Overview
This lecture explains how to test for the presence of four key gases—oxygen, hydrogen, carbon dioxide, and chlorine—using standard laboratory methods for GCSE exams.
Test for Oxygen Gas
- Use a glowing (not burning) splint and insert it into the gas sample.
- If the splint reignites, oxygen gas is present.
Test for Hydrogen Gas
- Produce hydrogen by reacting magnesium with hydrochloric acid.
- Insert a lit splint into the gas; if a squeaky pop sound is heard, hydrogen gas is present.
Test for Carbon Dioxide Gas
- Produce carbon dioxide by mixing calcium chloride (marble chips) with hydrochloric acid.
- Bubble the gas through colorless lime water.
- If the lime water turns cloudy, carbon dioxide is present.
Test for Chlorine Gas
- Produce chlorine gas via electrolysis of sodium chloride (salty water).
- Dampen blue litmus paper and hold it near the gas.
- The litmus paper first turns red and then bleaches white, indicating chlorine is present.
- Red litmus paper will also bleach white in chlorine gas.
Key Terms & Definitions
- Glowing Splint — Wooden splint with a glowing ember, not a visible flame.
- Lime Water — Aqueous solution of calcium hydroxide, used to detect carbon dioxide.
- Litmus Paper — Indicator paper that changes color in acids and bases; also used for gas testing.
- Bleach (in testing) — Turning white due to chemical reaction with chlorine.
- Squeaky Pop — Sound produced when hydrogen ignites.
Action Items / Next Steps
- Review and memorize each gas test and its positive result for your GCSE exams.
- Practice writing out the steps of each test for revision.