Identifying Three Unlabelled Organic Liquids
Three unlabelled colourless liquids are kept in a school laboratory: A, B and C. A has a sharp smell, turns blue litmus red and gives brisk effervescence with sodium hydrogencarbonate solution. B is neutral to litmus, burns with a clean blue flame and reacts with sodium metal to give hydrogen gas. C is almost odourless, immiscible with water, and has a sweet fruity smell; when warmed with sodium hydroxide solution it gives a liquid that smells like B and a sodium salt. A student claims that all three are hydrocarbons because they are liquids containing carbon and hydrogen and because B burns like petrol. The teacher asks for a systematic identification using the tests already performed, and for an explanation of what each test actually proves.
1. Identify A, B and C, giving the evidence for each.
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A is ethanoic acid (acetic acid). Evidence: sharp smell, turns blue litmus red (acidic), and gives CO2 with NaHCO3: CH3COOH + NaHCO3 → CH3COONa + H2O + CO2. B is ethanol. Evidence: neutral to litmus, burns with a blue flame, and reacts with sodium metal to give hydrogen: 2C2H5OH + 2Na → 2C2H5ONa + H2. C is an ester, ethyl ethanoate. Evidence: sweet fruity smell, immiscible with water, and alkaline hydrolysis gives ethanol (B) and a sodium salt: CH3COOC2H5 + NaOH → CH3COONa + C2H5OH. The hydrolysis product ‘smells like B’ confirms the alcohol part; the sodium salt is from the acid part.
2. Explain what the sodium hydrogencarbonate test proves and why the sodium-metal test alone cannot identify B.
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The NaHCO3 test proves the presence of an acid stronger than carbonic acid: only an acid can release CO2 from hydrogencarbonate. It does not prove the acid is ethanoic acid specifically, but together with the smell and esterification it supports that identification. The sodium-metal test proves only that the liquid has a replaceable hydrogen attached to oxygen (an alcohol or an acid); both ethanol and ethanoic acid react with sodium. It cannot distinguish them, and it does not prove B is ethanol. The neutral litmus and absence of CO2 with NaHCO3 separate B from A, while the hydrolysis result identifies C.
3. A student claims all three are hydrocarbons because they contain carbon and hydrogen and B burns like petrol. Critically assess this claim.
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The claim is false. A hydrocarbon contains only carbon and hydrogen. A and B contain oxygen as well: ethanoic acid is C2H4O2 and ethanol is C2H6O. C, ethyl ethanoate, is C4H8O2, also containing oxygen. Burning with a blue flame is not proof of a hydrocarbon; ethanol burns with a blue flame too. A better test is reaction with sodium (alcohols react, hydrocarbons generally do not) and behaviour with water. The student has confused ‘contains C and H’ with ‘contains only C and H’. The systematic tests show three different functional groups: carboxylic acid, alcohol and ester.