3.5.5Movement and Interactions
Tests For Gases — AQA GCSE Combined Science Synergy 8465 - Physical Sciences Revision Notes & Topical Questions
5 min read 4 parts Lesson 58 of 59
Section 1 of 4Identification of common gases
| Gas | Test Method | Positive Result |
|---|---|---|
| Hydrogen (H2) | Hold a burning wooden splint near mouth of test tube | Burns with a characteristic "squeaky pop" sound |
| Oxygen (O2) | Insert a glowing wooden splint into the gas | Glowing splint relights |
| Carbon Dioxide (CO2) | Bubble gas through limewater (aqueous calcium hydroxide) | Limewater turns milky / cloudy white precipitate |
| Chlorine (Cl2) | Hold damp blue litmus paper in the gas | Litmus paper turns red then bleaches completely white |
| Ammonia (NH3) | Hold damp red litmus paper in the gas | Damp red litmus paper turns blue (pungent smell) |
Section 2 of 4Tests for cations and anions
Tests for cations and anions
Flame tests for cations
- Lithium (Li+): Crimson red.
- Sodium (Na+): Yellow.
- Potassium (K+): Lilac.
- Calcium (Ca2+): Orange-red (brick red).
- Copper(II) (Cu2+): Green-blue.
Precipitate tests with NaOH(aq)
- Cu2+: Light blue precipitate [Cu(OH)2].
- Fe2+: Green precipitate [Fe(OH)2].
- Fe3+: Red-brown precipitate [Fe(OH)3].
- Al3+: White precipitate that dissolves in excess NaOH.
- Ca2+, Mg2+: White precipitate insoluble in excess NaOH.
Tests for anions
- Carbonate (CO32-)
- Add dilute HCl;
- effervescence of CO2 turns limewater milky.
- Add dilute HCl;
- Sulfate (SO42-)
- Acidify with dilute HCl, add barium chloride solution;
- white precipitate of BaSO4 forms.
- Acidify with dilute HCl, add barium chloride solution;
- Halides (Cl-, Br-, I-)
- Acidify with dilute nitric acid (HNO3), add silver nitrate (AgNO3):
Chloride gives white AgCl;
- Bromide gives cream AgBr;
- Iodide gives yellow AgI.
- Acidify with dilute nitric acid (HNO3), add silver nitrate (AgNO3):
Chloride gives white AgCl;
Section 3 of 4Key terms
| Term | Meaning |
|---|---|
| Qualitative analysis | Chemical procedures that identify the components of a mixture without determining numerical amounts. |
| Precipitate | An insoluble solid that separates from solution during a chemical double displacement reaction. |
| Limewater | A dilute aqueous solution of calcium hydroxide used as a test reagent for carbon dioxide. |
Section 4 of 4Exam technique
Avoid these mistakes
Using hydrochloric acid to acidify silver nitrate — adding HCl introduces chloride ions, producing a false positive white precipitate; always use nitric acid.
Confusing tests for oxygen and hydrogen — hydrogen pops with a lighted splint; oxygen relights a glowing splint.
Last updated: