GCSE Chemistry: Tests for Ions and Flame Emission Spectroscopy

Identifying the ions in an unknown compound is one of the most practical parts of separate GCSE Chemistry. It is pure recall on the surface — a flame colour here, a precipitate colour there — but questions usually combine two or three tests, ask why each reagent is added in a particular order, or ask you to interpret a spectrum. This material covers the ion tests; the gas tests, chromatography and formulations are practised in the category's chemical analysis material.

The quiz starts with flame tests: lithium crimson, sodium yellow, potassium lilac, calcium orange-red and copper green. Sodium hydroxide solution is the next test for metal ions: aluminium, calcium and magnesium ions give white precipitates, and you learn to tell aluminium apart because only its hydroxide dissolves in excess sodium hydroxide; copper(II), iron(II) and iron(III) give blue, green and brown precipitates. Two results are then combined to name a metal ion.

For the negative ions you test a carbonate with dilute acid and limewater, a halide with dilute nitric acid and silver nitrate (white chloride, cream bromide, yellow iodide), and a sulfate with dilute hydrochloric acid and barium chloride. Two questions ask about the reasoning behind the method: why an acid is added first, and why hydrochloric acid must never be used before silver nitrate. A combined question asks you to name a salt from a flame colour and a precipitate.

The last questions move to instrumental analysis. Instrumental methods are accurate, sensitive and rapid, and in flame emission spectroscopy the light from a sample in a flame is passed through a spectroscope to give a line spectrum. You match a sample's lines against reference spectra given in the question to find which metal ions a mixture contains — something a flame test cannot do reliably, because one strong colour can hide another.

The flashcards give every test with its positive result, ready to learn by heart. In the oral exam an examiner asks one question at a time — how you would carry out a test, what you would see, how you would identify an unknown salt — and gives brief, precise feedback at the end. The printable written work has eight longer tasks answered by hand: describing tests with their results, planning how to identify unknown salts, explaining steps of the method and comparing a flame test with flame emission spectroscopy, each checked against a model answer.

The content is based on the chemical analysis section of the DfE GCSE chemistry subject content; for example, AQA treats the identification of ions and flame emission spectroscopy as chemistry-only content, with the ion tests as one of its required practicals.

  • Recall the flame test colours of lithium, sodium, potassium, calcium and copper ions
  • Identify aluminium, calcium, magnesium, copper(II), iron(II) and iron(III) ions using sodium hydroxide solution
  • Describe the tests for carbonate, halide and sulfate ions and their positive results
  • Explain why dilute acid is added first and which acid must be used in each test
  • Identify an unknown ionic compound from the results of several tests
  • Describe flame emission spectroscopy and interpret a line spectrum against reference spectra
  • State the advantages of instrumental methods over chemical tests

Practice material written by Zestly, based on the DfE GCSE chemistry subject content (chemical analysis: identification of ions by chemical and spectroscopic means); chemistry-only placement given as an example from the AQA GCSE Chemistry (8462) specification. Spectral line positions in the questions are rounded values chosen for practice.

Sample question

Line spectra on a wavelength scale marked from 400 to 800 nm. Sample: lines at 589, 610 and 671 nm. Reference spectra: lithium 610 and 671 nm; sodium 589 nm; potassium 405 and 767 nm; calcium 423 and 616 nm.

In flame emission spectroscopy a sample gives lines at 589 nm, 610 nm and 671 nm. Reference line spectra (rounded): lithium 610 nm and 671 nm; sodium 589 nm; potassium 405 nm and 767 nm; calcium 423 nm and 616 nm. Which metal ions are present in the sample?

See the answer

Lithium and sodium

Each metal ion gives its own set of lines. The 589 nm line matches sodium, and the 610 nm and 671 nm lines together match lithium. Calcium would also need a line at 423 nm and potassium lines at 405 nm and 767 nm, which are absent. Unlike a flame test, where the strong yellow of sodium can mask other colours, the line spectrum identifies both ions in the mixture.

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