Flame tests — AQA GCSE Chemistry
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Flame tests explained
Flame tests covers 4 specification points, including Flame tests can be used to identify some metal… and Lithium compounds result in a crimson flame….
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Open a point below for its explanation and revision guidance.
Flame tests can be used to identify some metal ions (cations).
A flame test identifies certain metal cations from the colour they give to a flame. A clean nichrome or platinum wire is dipped in concentrated hydrochloric acid, heated in a non-luminous flame until no colour is seen, then dipped in the sample and returned to the flame. The heat excites electrons in the metal ions; as they relax, light of a characteristic wavelength is emitted, giving the flame its colour. The method is quick and needs little sample, but it only works for some cations and cannot detect non-metal ions. A positive result is a distinctive flame colour that matches a known cation, so the test is a qualitative identification rather than a measurement of amount.
Lithium, sodium, potassium, calcium and copper compounds produce distinctive colours in flame tests: lithium compounds result in a crimson flame sodium compounds result in a yellow flame potassium compounds result in a lilac flame calcium compounds result in an orange-red flame copper compounds result in a green flame.
Each of these five metal cations gives a characteristic flame colour when its compound is heated in a flame test. Lithium compounds give a crimson flame, sodium compounds a yellow flame, potassium compounds a lilac flame, calcium compounds an orange-red flame and copper compounds a green flame. To observe them, clean a nichrome or platinum wire in concentrated hydrochloric acid, heat it until the flame is colourless, dip it in the sample and hold it in the hot part of a non-luminous flame. The colour comes from light emitted by excited metal ions as they return to lower energy levels. Sodium yellow is intense and easily masks other colours, so careful cleaning and viewing through a blue glass can help when potassium is suspected.
If a sample containing a mixture of ions is used some flame colours can be masked.
When a sample contains more than one metal cation, the flame test can give a misleading result because one colour may hide another. Sodium is the common example: its intense yellow emission can dominate the flame and mask the lilac of potassium or the crimson of lithium. Masking happens because the eye sees the brightest colour, and the weaker emission from the other cation is not visible against it. To reduce the problem of masking in a mixture, a blue glass can be placed between the eye and the flame to absorb yellow light so that a lilac colour becomes visible, or a spectroscope can be used. A flame test alone cannot prove which cations are present in a mixture. Note that masking within a mixture is different from contamination, which is prevented by cleaning the wire between tests.
Students should be able to identify species from the results of the tests in 4.8.3.1 to 4.8.3.5.
This outcome asks you to work backwards from observations to identify an unknown species. The tests in 4.8.3.1 to 4.8.3.5 cover flame tests for metal cations, sodium hydroxide precipitates for metal cations, carbonate identification with dilute acid, sulfate identification with barium chloride, and halide identification with silver nitrate. For example, a white precipitate with sodium hydroxide that dissolves in excess means Al³⁺; a cream precipitate with silver nitrate means Br⁻. You must combine evidence from several tests, because one observation alone may fit more than one species. State the species with its charge, such as Cu²⁺ or SO₄²⁻, and link each observation to the test that produced it.
Your focus
- Describe how to carry out a flame test using a clean wire and concentrated hydrochloric acid.
- Relate a named flame colour to the cation responsible for it.
- Explain why flame tests identify only some cations and how contamination affects results.
Show all 12 objectives
- Recall the flame colour produced by lithium, sodium, potassium, calcium and copper compounds.
- Use observed flame colours to identify an unknown metal cation.
- Explain why intense sodium yellow can mask other flame colours and how this is managed.
- Explain why one flame colour can mask another in a mixture of cations.
- Describe a practical measure that reduces masking, such as using a blue glass.
- Evaluate the reliability of a flame test for identifying cations in a mixture.
- Match a stated observation from a named test to the correct species.
- Combine results from more than one test to identify an unknown species.
- Write identified ions with correct symbols and charges.
Flame tests exam tips
Marking Points
- States that a clean wire, such as nichrome or platinum, is used to hold the sample in the flame.
- Describes cleaning the wire with concentrated hydrochloric acid and heating until no colour is seen, to avoid contamination.
- Explains that the characteristic flame colour is produced by the metal cation in the sample.
- Explains that heating excites electrons and emitted light gives the observed colour.
- States that the test identifies some metal cations but not non-metal ions or all metal ions.
- Links a named flame colour to a specific cation to identify it.
- Recalls the flame colour for each cation: lithium crimson, sodium yellow, potassium lilac, calcium orange-red and copper green.
- Describes the procedure of cleaning the wire, dipping it in the sample and holding it in a non-luminous flame.
- Explains that the colour is characteristic of the metal cation present in the compound.
- Uses the observed colour to identify an unknown cation by comparison with the known colours.
- Recognises that sodium yellow is intense and can mask other flame colours.
- States that the test is qualitative and does not measure the amount of the cation.
- States that a mixture of cations can give a flame colour that hides the colour of another cation.
- Identifies sodium yellow as a common colour that masks other flame colours such as potassium lilac.
- Explains that the brightest emission dominates what is seen, so the weaker colour is not observed.
- Suggests a control measure for masking such as using a blue glass to filter yellow light or using a spectroscope.
- Concludes that a flame test alone may not identify all cations in a mixture and that further chemical tests may be needed.
- Distinguishes masking by another ion in a mixture from contamination caused by a dirty wire.
- Select the correct test for the target species, for example a flame test for a metal cation or silver nitrate for a halide.
- Describe the observation precisely, including colour and whether a precipitate forms or dissolves in excess.
- Match the observation to the species, such as lilac flame for K⁺ or white precipitate soluble in excess for Al³⁺.
- Use results from more than one test where a single observation is ambiguous, and reject species that conflict with any result.
- Name the species with correct charge notation, for example Fe²⁺, Fe³⁺, CO₃²⁻ or SO₄²⁻.
Examiner Tips
- 💡Describe the cleaning step before the test, because it is often worth a marking point.
- 💡Use the phrase characteristic flame colour and name the cation it indicates.
- 💡State one limitation of flame tests, such as masking by other ions or inability to detect non-metal ions.
- 💡Learn the five colour-cation pairs as a table and recall them without prompts.
- 💡When identifying an unknown, state the colour observed and then name the cation it indicates.
- 💡Mention that a blue glass may be used to filter out intense sodium yellow when testing for potassium.
- 💡Use the word masked and give sodium yellow hiding potassium lilac as the standard example.
- 💡Suggest a practical measure such as a blue glass to view potassium in the presence of sodium.
- 💡End with a limitation statement: a flame test on a mixture may not identify every cation present.
- 💡Underline the key observation in the question before choosing a species, so colour and solubility are not overlooked.
- 💡When two species give similar results, write both and then eliminate one using a second test result.
- 💡Give the species symbol and charge, not just the element name, to show the ion has been identified.
Common Mistakes
- Using the same dirty wire for several samples: the error is contamination that gives a misleading colour, and the correction is to clean the wire with acid and heat it until the flame is colourless before each test.
- Claiming the test identifies any ion: the error is overgeneralising, and the correction is to state that flame tests identify only some metal cations.
- Treating the flame colour as a quantitative measure: the error is assuming the colour shows how much ion is present, and the correction is to describe the test as qualitative identification only.
- Mixing up potassium lilac with lithium crimson: the error is swapping two similar-looking colours, and the correction is to learn the pairs as potassium lilac and lithium crimson.
- Calling the sodium flame orange or golden: the error is an imprecise colour name, and the correction is to use the accepted term yellow.
- Describing copper as blue-green or calcium as red: the error is using non-standard colour names, and the correction is to use green for copper and orange-red for calcium.
- Assuming the flame colour shows every cation present: the error is treating one colour as complete identification, and the correction is to state that some colours may be masked and further tests may be needed.
- Confusing masking with contamination: the error is suggesting that cleaning the wire reduces masking in a mixture, and the correction is to distinguish an impurity on the wire from a second cation in the sample itself.
- Ignoring the problem and reporting only the dominant colour: the error is an incomplete identification, and the correction is to note that a masked cation may still be present.
- Confusing the white precipitates of Al³⁺, Ca²⁺ and Mg²⁺; correct this by adding excess sodium hydroxide, because only aluminium hydroxide dissolves.
- Treating a flame colour as proof of a mixture without checking other tests; correct this by using the full set of results before naming the species.
- Writing ion names without charge, such as saying aluminium rather than Al³⁺; correct this by including the charge in the identified species.