Tests for gases
The four gas tests
| Gas | Test | Positive result |
|---|---|---|
| Hydrogen | Hold a burning splint at the mouth of the test tube | It burns with a |
| Oxygen | Put a into the test tube | The splint relights |
| Bubble the gas through (calcium hydroxide solution) | The limewater turns milky (cloudy) | |
| Chlorine | Hold damp litmus paper in the gas | The paper is bleached white |
Blue litmus turns red first, because chlorine is acidic, and is then bleached.
Key terms
Why the tests work
- Hydrogen burns rapidly in air with a pop: it reacts with oxygen to make water.
- Oxygen helps things burn, so it makes a glowing splint burst back into flame.
- reacts with limewater to form calcium carbonate, an insoluble white solid that makes it cloudy.
- Chlorine is a , so it removes the colour from damp litmus paper.
Chlorine is toxic, so the chlorine test should be done in a fume cupboard.
Key terms
Where each gas comes from
Exam questions often describe a reaction and ask you to identify the gas it makes:
| Reaction | Gas made |
|---|---|
| A metal such as magnesium with an acid | Hydrogen |
| Hydrogen peroxide decomposing, with a catalyst | Oxygen |
| An acid with a carbonate, such as calcium carbonate | Carbon dioxide |
| Electrolysis of sodium chloride solution, at the anode | Chlorine |
The limewater test, as a symbol equation: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. The calcium carbonate formed is the white solid that makes it cloudy.
Writing a full answer
A test answer needs both parts: what you do, and what you see if the gas is there.
- Hydrogen: hold a lighted splint at the mouth of the tube; it burns with a .
- Oxygen: put a glowing splint into the tube; it relights.
The two splints are easy to swap. Oxygen needs a glowing splint, because it makes the splint burst back into flame. A lighted splint would already be burning, so you'd see nothing new.
Exam tip
'It pops' or 'the limewater changes' won't get the mark. Give the exact result: a squeaky pop, or limewater turning milky.