Tests for Common Gases — AQA GCSE Chemistry
There are four standard laboratory tests for common gases that you must know.
The four gas tests
| Gas | Test | Positive result |
|---|---|---|
| Hydrogen | hold a lighted splint at the mouth of the tube | a squeaky pop |
| Oxygen | insert a glowing splint into the tube | the splint relights |
| Carbon dioxide | bubble the gas through limewater | limewater turns cloudy/milky |
| Chlorine | hold damp litmus paper in the gas | the paper is bleached white (turns red first if blue) |
Why these work
- Hydrogen is flammable and burns explosively in a small amount — the "pop".
- Oxygen supports combustion, so it relights a glowing splint.
- Carbon dioxide reacts with calcium hydroxide in limewater to form insoluble calcium carbonate (the cloudiness).
- Chlorine is a bleach, so it removes the colour from litmus paper.
Exam tips
- Memorise all four tests and their exact positive results.
- Hydrogen → squeaky pop; oxygen → relights a glowing splint.
- Carbon dioxide → limewater cloudy; chlorine → bleaches damp litmus.
- Use the correct splint state: lighted for hydrogen, glowing for oxygen.