrate). Heated with an aldehyde, it gives a reddish-brown precipitate.
The NCERT Class 12 chemistry lab manual gives the equations:
RCHO + 2[Ag(NH₃)₂]⁺ + 2OH⁻ → 2Ag + 3NH₃ + H₂O + RCOONH₄
RCHO + 2Cu²⁺ (complexed) + 5OH⁻ → RCOO⁻ + Cu₂O + 3H₂O
Both are mild oxidising agents. The aldehyde is oxidised to a carboxylate, which is why compounds that cannot be oxidised this easily fail.
Which compounds pass and which fail?
Aliphatic aldehydes (methanal, ethanal, propanal): pass both.
Aromatic aldehydes (benzaldehyde): pass Tollens', fail Fehling's. NCERT: "Aromatic aldehydes do not respond to this test."
Ketones (propanone, acetophenone): fail both. NCERT Example 8.4 identifies acetophenone partly because it "neither reduces Tollens' or Fehlings' reagent".
Formic acid: passes Tollens'. The lab manual asks why as a discussion question; the answer is that HCOOH has an H attached to C=O, like an aldehyde.
Glucose and fructose: pass both. NCERT Biomolecules: "All monosaccharides whether aldose or ketose are reducing sugars," meaning they reduce Fehling's solution and Tollens' reagent.
Maltose and lactose: pass, since NCERT counts them as reducing sugars.
Sucrose: fails. Its reducing groups are tied up in the glycosidic bond, so NCERT calls it non-reducing.
Worked example 1: how much silver?
1.16 g of propanal (C₃H₆O, molar mass 58.08 g/mol) is fully oxidised by excess Tollens' reagent. What mass of silver forms?
Moles of propanal: 1.16 / 58.08 = 0.0200 mol.
The equation gives 2 Ag per RCHO: 2 × 0.0200 = 0.0400 mol Ag.
Mass, using the standard atomic weight of silver, 107.87: 0.0400 × 107.87 = 4.31 g.
With excess Fehling's reagent instead, 1 Cu₂O forms per RCHO: 0.0200 × 143.1 g/mol = 2.86 g of Cu₂O.
Worked example 2: five unlabelled bottles
The bottles hold ethanal, benzaldehyde, propanone, glucose and sucrose.
Tollens' positive: ethanal, benzaldehyde, glucose. Negative: propanone, sucrose.
Fehling's on the three positives: ethanal and glucose pass; benzaldehyde fails. Benzaldehyde is identified.
Ethanal versus glucose: ethanal has a CH₃CO group and gives the iodoform test, which NCERT uses to detect that group. Glucose is a solid.
Propanone versus sucrose: propanone gives iodoform. Sucrose, after hydrolysis, gives glucose and fructose (NCERT), and the hydrolysed solution then reduces Fehling's.
Which mistakes do students make?
Counting 1 Ag per aldehyde. It is 2.
Giving benzaldehyde a positive Fehling's.
Failing fructose because it is a ketose.
Passing sucrose because it contains glucose and fructose units.
Forgetting formic acid in "which of these gives a silver mirror" lists.
A practice routine
Write the two equations from memory and balance-check them.
Sort ten compounds into pass/fail for each reagent, including fructose, sucrose and formic acid.
Find the mass of silver from 0.88 g of ethanal (M = 44.05 g/mol). (Check: about 4.31 g.)
Read our post on functional group tests in the lab for the order in which to run tests.
When the edge cases blur together, the Fermi Concept Library covers every concept in chemistry, so you can revise aldehydes and reducing sugars side by side.
Tonight, make the ten-compound pass/fail list from step 2 without notes, then check it against this post.
