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)
Memory Aid
"POP = Hydrogen, RELIGHT = Oxygen, MILKY = CO₂, BLEACH = Chlorine" Or remember it as a story: "Hydrogen POPS, Oxygen RELIGHTS, CO₂ CLOUDS limewater, Chlorine BLEACHES" For the litmus confusion: "BLUE bleaches (chlorine), RED turns blue (ammonia)" — opposite colours, opposite results. Quick Check: A student bubbles a gas through limewater and the solution turns milky. Name the gas and write the ionic equation for the reaction. Show Answer The gas is carbon dioxide (CO₂). Equation: CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l)