AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Students should be able to: identify the functional groups using reactions…
Full specification wording
Identification of functional groups by test-tube reactions: 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.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 2 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
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.
What earns the marks
- Choosing a test that gives different observations for the two compounds being distinguished.
- Naming the reagent fully, including acidified or ammoniacal where required.
- The observation with each compound, including the negative result.
- A conclusion that follows directly from the observations recorded.
Common mistakes and how to put them right
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.
Exam tips
- 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.
Try the lesson
The start of the Identification of functional groups by test-tube reactions: Students should be able to: identify the functional groups using reactions in the specification. lesson, made from checked teaching notes for this specification point.
Have a go first
A student warms an unknown liquid with sodium carbonate solution and observes steady effervescence. Which functional group does this test indicate is present?
Listen
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. A strong answer earns marks for these points. choosing a test that gives different observations for the two compounds being distinguished. naming the reagent fully, including acidified or ammoniacal where required. the observation with each compound, including the negative result. a conclusion that follows directly from the observations recorded. Exam tip: Set your answer out as a table: reagent, what you do, what you see with each compound, conclusion.
Key words
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planning a test sequence
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positive and negative results
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naming reagents fully
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conclusion from observation
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See every definition
- planning a test sequence
- Ordering a set of tests so that one result does not interfere with the next, testing for a carboxylic acid before using dichromate.
- positive and negative results
- Recording every result, including the ones where nothing happens, rather than reporting only the positive tube and leaving the other unexplained.
- naming reagents fully
- Giving the reagent's full name, including acidified or ammoniacal where required.
- conclusion from observation
- A conclusion that follows directly from the observations recorded and is justified by the observation that produced it, not asserted.
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