This topic summary covers Knowledge Organiser: Gas Tests 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 13 of 13 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Knowledge Organiser: Gas Tests
Hydrogen
- Test: burning (lit) splint held to mouth of tube
- Result: squeaky pop (H₂ ignites explosively)
- Equation: 2H₂ + O₂ → 2H₂O
- Note: BURNING splint — NOT glowing (that is oxygen)
Oxygen
- Test: glowing splint
- Result: splint relights
- Explanation: supports combustion
- Note: GLOWING not burning
Carbon Dioxide
- Test: bubble through limewater
- Result: turns milky/cloudy
- Product: CaCO₃ (white precipitate)
- Excess CO₂ clears the solution
Chlorine and Ammonia
- Cl₂: damp BLUE litmus → bleaches white
- NH₃: damp RED litmus → turns blue
- NH₃ also: white fumes with HCl rod
- Both need DAMP litmus
Key Equations
- 2H₂ + O₂ → 2H₂O (hydrogen combustion — squeaky pop)
- CO₂ + Ca(OH)₂ → CaCO₃ + H₂O (CO₂ turns limewater milky)
- NH₃ + HCl → NH₄Cl (white fumes of ammonium chloride)
Common Mistakes
- Using a burning splint for oxygen: The oxygen test uses a GLOWING (not burning) splint — a glowing splint relights in oxygen
- Saying limewater turns white for any gas: Only CO₂ turns limewater milky/cloudy — other gases do not react with Ca(OH)₂
- Using dry litmus for chlorine/ammonia: Both tests require DAMP litmus paper — dry litmus does not respond to these gases
- Confusing chlorine and ammonia litmus tests: Chlorine bleaches litmus (any colour → white); ammonia turns red litmus → blue (alkaline)
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 oxygen gas (O₂)?
Use a glowing splint (blow out flame first). POSITIVE RESULT: Glowing splint relights/bursts into flame (O₂ supports combustion)
Test for hydrogen gas (H₂)?
Use a burning splint. POSITIVE RESULT: 'Squeaky pop' sound (H₂ burns rapidly: 2H₂ + O₂ → 2H₂O)