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2.8.1Atoms, Elements and Compounds

Tests for Gases — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 18 of 26 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Part 1 of 5Gases Reacting with Test Materials

2.8.1 Tests for Gases

Each gas is identified by a specific test, and the observation must be stated precisely.

Ammonia
  • Damp red litmus paper turns blue, showing that the gas is alkaline.
Carbon dioxide
  • Bubbling the gas through limewater makes it turn cloudy white, due to insoluble calcium carbonate forming.
Chlorine
  • Damp blue litmus paper turns red and is then bleached white by the gas.
Hydrogen and oxygen
  • Hydrogen gives a squeaky pop with a lighted splint, while oxygen relights a glowing splint.
GasTestPositive result
AmmoniaDamp red litmus paperTurns blue
Carbon dioxideLimewaterTurns cloudy white
ChlorineDamp blue litmus paperTurns red, then bleached white
HydrogenLighted splint at the mouth of the tubeSqueaky pop
OxygenGlowing splint inside the tubeRelights

Part 2 of 5Procedure for Gas Tests

Careful technique prevents false positive results and gives clear observations.

1
For ammonia, hold the litmus paper near the mouth of the test tube, without touching the sides, to avoid contamination.
2
For carbon dioxide, pass the gas into limewater using a delivery tube and watch for cloudiness.
3
For hydrogen, hold a lighted splint at the open mouth of the tube, not inside it, because the gas needs air to burn.
4
For oxygen, insert a glowing splint into the tube, watching for it to relight.

Part 3 of 5Common Confusions

Common Confusions
Extinguishing a splint
  • Extinguishing a burning splint does not identify carbon dioxide, since nitrogen also does this.
Smell
  • A characteristic smell, such as the choking smell of chlorine, is not an acceptable test.
Ammonia and ammonium
  • Ammonia is the gas NH₃, while ammonium is the NH₄⁺ ion in solution.
Hydrogen and oxygen
  • A lighted splint has an H for hydrogen, and a glowing splint has an O for oxygen.

Part 4 of 5Worked Example

Question

  • A colourless gas turns limewater cloudy and extinguishes a lighted splint.
  • Identify the gas and describe the tests.

  1. Limewater turning cloudy shows the gas is carbon dioxide.
  2. Extinguishing a splint is not a definitive test, since nitrogen does this too, so it cannot confirm the identity.
  3. The definitive test is the limewater observation, so state that cloudiness confirms carbon dioxide.

Answer

  • Carbon dioxide, confirmed by limewater turning cloudy white.
  • The splint result is not used as proof.

Part 5 of 5Exam Checklist & Tips

2.8.1 – Key Points for the Exam

  • Limewater Aqueous calcium hydroxide, used to test for carbon dioxide.
  • Litmus An indicator that is red in acid and blue in alkali.
  • Squeaky pop The sound made when hydrogen burns rapidly with oxygen.
  • Glowing splint A wooden splint with a glowing ember, which relights in oxygen.

Common Mistakes

  • Stating that chlorine turns damp red litmus blue; it turns damp blue litmus red and then bleaches it.
  • Using the splint test for carbon dioxide; a burning splint is extinguished by other gases too.
  • Describing a gas by its smell instead of a chemical test.
  • Stating that hydrogen relights a glowing splint; it is oxygen that relights it.

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