8.3.6Chemical Analysis
Required Practical- Identifying Ions
5 min read 4 parts Lesson 15 of 17 Unit quiz
Section 1 of 4Cation Tests
The flame test and the hydroxide test help to identify the metal ion in a sample.
Flame test
- Flame test Dip a clean nichrome wire into the sample and place it in a hot, blue flame;
- lithium gives a crimson flame and sodium gives a yellow flame.
Potassium
- Potassium Potassium ions give a lilac flame, which is pale compared with the strong yellow of sodium.
Copper
- Copper Copper ions give a blue-green flame, which is different from the colours of the alkali metals.
Cleaning the wire
- Cleaning the wire The wire is cleaned in dilute hydrochloric acid and rinsed before each test,
- so no colour from an earlier sample contaminates the result.
Section 2 of 4Anion Tests
Each anion test uses a specific reagent and gives a characteristic observation.
1
Carbonate ions: add dilute acid and pass the gas through limewater, which turns cloudy.
2
Halide ions: add dilute nitric acid, then silver nitrate, to form a precipitate whose colour identifies the halide.
3
Sulfate ions: add dilute hydrochloric acid, then barium chloride solution, to form a white precipitate.
| Anion | Reagents | Observation |
|---|---|---|
| Carbonate, CO₃²⁻ | Dilute acid, then limewater | Gas turns limewater cloudy |
| Chloride, Cl⁻ | Dilute nitric acid, then silver nitrate | White precipitate |
| Bromide, Br⁻ | Dilute nitric acid, then silver nitrate | Cream precipitate |
| Iodide, I⁻ | Dilute nitric acid, then silver nitrate | Yellow precipitate |
| Sulfate, SO₄²⁻ | Dilute hydrochloric acid, then barium chloride | White precipitate |
Section 3 of 4Key Equations
Writing ionic equations for the tests shows which ions are responsible for each observation.
Key Equations
Silver halide
- Silver halide Ag⁺(aq) + Cl⁻(aq) → AgCl(s) gives the white precipitate of silver chloride.
Barium sulfate
- Barium sulfate Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) gives the white precipitate of barium sulfate.
Carbonate
- Carbonate CO₃²⁻(aq) + 2H⁺(aq) → CO₂(g) + H₂O(l) releases carbon dioxide gas.
Limewater
- Limewater CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l) makes the limewater cloudy.
Section 4 of 4Exam Checklist & Tips
8.3.6 – Key Points for the Exam
- Flame test A test that identifies a metal ion from the colour it gives to a flame.
- Cation A positively charged ion.
- Anion A negatively charged ion.
- Limewater Aqueous calcium hydroxide, which turns cloudy in the presence of carbon dioxide.
- Precipitate An insoluble solid formed during a reaction between two solutions.
Common Mistakes
- Forgetting to clean the wire between tests, so a colour from a previous sample contaminates the flame result.
- Using hydrochloric acid before silver nitrate when testing for halides;
- chloride ions from the acid would give a false positive for chloride.
- Confusing the sulfate test with the halide test and using silver nitrate where barium chloride is needed.
- Writing carbon dioxide as a liquid in the carbonate equation;
- it is a gas, so (g) is the correct state symbol.