Identifying four unlabelled liquids: when one positive test is not enough
A technician finds four unlabelled bottles of colourless liquids in a school store cupboard, marked only A, B, C and D. The stock list says they are ethanoic acid, ethanol, ethanal and propanone, but does not say which is which. The only tests available are: solid sodium hydrogencarbonate; acidified potassium dichromate(VI) solution; Tollens' reagent; and iodine solution with sodium hydroxide solution. Results: A gives brisk effervescence with sodium hydrogencarbonate and no change with the other three tests. B gives no effervescence, turns acidified dichromate from orange to green only when warmed, gives no silver mirror, and gives a pale yellow precipitate with iodine/NaOH. C gives no effervescence, turns dichromate green at room temperature without warming, gives a silver mirror with Tollens' reagent, and gives a pale yellow precipitate with iodine/NaOH. D gives no effervescence, no change with dichromate or Tollens' reagent, and a yellow precipitate with iodine/NaOH. A colleague concludes that 'D must be propanone, and the iodoform test on its own is enough to prove it'.
1. Assign A, B, C and D, justifying each from the evidence, and evaluate the colleague's claim about the iodoform test.
Read the worked answer
A = ethanoic acid. Only a carboxylic acid gives CO₂ with hydrogencarbonate. It is not oxidised by dichromate, is not an aldehyde (no mirror) and has no CH₃CO–/CH₃CH(OH)– group (no iodoform).
C = ethanal. It gives a silver mirror with Tollens' reagent, which is the specific test for an aldehyde, and it is oxidised by acidified dichromate at room temperature - aldehydes are oxidised readily, without the warming needed for an alcohol. It also gives iodoform, as CH₃CHO contains the CH₃CO– unit.
B = ethanol. No silver mirror, so not an aldehyde; dichromate turns green only on warming, the behaviour of a primary alcohol being oxidised to an aldehyde and then an acid; positive iodoform from the CH₃CH(OH)– unit.
D = propanone. No reaction with dichromate (ketones resist oxidation under these conditions) and no silver mirror, but a positive iodoform test from the CH₃CO– group.
The claim is wrong as stated. The iodoform test is positive for B, C and D, so on its own it cannot distinguish them; it only shows the presence of a CH₃CO– or CH₃CH(OH)– group. D is propanone because of the combination - iodoform positive, dichromate and Tollens' negative. A single positive test is never an identification when other candidates would give the same result.
2. Write an equation for the reaction of A with sodium hydrogencarbonate and for the reaction of C with Tollens' reagent, and state the observation in each case.
Read the worked answer
A with hydrogencarbonate: CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂ Observation: steady effervescence of a colourless gas that turns limewater milky - this is the standard confirmation that the gas is CO₂, not just any gas.
C with Tollens' reagent (ammoniacal silver nitrate, simplified): CH₃CHO + 2Ag⁺ + H₂O → CH₃COOH + 2Ag + 2H⁺ Observation: a silver mirror on the wall of a clean test tube, formed on warming gently in a water bath.
Note what the second equation shows: the aldehyde is oxidised to the acid while Ag⁺ is reduced to Ag. Tollens' reagent is a mild oxidising agent, which is why it stops at the aldehyde and does not oxidise ethanol - a useful point when explaining why B gives no mirror.
3. The test on B with Tollens' reagent gave no mirror. Give two reasons, other than B not being an aldehyde, why a genuine aldehyde could fail to give a mirror, and state how you would check.
Read the worked answer
Reasons unrelated to the identity of the sample:
- The reagent has deteriorated. Tollens' reagent is not stable - it decomposes on standing, depositing silver in the bottle, so an old reagent has little Ag⁺ left to be reduced.
- The tube is dirty. Grease, fingerprints or acid residues on the glass stop the silver depositing as a coherent mirror; the silver may appear as a grey or black precipitate instead, which is easy to miss or to misread.
Other valid reasons: the tube was not warmed; too little sample was used; the reagent was not sufficiently alkaline, so the reaction is too slow in the time allowed.
How to check: repeat with freshly prepared Tollens' reagent in a tube cleaned with alkali and rinsed with distilled water, warmed in a water bath, and confirm with an independent aldehyde test such as Fehling's solution (brick-red precipitate of Cu₂O). If two independent tests on a known aldehyde both work, a negative result on the sample becomes meaningful; a single negative result on an old reagent does not.