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
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5Gases Reacting with Test Materials
Each gas is identified by a specific test, and the observation must be stated precisely.
Ammonia
- Ammonia Damp red litmus paper turns blue, showing that the gas is alkaline.
Carbon dioxide
- Carbon dioxide Bubbling the gas through limewater makes it turn cloudy white, due to insoluble calcium carbonate forming.
Chlorine
- Chlorine Damp blue litmus paper turns red and is then bleached white by the gas.
Hydrogen and oxygen
- Hydrogen and oxygen Hydrogen gives a squeaky pop with a lighted splint, while oxygen relights a glowing splint.
| Gas | Test | Positive result |
|---|---|---|
| Ammonia | Damp red litmus paper | Turns blue |
| Carbon dioxide | Limewater | Turns cloudy white |
| Chlorine | Damp blue litmus paper | Turns red, then bleached white |
| Hydrogen | Lighted splint at the mouth of the tube | Squeaky pop |
| Oxygen | Glowing splint inside the tube | Relights |
Section 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.
Section 3 of 5Common Confusions
Common Confusions
Extinguishing a splint
- Extinguishing a splint Extinguishing a burning splint does not identify carbon dioxide, since nitrogen also does this.
Smell
- Smell A characteristic smell, such as the choking smell of chlorine, is not an acceptable test.
Ammonia and ammonium
- Ammonia and ammonium Ammonia is the gas NH₃, while ammonium is the NH₄⁺ ion in solution.
Hydrogen and oxygen
- Hydrogen and oxygen A lighted splint has an H for hydrogen, and a glowing splint has an O for oxygen.
Section 4 of 5Worked Example
Question
- A colourless gas turns limewater cloudy and extinguishes a lighted splint.
- Identify the gas and describe the tests.
- Limewater turning cloudy shows the gas is carbon dioxide.
- Extinguishing a splint is not a definitive test, since nitrogen does this too, so it cannot confirm the identity.
- 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.
Section 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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