This comparison covers All Gas Tests: Quick Reference within Gas Tests for GCSE Chemistry. Revise Gas Tests in Chemical Analysis for GCSE Chemistry with 20 exam-style questions and 15 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 6 of 13 in this topic. Use this comparison to connect the idea to the wider topic before moving on to questions and flashcards.
⚖️ All Gas Tests: Quick Reference
| Gas | Test Method | Positive Result | Equation |
|---|---|---|---|
| H₂ | Burning splint | Squeaky pop sound | 2H₂ + O₂ → 2H₂O |
| O₂ | Glowing splint | Splint relights | Supports combustion |
| CO₂ | Limewater | Turns milky/cloudy | CO₂ + Ca(OH)₂ → CaCO₃ + H₂O |
| Cl₂ | Damp blue litmus | Bleached white | Cl₂ + H₂O → HClO + HCl |
| NH₃ | Damp red litmus OR HCl rod | Turns blue OR white fumes | NH₃ + HCl → NH₄Cl |
Gas tests: exact wording, distractors and fail conditions
The wording examiners want, what each test is confused with, and when it fails.
- Hydrogen (squeaky pop)Exact wording for the mark: 'lighted/burning splint makes a squeaky pop'. Confused with: the oxygen test — students write 'relights' for hydrogen by mistake. Fails if: the splint is only glowing, not lit — hydrogen needs a flame to ignite.
- Oxygen (relights glowing splint)Exact wording: 'glowing splint relights'. Confused with: a lit splint burning normally in air, which proves nothing. Fails if: the splint has gone out completely — it must still be glowing.
- Carbon dioxide (limewater)Exact wording: 'limewater turns milky/cloudy' as calcium carbonate precipitates. Fails if: too much CO2 is bubbled through for too long — excess CO2 reacts further to form soluble calcium hydrogencarbonate and the limewater clears again.
- Chlorine (bleaches litmus)Exact wording: 'damp blue litmus paper is bleached/turns white'. Confused with: an acidic gas like HCl, which turns litmus red but does NOT bleach it. Fails if: the litmus is dry — chlorine only bleaches once dissolved in the water on the paper.
- Ammonia (turns litmus blue)Exact wording: 'damp red litmus paper turns blue'. Fails if: the litmus is dry — ammonia needs the water on the paper to form the alkaline solution that causes the colour change.
- Splint state matters more than students think'Lit' and 'glowing' are not interchangeable in mark schemes. A LIT splint (with a flame) tests hydrogen; a GLOWING splint (an ember, no flame) tests oxygen. Writing 'glowing' when you mean 'lit' is marked wrong even if the rest is correct.
Practice questions for Gas Tests
Which observation confirms a gas is hydrogen when tested with a burning splint?
Explain why hydrogen gas produces a squeaky pop when tested with a burning splint.
Quick recall flashcards
Test for hydrogen gas (H₂)?
Use a burning splint. POSITIVE RESULT: 'Squeaky pop' sound (H₂ burns rapidly: 2H₂ + O₂ → 2H₂O)
Test for oxygen gas (O₂)?
Use a glowing splint (blow out flame first). POSITIVE RESULT: Glowing splint relights/bursts into flame (O₂ supports combustion)