Identification of Gases
Section: 12. Experimental Techniques & Chemical Analysis | Syllabus: Cambridge IGCSE Chemistry (0620)
Tests for Gases
| Gas | Test | Positive Result |
|---|---|---|
| Ammonia, NH₃ | Hold damp red litmus paper in the gas | Turns blue |
| Carbon dioxide, CO₂ | Bubble through limewater | Limewater turns milky |
| Chlorine, Cl₂ | Hold damp litmus paper in the gas | Litmus is bleached (turns white) |
| Hydrogen, H₂ | Hold a lighted splint at the mouth of the tube | Burns with a squeaky pop |
| Oxygen, O₂ | Hold a glowing splint in the gas | Relights the glowing splint |
| Sulfur dioxide, SO₂ | Pass into acidified aqueous potassium manganate(VII) | Purple potassium manganate(VII) is decolourised |
Ammonia and Chlorine
Ammonia is alkaline: it turns damp red litmus paper blue, and has a sharp, pungent smell.
Chlorine is a pale yellow-green gas that acts as a bleach: damp litmus paper placed in chlorine is bleached white (it may first turn red, since chlorine dissolves in the water on the paper to form an acidic solution, before being bleached).
Carbon Dioxide
Carbon dioxide is tested by bubbling it through limewater (aqueous calcium hydroxide). If CO₂ is present, the limewater turns milky as an insoluble white precipitate of calcium carbonate forms.
CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l)
Worked Example
- Question (based on a real exam question): a gas produced in a reaction turns limewater milky. Identify the gas.
- Answer: carbon dioxide, CO₂ - this is the standard confirmatory test for the gas.
Hydrogen and Oxygen
Hydrogen is tested with a lighted splint held at the mouth of the test tube: it burns rapidly with a characteristic "squeaky pop".
Oxygen is tested with a glowing (not flaming) splint inserted into the gas: because oxygen supports combustion, the splint relights.
Sulfur Dioxide
Sulfur dioxide is a reducing agent, so passing it into acidified aqueous potassium manganate(VII) reduces the purple manganate(VII) ion, decolourising the solution - the same colour-change principle used to test for sulfite ions in solution.
Common Mistakes
- ❌ Using a dry piece of litmus paper for the chlorine or ammonia test
- ✓ the paper must be damp - both tests rely on the gas first dissolving in the water on the paper
- ❌ Using a flaming splint to test for oxygen
- ✓ the splint must only be glowing (blown out first) - a still-flaming splint already has enough oxygen from the air, so relighting cannot be observed
- ❌ Confusing the sulfur dioxide test with the carbon dioxide test
- ✓ CO₂ is tested with limewater (turns milky); SO₂ is tested with acidified potassium manganate(VII) (turns from purple to colourless)
Interactive revision notes, videos and practice questions load below.