Required Practical: Tests for Gases, Ions and Water — AQA GCSE Chemistry
This required practical uses chemical tests to identify gases, ions and water in samples (often called "identifying ions by tests").
Tests for gases
| Gas | Test | Result |
|---|---|---|
| Hydrogen | lighted splint | squeaky pop |
| Oxygen | glowing splint | relights |
| Carbon dioxide | limewater | turns cloudy |
| Chlorine | damp litmus paper | bleached white |
Tests for ions (Separate)
- Carbonate ions (CO₃²⁻): add dilute acid → fizzing, and the gas turns limewater cloudy (CO₂).
- Sulfate ions (SO₄²⁻): add dilute hydrochloric acid then barium chloride → a white precipitate.
- Halide ions (Cl⁻, Br⁻, I⁻): add dilute nitric acid then silver nitrate → a precipitate (chloride = white, bromide = cream, iodide = yellow).
- Metal ions: flame tests and sodium hydroxide precipitates (see flame test notes).
Test for water
- Chemical test: anhydrous copper sulfate turns from white to blue in the presence of water.
- Test for pure water: it boils at exactly 100 °C and freezes at 0 °C.
Exam tips
- Learn the four gas tests and their results.
- Anion tests: carbonate → fizz/CO₂; sulfate → barium chloride (white ppt); halides → silver nitrate (white/cream/yellow).
- Add dilute acid first in sulfate and halide tests (to remove carbonate).
- Water turns anhydrous copper sulfate blue.