Identifying an Unknown Liquid: Which Test Is Decisive
An unknown colourless liquid Z is issued for identification. It boils at 78 °C, mixes completely with water, and gives a neutral result on moist pH paper. The following observations are recorded: sodium carbonate solution - no effervescence; acidified potassium dichromate solution, warmed - orange turns green; Fehling's solution, warmed - no red precipitate; iodine in sodium hydroxide solution, warmed - yellow precipitate; bromine water - remains orange, no decolourisation; 2,4-dinitrophenylhydrazine - no precipitate. The student's partner argues that Z must be ethanoic acid, 'because it dissolves in water and doesn't react with anything'. The student is not convinced and must justify a different identification, quoting the evidence test by test rather than relying on the smell.
1. Identify Z and justify your answer, explaining what each negative and positive test rules out.
Read the worked answer
Z is ethanol.
- Neutral on pH paper and no effervescence with sodium carbonate: both rule out a carboxylic acid. Ethanoic acid would turn pH paper red and would release CO₂ from carbonate.
- No precipitate with 2,4-DNP: rules out a carbonyl compound - no aldehyde or ketone (so not ethanal or propanone).
- No red precipitate with Fehling's: confirms no aldehyde present.
- Yellow precipitate with iodine/NaOH (iodoform test): positive only for a CH₃CO– group or a CH₃CH(OH)– group. Combined with the absence of a carbonyl compound, this points to an alcohol of the type CH₃CH(OH)–, i.e. ethanol (propan-2-ol would also respond, but its boiling point is about 82 °C, not 78 °C).
- Dichromate orange → green: Z is oxidised, which is characteristic of a primary or secondary alcohol.
- Bromine water not decolourised: no C=C double bond, so it is not an alkene.
Boiling point 78 °C matches ethanol (78.3 °C), and full miscibility with water fits a small alcohol. The partner's reasoning is faulty: mixing with water and 'not reacting' are not evidence for an acid - an acid would have reacted with the carbonate.
2. Write the equation for the reaction the partner expected with sodium carbonate, and explain how the observation would have differed if Z really were ethanoic acid.
Read the worked answer
2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂
(Equivalently, with hydrogencarbonate: CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂.)
If Z were ethanoic acid, adding sodium carbonate would produce steady effervescence of carbon dioxide - the classic test for a carboxylic acid - and the gas would turn limewater milky. Moist pH paper would also have shown a clearly acidic (red/orange) result, and the smell would be sharp and vinegar-like.
Neither happened. The absence of effervescence is a genuine negative result and must be used as evidence, not ignored.
3. Write the equation for the reaction of Z with acidified dichromate, and explain why the observation (orange to green) is consistent with your identification.
Read the worked answer
3CH₃CH₂OH + Cr₂O₇²⁻ + 8H⁺ → 3CH₃CHO + 2Cr³⁺ + 7H₂O
With excess oxidising agent the aldehyde is oxidised further: CH₃CHO + [O] → CH₃COOH
- Ethanol is a primary alcohol, so it is oxidised first to ethanal and then to ethanoic acid.
- The dichromate ion (orange) is reduced to Cr³⁺ (green), which is exactly the colour change observed. The colour change is evidence that Z is being oxidised, and it is consistent with an alcohol rather than a ketone-forming secondary alcohol or an unreactive ether.
- The Fehling's result stays negative because the oxidation was carried out with dichromate, not with Fehling's reagent; Fehling's would only give a red precipitate with the aldehyde product if it were tested separately.