Case Closed

Case Closed: the identifying ions required practical, hands on

This free game is part of our AQA GCSE Chemistry (8462) course for exams in EXAM YEARS 2027-2029.

Identifying ions for AQA GCSE Chemistry

Identifying the ions in an unknown ionic compound is required practical 7 in AQA GCSE Chemistry (8462). It brings together the flame tests, the sodium hydroxide test for metal ions, and the tests for carbonate, halide and sulfate ions (spec 4.8.3.1 to 4.8.3.5). In a textbook it is a table of colours. In the exam it is a set of results you have to turn into the name of a compound, with the evidence for each ion.

Case Closed puts you on the night shift in a detective's Chemical Evidence Unit. Every case is an unknown solid made of one positive ion and one negative ion. You run the tests yourself, watch the results happen, and then accuse the metal ion and the negative ion on a suspect board. The one rule is no proof, no case: if your results still fit two ions, the accusation is "not proven" and Inspector Morgan tells you which test would tell them apart.

Flame tests and sodium hydroxide

For the flame test you open the Bunsen burner's air hole for the hot blue flame, then hold the wire loop in it. Lithium compounds give a crimson flame, sodium yellow, potassium lilac, calcium orange-red and copper green. From level 3 the loop has been used, so you clean it first: an old sample of sodium on the wire flashes yellow, which shows how one flame colour can mask another in a mixture of ions. Test in the yellow safety flame and the colour is hidden.

With sodium hydroxide solution you hold to add drops and watch the precipitate form where they land. Copper(II) gives a blue precipitate, iron(II) green and iron(III) brown. Aluminium, calcium and magnesium all give a white precipitate, so you keep adding sodium hydroxide until it is in excess: only aluminium hydroxide dissolves. To tell calcium from magnesium you need the flame test as well. You also balance the equations for the insoluble hydroxides, such as CuSO₄ + 2NaOH → Cu(OH)₂ + Na₂SO₄.

Carbonates, halides and sulfates

Add dilute hydrochloric acid to a carbonate and it fizzes; the gas bubbles through limewater, which turns milky because the gas is carbon dioxide. For halides you choose the acid first. Dilute nitric acid then silver nitrate gives a white precipitate with chloride, cream with bromide and yellow with iodide. Pick hydrochloric acid instead and you get a white precipitate you didn't earn, because the acid itself contains chloride ions. For sulfates you add dilute hydrochloric acid then barium chloride: a white precipitate means sulfate ions. Dilute sulfuric acid would add sulfate ions and give a false result.

The practical details are built in: a fresh sample in a clean tube for every test, dissolving the solid in distilled water first, writing colourless rather than "clear", and what to do when a solid such as calcium carbonate won't dissolve (do the flame test and the acid test on the solid itself).

Five levels, about 8 minutes

The game starts with the lesson's own worked examples: potassium carbonate and copper(II) sulfate, with crib notes on the suspects. Then come the white precipitates (aluminium bromide and magnesium iodide), a clean technique level with a dirty loop and acid traps, and a set of photo files where you read a trainee's photos of flames and precipitates and name the compound, including the lesson's exam question on calcium chloride. You also put the flame test method in order, as in a 3-mark exam answer. It ends with a 75-second rush of unknown solids.

After every case a "case closed" card shows the evidence written the way examiners want it: each ion named, with the result that proves it. A game takes about 8 minutes, works on a computer, tablet or phone, and saves your best score.