Identification of functional groups by test-tube reactions — AQA A-Level Chemistry
Test yourself on Identification of functional groups by test-tube reactions with AQA A-Level practice questions.
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Identification of functional groups by test-tube reactions explained
Each functional group on this course has its own test-tube reaction, and each needs the right reagent, the right condition and a stated observation.
Read the full explanation
An alkene decolourises orange bromine water at room temperature. A primary or secondary alcohol, warmed with acidified potassium dichromate(VI), turns it from orange to green. A carboxylic acid added to sodium carbonate or hydrogencarbonate solution fizzes, and the gas turns limewater milky. An aldehyde gives a silver mirror with Tollens' reagent or a red precipitate with Fehling's solution, both warmed in a water bath. A halogenoalkane, warmed with sodium hydroxide, acidified with nitric acid and treated with silver nitrate, gives a white, cream or yellow precipitate.
Students should be able to: identify the functional groups using reactions in the specification.
Identifying an unknown is a process of elimination, so plan the order of tests and record every result, including the ones where nothing happens. Test for a carboxylic acid first with sodium carbonate solution, because an acid also reacts with other reagents and would confuse later results. Bromine water then separates the alkene. Acidified potassium dichromate(VI) picks out anything that can be oxidised, which covers primary and secondary alcohols and aldehydes, so follow it with Tollens' reagent or Fehling's solution to decide whether an aldehyde is present. A tertiary alcohol is identified by the dichromate staying orange. Each conclusion must be justified by the observation that produced it, not asserted.
Required practical 6 Tests for alcohol, aldehyde, alkene and carboxylic acid.
In this practical you work through four tests on unknown liquids. Alkene: shake with bromine water at room temperature and look for orange to colourless. Alcohol: warm with acidified potassium dichromate(VI) in a water bath and look for orange to green. Aldehyde: warm with Fehling's solution and look for the blue solution giving a red precipitate, or with Tollens' reagent for a silver mirror. Carboxylic acid: add sodium hydrogencarbonate solution, look for effervescence and bubble the gas through limewater, which turns milky. Warming is done in a water bath, not over a flame, because the liquids are flammable. AQA has set a flawed method in which Fehling's was cooled and the gas tested with a splint; both are errors.
Your focus
- Match each functional group on the specification to its test reagent, condition and positive observation.
- Describe the observation for a compound that gives a negative result in a named test.
- Select a single test that would distinguish two named compounds and justify the choice.
Show all 9 objectives
- Select a single test that distinguishes two given organic compounds and state both observations.
- Order a set of tests so that one result does not interfere with the next.
- Deduce the functional group present in an unknown from a set of given test results.
- Carry out and describe the four specified tests, giving the observation for each functional group.
- Identify errors in a given practical method and state the correction for each.
- Justify heating in a water bath rather than over a flame when testing organic liquids.
Identification of functional groups by test-tube reactions exam tips
Marking Points
- one mark for naming the reagent as something that could be taken from a bottle, such as acidified potassium dichromate(VI)
- one mark for the condition, including warming in a water bath where the test requires it
- one mark for the observation on a positive result, giving both the starting and the final appearance
- one mark for stating what is seen with a compound that does not react
- one mark for choosing a test that gives different observations for the two compounds being distinguished
- one mark for naming the reagent fully, including acidified or ammoniacal where required
- one mark for the observation with each compound, including the negative result
- one mark for a conclusion that follows directly from the observations recorded
- one mark for each reagent correctly matched to its functional group and observation
- one mark for warming in a water bath rather than over a flame, because the organic liquids are flammable
- one mark for bubbling the gas from the carbonate test through limewater rather than using a lighted splint
- one mark for stating the observation with a compound that gives a negative result
Examiner Tips
- 💡Write reagent, condition and observation as three separate items; a missing condition often costs a mark.
- 💡Say "no change" explicitly for the compound that does not react.
- 💡Name reagents in full, including the word acidified or ammoniacal where it applies.
- 💡Set your answer out as a table: reagent, what you do, what you see with each compound, conclusion.
- 💡Test for the acid before using dichromate, since an acid interferes with later observations.
- 💡One well-chosen test usually earns full marks; a list of every test you know does not.
- 💡When asked to spot mistakes in a method, check the temperature of each step first; that is where errors are usually hidden.
- 💡Give the corrected instruction as well as naming the mistake.
- 💡Carry the safety reason with you: flammable liquids mean a water bath, not a flame.
Common Mistakes
- testing the gas from a carbonate with a lighted splint instead of bubbling it through limewater
- cooling Fehling's solution instead of warming it, which AQA has flagged as a method error
- writing that bromine water "turns clear" rather than colourless
- giving the product formed instead of what would actually be observed
- choosing a test that gives the same result for both compounds, such as bromine water for two alkenes
- reporting only the positive tube and leaving the other unexplained
- using a test not listed in the specification, such as an iodoform reaction
- describing a colour change without saying which substance produced it
- heating a flammable organic liquid directly with a Bunsen flame
- cooling the Fehling's tube in iced water, the error AQA planted in a candidate method
- using a lighted splint on the gas from sodium hydrogencarbonate instead of limewater
- warming the bromine water test, which is carried out at room temperature