What is Tollen's reagent ?

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Q. What is Tollen's reagent ?

Punjab PsebPSEB Punjab Class 12 Board 2018Subjective· 1mImportance★★★★★

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Concept understanding — Tollens Test

The Tollens Test: The Silver Mirror That Spots an Aldehyde

Imagine you have two unlabelled bottles — one contains an aldehyde, the other a ketone. They look identical, smell similar, and both are carbonyl compounds. How do you tell them apart in a single, dramatic step? The Tollens test gives you a visible, unmistakable answer: a beautiful silver mirror coating the inside of your test tube.

The Intuition: Why Aldehydes Are Special

The key difference lies in the carbonyl carbon. In an aldehyde, that carbon is bonded to at least one hydrogen atom. In a ketone, it is bonded to two carbon groups. That hydrogen on the aldehyde is not just any hydrogen — it is weakly acidic and, more importantly, easily oxidised. The aldehyde can lose this hydrogen and become a carboxylic acid, donating electrons in the process.

Tollens reagent is a solution of silver ions ( $\text{Ag}^+$ ) held in a complex with ammonia. Silver ions are excellent oxidising agents — they want to gain electrons and become neutral silver metal. When an aldehyde meets these silver ions, the aldehyde gets oxidised (loses its hydrogen) and the silver ions get reduced (gain electrons), forming metallic silver.

Tip

Think of it as a trade: the aldehyde gives away electrons (gets oxidised), and the silver ions accept those electrons (get reduced). The silver atoms then clump together on the glass surface, forming that characteristic mirror.

The Precise Statement

Tollens test is a qualitative chemical test that distinguishes aldehydes from ketones. When an aldehyde is warmed with ammoniacal silver nitrate (Tollens reagent), the aldehyde is oxidised to a carboxylate ion, and the silver(I) ions are reduced to metallic silver, which deposits as a shiny mirror on the walls of the test tube. Ketones do not react under these mild conditions.

\text{RCHO} + 2[\text{Ag(NH}_3\text{)}_2]^+ + 3\text{OH}^- \longrightarrow \text{RCOO}^- + 2\text{Ag} \downarrow + 4\text{NH}_3 + 2\text{H}_2\text{O}

How the Reagent Is Made

You cannot just use silver nitrate directly — the silver ions would precipitate as silver oxide in the alkaline conditions needed for the reaction. So you first dissolve silver nitrate in water, add sodium hydroxide to get a brown precipitate of silver oxide, and then add just enough ammonia to redissolve that precipitate. The result is a clear solution containing the complex ion $[\text{Ag(NH}_3\text{)}_2]^+$ .

Watch out

Never store Tollens reagent. It forms explosive silver nitride ( $\text{Ag}_3\text{N}$ ) on standing. Prepare it fresh, use it immediately, and destroy any leftover by adding dilute acid.

The Procedure in the Lab

Clean a test tube thoroughly — any grease will ruin the mirror.
Add a few drops of the unknown compound.
Add about 2 mL of freshly prepared Tollens reagent.
Warm the mixture gently in a water bath (do not boil). …

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Tollens' reagent is the diamminesilver(I) hydroxide complex, $[Ag(NH_3)_2]^+OH^-$ ; aldehydes reduce the $Ag^+$ in it to metallic silver (a bright mirror), while ketones (other than $\alpha$ -hydroxy ketones) do not react.

Preparation: Aqueous ammonia is added dropwise to silver nitrate solution. A precipitate of silver oxide forms first:

$2AgNO_3 + 2NH_4OH \rightarrow Ag_2O\downarrow + 2NH_4NO_3 + H_2O$

This precipitate then dissolves in excess ammonia to give the clear Tollens' reagent:

$Ag_2O + 4NH_4OH \rightarrow 2[Ag(NH_3)_2]^+OH^- + 3H_2O$

Use (Tollens' test): When warmed gently with an aldehyde, the $Ag^+$ ion (a mild oxidising agent when complexed this way) oxidises the aldehyde to a carboxylate and is itself reduced to metallic silver, which deposits as a shiny mirror on the test-tube walls:

…

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

Showing the 12 most recent of 18 on this concept.

CBSE 2026Set ANNUAL1 markMCQ
$Q. Which of the following gives silver mirror with Tollen's reagent?$
(a) Formic acid
(b) Acetic acid
(c) Acetone
(d) Ethyl alcohol
›Reveal solutionSolution

Formic acid is unusual among carboxylic acids in giving Tollens' test because it structurally contains an aldehyde-like H–C=O unit.

Tollens' reagent ( $[Ag(NH_3)_2]^+OH^-$ , ammoniacal silver nitrate) is reduced to metallic silver (depositing as a bright 'silver mirror' on the test tube) by compounds that can be oxidised easily, classically aldehydes ( $R-CHO \rightarrow R-COOH$ , with $Ag^+ \rightarrow Ag$ ).

…

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CBSE 2026Set ANNUAL1 markMCQ
$Q. Tollen's reagent is used to distinguish between$
(a) Alcohol and ether
(b) Aldehydes and ketones
(c) Carboxylic acid and esters
(d) 1 degree, 2 degree and 3 degree amines
›Reveal solutionSolution

Tollen's test distinguishes aldehydes from ketones because only aldehydes are easily oxidised, reducing the diamminesilver(I) complex to metallic silver (a bright mirror).

Tollen's reagent is an ammoniacal solution of silver nitrate, containing the complex ion [Ag(NH3)2]+. When warmed gently with an aldehyde, the aldehyde is oxidised to a carboxylate ion, while Ag+ is reduced to metallic silver, which deposits as a shiny 'silver mirror' on the test-tube wall:

RCHO + 2[Ag(NH3)2]+ + 3OH- -> RCOO- + 2Ag(s) + 4NH3 + 2H2O

…

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CBSE 2026Set ANNUAL1 mark
$Q. Among aldehydes or ketones which gives positive Tollen's Test?$
›Reveal solutionSolution

Tollens' reagent (ammoniacal silver nitrate) oxidizes aldehydes to carboxylate ions while itself being reduced to metallic silver (silver mirror); ketones do not respond (with the exception of α-hydroxy ketones).

Tollens' reagent, $[Ag(NH_3)_2]^+$ , is a mild oxidizing agent. Aldehydes are readily oxidized (they have an oxidizable C–H bond on the carbonyl carbon):

$RCHO + 2[Ag(NH_3)_2]^+ + 3OH^- \rightarrow RCOO^- + 2Ag\downarrow + 4NH_3 + 2H_2O$

…

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CBSE 2024Set FZ1 markMCQ
$Q. Which of the following compound is identified by Tollen's Reagent?$
(a) Alcohol
(b) Aldehyde
(c) Ketone
(d) Carboxylic acid
›Reveal solutionSolution

Tollen's reagent gives a silver mirror only with aldehydes → option (b).

Concept. Tollen's reagent is a mild oxidising agent, the diamminesilver(I) ion $[Ag(NH_3)_2]^+$ . Aldehydes are readily oxidised to carboxylate; in doing so they reduce $Ag^+$ to metallic silver, which deposits as a bright silver mirror.

Reaction.

$RCHO + 2[Ag(NH_3)_2]^+ + 3OH^- \longrightarrow RCOO^- + 2Ag\downarrow + 4NH_3 + 2H_2O$ …

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CBSE 2024Set ANNUAL1 mark
$Q. What is Tollen's reagent?$
›Reveal solutionSolution

Tollen's reagent is a solution of silver nitrate in excess ammonia, [Ag(NH3)2]+OH-; it selectively oxidises aldehydes and deposits metallic silver as a mirror on the container wall.

Preparation: AgNO3 solution is treated with dilute NaOH to precipitate Ag2O, then dissolved in excess aqueous ammonia to give the soluble complex ion [Ag(NH3)2]+ (diamminesilver(I) ion), along with OH-.

…

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CBSE 2023Set F1 markMCQ
$Q. Which of the following is Tollen's reagent?$
(a) [Ag(NH3)2]+ ion
(b) Cu(OH)2
(c) Ag2O
(d) CuO
›Reveal solutionSolution

Tollen's reagent is ammoniacal silver nitrate, containing the diamminesilver(I) ion, [Ag(NH3)2]+.

Tollen's reagent is prepared by adding aqueous ammonia to silver nitrate until the initial precipitate just dissolves, forming the complex ion [Ag(NH3)2]+ (diamminesilver(I)). Aldehydes reduce it, depositing metallic silver as a bright silver mirror (the silver …

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CBSE 2023Set ANNUAL1 markMCQ
$Q. Tollen's reagent is$
(a) mixture of CuSO4 and NaOH
(b) ammoniacal silver nitrate solution
(c) a solution of copper sulphate, sodium citrate and sodium carbonate
(d) anhydrous ZnCl2 and conc. HCl
›Reveal solutionSolution

Tollen's reagent is AgNO3 dissolved in excess ammonia, which forms the diamminesilver(I) complex used to oxidise aldehydes.

Tollen's reagent is prepared by adding aqueous ammonia to silver nitrate solution until the initially formed brown precipitate of Ag2O just redissolves, giving [Ag(NH3)2]+OH- (ammoniacal silver nitrate). Aldehydes reduce this complex to metallic silver, which deposits as a bright 'silver mirror' on the test-tube wall, while the aldehyde is …

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CBSE 2023Set TERM21 mark
$Q. Freshly prepared ammonical silver nitrate solution is called \dots\dots\dots\dots\dots$
›Reveal solutionSolution

Tollens' reagent is used to test for aldehydes via the 'silver mirror test'.

Tollens' reagent is prepared by adding aqueous ammonia to silver nitrate solution until the initially formed brown precipitate of $Ag_2O$ just dissolves, forming the complex ion $[Ag(NH_3)_2]^+OH^-$ (diamminesilver(I) hydroxide) — this freshly-prepared solution is called Tollens' reagent.

…

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CBSE 2023Set ANNUAL1 markMCQ
$Q. The compounds CH₃CH₂CHO and CH₃CH₂COOH cannot be distinguished by -$
(i) Tollen's test
(ii) Fehling's test
(iii) NaHCO₃ test
(iv) Hinsberg's test
›Reveal solutionSolution
Hinsberg's test is a test for classifying amines, not carbonyl compounds — it is simply irrelevant to this pair, so neither compound gives a diagnostic result and the two cannot be told apart by it.

Propanal ( $CH_3CH_2CHO$ ) and propanoic acid ( $CH_3CH_2COOH$ ) CAN be distinguished by:

Tollens' test: the aldehyde reduces Tollens' reagent to give a silver mirror; the carboxylic acid does not.

Fehling's test: the aldehyde gives a brick-red precipitate of $Cu_2O$ ; the carboxylic acid does not.

NaHCO₃ test: the carboxylic acid gives brisk effervescence of $CO_2$ (it's acidic enough to liberate $CO_2$ from bicarbonate); the aldehyde does not react. …
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CBSE 2022Set ANNUAL1 markMCQ
$Q. Tollen's reagent is used to identify -$
(a) aldehydes
(b) ketones
(c) carboxylic acids
(d) alcohols
›Reveal solutionSolution

Tollen's reagent (ammoniacal silver nitrate) is used to identify aldehydes via the silver-mirror test.

Tollen's reagent, [Ag(NH3)2]⁺OH⁻, oxidizes an aldehyde to the corresponding carboxylate ion while Ag⁺ is reduced to metallic silver, which deposits as a bright silver mirror on the walls of the test tube …

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CBSE 2022Set ANNUAL1 mark
$Q. What is Tollen's reagent?$
›Reveal solutionSolution

Tollen's reagent = ammoniacal AgNO3, containing [Ag(NH3)2]+, a mild oxidant that gives a silver mirror with aldehydes.

Tollen's reagent is prepared by adding aqueous ammonia to silver nitrate solution until the initial brown precipitate of Ag2O just dissolves, forming the soluble complex $[Ag(NH_3)_2]^+$ .

It is a weak oxidising agent. When warmed with an aldehyde, the aldehyde is oxidised to the corresponding carboxylate ion, while Ag+ is reduced to metallic silver, which deposits as a shiny mirror on the inner wall of the tube:

…

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CBSE 2021Set ANNUAL1 markMCQ
$Q. Aldehydes and ketones can be distinguished by -$
(a) Tollen's reagent
(b) Sodium bicarbonate test
(c) FeCl3 solution
(d) Carbylamine test
›Reveal solutionSolution

Aldehydes reduce Tollen's reagent (ammoniacal AgNO3) to give a silver mirror; ketones do not.

Tollen's reagent is a mild oxidising agent, $[Ag(NH_3)_2]^+$ . Aldehydes are easily oxidised to carboxylate ions because the carbonyl carbon still carries an H atom, so they reduce $Ag^+$ to metallic silver, which deposits as a bright silver mirror on the test-tube wall:

$RCHO + 2[Ag(NH_3)_2]^+ + 3OH^- \rightarrow RCOO^- + 2Ag\downarrow + 4NH_3 + 2H_2O$

…

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Q.What is Tollen's reagent ?

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