AQA GCSE Chemistry · Chemical AnalysisTriple only
Identifying ions required practical
Use chemical tests to identify the ions in unknown single ionic compounds.
Flame tests and sodium hydroxide identify the metal (positive) ion. Tests with acid, silver nitrate and barium chloride identify the negative ion. Put the two together to name the compound.
This one is only on the separate Chemistry GCSE, so it won’t come up in Combined Science.
Equipment
- Nichrome wire loop and a Bunsen burner
- Dilute hydrochloric acid (to clean the loop)
- Sodium hydroxide solution and dropping pipettes
- Dilute nitric acid and silver nitrate solution
- Dilute hydrochloric acid and barium chloride solution
- Limewater and test tubes
Variables
- Independent you change
- The unknown compound
- Dependent you measure
- The flame colour, precipitate colour or gas produced
- Control you keep the same
- Clean wire loop for every flame test
- Small amounts of each reagent
- Fresh sample for each test
Method
- 1
Flame test: dip a clean nichrome loop in acid, then in the solid, and hold it in a blue Bunsen flame. Note the colour.
- 2
Hydroxide test: add a few drops of sodium hydroxide solution to a solution of the compound. Note the colour of any precipitate.
- 3
Carbonate test: add dilute acid. If it fizzes, bubble the gas through limewater: milky means carbon dioxide.
- 4
Halide test: add dilute nitric acid, then silver nitrate. White = chloride, cream = bromide, yellow = iodide.
- 5
Sulfate test: add dilute hydrochloric acid, then barium chloride. A white precipitate means sulfate.
- 6
Combine the positive and negative ion results to name the compound.
Worked example
Results for four unknown compounds.
| Compound | Positive ion test | Negative ion test | Compound is |
|---|---|---|---|
| A | Lilac flame | Cream ppt with silver nitrate | Potassium bromide |
| B | Blue ppt with NaOH | White ppt with barium chloride | Copper(II) sulfate |
| C | Green ppt with NaOH | Fizzes with acid; limewater milky | Iron(II) carbonate |
| D | Crimson flame | White ppt with silver nitrate | Lithium chloride |
Working
Flames: Li⁺ crimson · Na⁺ yellow · K⁺ lilac · Ca²⁺ orange-red · Cu²⁺ green
NaOH: Cu²⁺ blue · Fe²⁺ green · Fe³⁺ brown · Al³⁺, Ca²⁺, Mg²⁺ white (Al³⁺ dissolves in excess)
Conclusion
Each compound is identified from one positive-ion test and one negative-ion test. Acid is added before silver nitrate or barium chloride to remove carbonate ions, which would otherwise also give a precipitate.
Risks
- Bunsen flame and hot wireTie hair back and let the loop cool.
- Sodium hydroxide and acids are corrosive or irritantWear eye protection and wash off splashes.
- Barium chloride is toxicUse small amounts and wash hands afterwards.
Common mistakes
- Not cleaning the loop. Leftover compound from the last test changes the flame colour.
- Using hydrochloric acid in the halide test. It adds chloride ions, giving a false white precipitate; use nitric acid.
- Using sulfuric acid in the sulfate test. It adds sulfate ions; use hydrochloric acid.
Better results
- Use a flame emission spectroscope to identify metal ions more reliably, even in mixtures.
- Add sodium hydroxide drop by drop, then in excess, to tell aluminium (dissolves) from calcium and magnesium.
- Look at the flame through blue glass to see a lilac potassium flame next to sodium’s yellow.
Launching November 2026
7 exam questions on this practical.
Practise the method, the variables, the maths and what the results mean, then revise the Chemical Analysis flashcards. It opens to students soon: join the waitlist and we’ll email you once, when it does.