Tests for common gases
Four gases come up over and over again in GCSE Chemistry: hydrogen, oxygen, carbon dioxide, chlorine. You must know each test, what to do, and what you should observe.
Hydrogen — squeaky pop
Test: light a wooden splint and place it at the mouth of a test tube containing the gas.
Observation: A squeaky pop sound (rapid combustion of H₂ + O₂ in air → H₂O).
Equation: 2H₂ + O₂ → 2H₂O.
Hydrogen is produced when reactive metals react with acids (Mg + 2HCl → MgCl₂ + H₂).
Oxygen — relighting glowing splint
Test: insert a glowing wooden splint (no flame, just glowing red ember) into the gas.
Observation: The splint relights with a flame.
This relies on the fact that oxygen supports combustion. (Air alone won't relight the splint.)
Oxygen is produced by the decomposition of hydrogen peroxide: 2H₂O₂ →(MnO₂) 2H₂O + O₂ (a classic GCSE demo).
Carbon dioxide — limewater test
Test: bubble the gas through limewater (saturated calcium hydroxide solution).
Observation: Limewater turns cloudy/milky white.
Equation: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O.
The white cloudiness is calcium carbonate (insoluble). With excess CO₂, the cloudiness clears as soluble Ca(HCO₃)₂ forms — but this isn't usually shown at GCSE.
Chlorine — damp blue litmus paper
Test: hold a piece of damp blue litmus paper in the gas.
Observation: The litmus paper turns red, then bleached white.
Why: Cl₂ reacts with water to form HCl + HOCl (hypochlorous acid), which is acidic AND bleaches the dye.
Summary table
| Gas | Test | Observation |
|---|---|---|
| H₂ | Lit splint | Squeaky pop |
| O₂ | Glowing splint | Relights |
| CO₂ | Limewater | Turns cloudy |
| Cl₂ | Damp blue litmus | Turns red then bleached |
✦Worked example— Worked example — identify a gas
A gas is collected from heating CaCO₃. Limewater turns cloudy. What is the gas?
CaCO₃ → CaO + CO₂. Cloudy limewater = positive test for CO₂. The gas is CO₂.
⚠Common mistakes
- Using a flaming splint for O₂ — must be a glowing splint (no flame).
- Using dry litmus paper for chlorine — must be damp.
- Forgetting "squeaky" pop — students sometimes write "popping noise"; mark schemes want "squeaky pop".
- Confusing "relights" with "burns brighter" — for the oxygen test, "relights" is the key word.
Links
Used throughout the course (testing for products of reactions). Builds on C4.4 (acid + carbonate gives CO₂) and C8.2.
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