Testing for Anions T
With the cation found, the other half of a compound is the negative ion (anion). Three to know: carbonates, halides and sulfates. Each has its own reagent and its own unmistakable result.
| Anion | Add this… | Positive result |
|---|---|---|
| Carbonate (CO32–) | dilute acid | fizzes; the gas (CO2) turns limewater milky |
| Halide (Cl–, Br–, I–) | dilute nitric acid, then silver nitrate | silver halide precipitate — white (Cl), cream (Br) or yellow (I) |
| Sulfate (SO42–) | dilute hydrochloric acid, then barium chloride | white precipitate (barium sulfate) |
The silver halides step through white → cream → yellow as you go down the group (Cl → Br → I). Barium sulfate is a separate white precipitate, made with barium chloride.
- Carbonate: CO32–(aq) + 2H+(aq) → CO2(g) + H2O(l) — then CO2(g) + Ca(OH)2(aq) → CaCO3(s) + H2O(l) (the white solid that turns limewater milky).
- Halide: Ag+(aq) + X–(aq) → AgX(s) — e.g. Ag+ + Cl– → AgCl.
- Sulfate: Ba2+(aq) + SO42–(aq) → BaSO4(s).
Carbonate ions would also form a precipitate with silver nitrate or barium chloride, giving a false positive. Adding acid first reacts away any carbonate (it fizzes off as CO2) so it can’t interfere. The catch is choosing an acid that doesn’t add the very ion you’re testing for:
- Halide test → nitric acid (not hydrochloric — HCl would add chloride ions and give a false positive).
- Sulfate test → hydrochloric acid (not sulfuric — that would add sulfate ions).
Required practical 7: identifying the ions in an unknown salt T
Use chemical tests to identify the cation and the anion in unknown single ionic compounds, covering all the tests in sections 5–7. The skill is to combine them: one test names the metal, another names the non-metal, and together they name the salt.
- If the sample is solid, dissolve a little in distilled water. Work tidily — you’ll have several tubes and reagents, so a neat results table is essential.
- Find the cation: a flame test, and/or sodium hydroxide for the precipitate colour.
- Find the anion: carbonate (acid), then halide (nitric acid + silver nitrate) and/or sulfate (hydrochloric acid + barium chloride) as needed.
- You won’t need every test on every sample — choose the ones that narrow it down. Repeat any test that gives an unclear result.
This practical is largely qualitative — you record what you see rather than measure a quantity.
- Independent variable: which chemical test you apply (flame test, sodium hydroxide, carbonate, halide or sulfate test).
- Dependent variable (what you observe): the colour, precipitate or gas produced — the result that names the ion.
- Control variables: test a fresh separate portion of the sample for each test; acidify before the halide and sulfate precipitation tests to remove carbonate ions that would give a false positive.
A white solid gives these results:
| Test | Observation | Conclusion |
|---|---|---|
| Flame test | yellow flame | sodium, Na+ |
| Dilute nitric acid, then silver nitrate | cream precipitate | bromide, Br– |
Cation Na+ (1+) and anion Br– (1–) balance one-to-one, so the salt is sodium bromide, NaBr. (To get a formula, always balance the charges: a 2+ ion with a 2– ion is 1:1; a 2+ ion with 1– ions needs two of them, e.g. CaCl2.)
🧪 Identify the unknown salt
You’re handed an unknown ionic compound. Run bench tests to find its cation (metal ion) and anion (non-metal ion), then name it — in as few tests as you can. For each result, try to recall what it tells you before you tap Reveal. This is exactly the Required practical 7 skill.
Pick a test to start.
| Test | Observation | What does it tell you? |
|---|---|---|
| No tests run yet — click a test above. | ||
- Silver bromide is “cream”, not yellow. Yellow is silver iodide. Cream is the trickiest colour to name — learn the white → cream → yellow sequence.
- Wrong acid. Nitric acid for the halide test (HCl adds chloride); hydrochloric acid for the sulfate test (sulfuric adds sulfate).
- Forgetting to acidify. Skipping the acid lets carbonate ions give a false precipitate.
- Letting the gas escape. In the carbonate test, connect to the limewater quickly so the CO2 doesn’t escape before it can turn it milky.
🧪 Exam-style questions
Dilute nitric acid and silver nitrate solution are added to a salt solution. A cream precipitate forms. Which ion is present? Tick (✓) one box.
Describe how you would test a solution for sulfate ions, and give the result you would see.
Show answer
- Add dilute hydrochloric acid to the solution. 1 mark (Allow: any reference to acidifying first to remove carbonate ions.)
- Then add barium chloride solution. 1 mark
- A white precipitate (of barium sulfate) forms. 1 mark
Do not accept: dilute sulfuric acid (it adds sulfate ions), or silver nitrate (that is the halide test).
Why is dilute nitric acid added before the silver nitrate in the test for halide ions? Tick (✓) one box.
A salt gives no flame colour, a green precipitate with sodium hydroxide, and a white precipitate when dilute hydrochloric acid then barium chloride are added. What is the salt? Tick (✓) one box.
A chemist needs to find the concentration of a solution of barium hydroxide (an alkali). It could be found by one of two methods.
Method 1: an excess of sodium sulfate solution is added to 25 cm³ of the barium hydroxide solution; a precipitate of barium sulfate forms, which is filtered, dried and weighed; the concentration is calculated from the mass of barium sulfate produced.
Method 2: 25 cm³ of the barium hydroxide solution is titrated with hydrochloric acid of known concentration; the concentration is calculated from the titration result.
Compare the advantages and disadvantages of the two methods.
Show answer
Any five from the points below (1 mark each, maximum 5; converse arguments are allowed):
Method 1 — precipitate and weigh:
- weighing is accurate
- not all the barium sulfate may be precipitated (allow: not all the barium hydroxide has reacted)
- some precipitate may be lost (e.g. when filtering)
- the precipitate may not be (fully) dry
- it takes longer
- it requires energy (to dry the precipitate)
Method 2 — titration:
- it is accurate
- it works for low concentrations (allow: reliable / precise)
Source: AQA GCSE Chemistry.