Identifying Four Colourless Liquids with Limited Tests
Four unlabelled colourless liquids are known to be ethanol, ethanoic acid, ethanal and ethyl ethanoate. A student is given only these tests: (i) add a few drops of bromine water and shake; (ii) add a small spatula of sodium carbonate and look for bubbles; (iii) add a piece of sodium metal. The student's notes read: P - no change with bromine water, no bubbles with sodium carbonate, steady bubbling with sodium metal; Q - bromine water decolourised almost immediately, no bubbles with sodium carbonate, no visible reaction with sodium metal; R - no change with bromine water, bubbles with sodium carbonate, steady bubbling with sodium metal; S - no change with bromine water, no bubbles with sodium carbonate, no visible reaction with sodium metal, strong sweet smell. The student concludes that S must be ethanoic acid because it has the strongest smell. A classmate says the conclusion is unsafe because the tests were not used in a logical order and because smell is not a reliable identification test.
1. Identify P, Q, R and S, giving the reasoning for each.
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P is ethanol: no reaction with bromine water shows it is not an aldehyde and has no C=C bond; no CO₂ with carbonate shows it is not a carboxylic acid; sodium gives hydrogen from the –OH group. Q is ethanal: it decolourises bromine water because it is oxidised to ethanoic acid; it gives no CO₂ because it is not acidic; it has no O–H group, so sodium gives no visible reaction. R is ethanoic acid: no reaction with bromine water; carbonate gives CO₂ because it is an acid; sodium also gives hydrogen from the acidic –OH group. S is ethyl ethanoate: no reaction with bromine water, no CO₂ with carbonate, no hydrogen with sodium; the sweet smell is characteristic of an ester.
2. Explain why sodium carbonate and bromine water are useful discriminating tests here, and why sodium metal alone is not sufficient.
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Sodium carbonate releases carbon dioxide only with an acid, so it identifies ethanoic acid definitively among these four liquids. Bromine water is decolourised by ethanal because the aldehyde is oxidised to a carboxylic acid; ethanol, ethanoic acid and ethyl ethanoate do not decolourise it under these conditions. Sodium metal reacts with compounds containing an –OH group, producing hydrogen, so it detects ethanol and ethanoic acid. However, sodium cannot distinguish ethanol from ethanoic acid because both give hydrogen; it must be used after the carbonate test has identified the acid. Ethyl ethanoate and ethanal give no visible reaction with sodium, so sodium alone would not distinguish them either.
3. Critique the student's conclusion about S and suggest a better order for the tests.
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S cannot be ethanoic acid because it gives no carbon dioxide with sodium carbonate and no hydrogen with sodium metal, both of which ethanoic acid would give. Its sweet smell fits ethyl ethanoate. Smell is not a reliable identification test; if a smell must be checked, the vapour should be wafted towards the nose, never inhaled directly. A better order is: use sodium carbonate first to identify the carboxylic acid; then use bromine water to distinguish the aldehyde from the other three; finally use sodium metal to identify the alcohol, leaving the ester as the remaining liquid.