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Cambridge IGCSE Chemistry · 0620
Chapter 12: Experimental Techniques and Chemical Analysis — Part 5
Topic 12.5 · Identification of ions and gases
Tests for Anions (Negative Ions)
| Anion | Test | Result |
|---|---|---|
| Carbonate (CO32-) | Add dilute acid | Effervescence; gas (CO2) turns limewater cloudy. |
| Chloride (Cl-) | Acidify with dilute HNO3, add aq. AgNO3 | White precipitate. |
| Bromide (Br-) | Acidify with dilute HNO3, add aq. AgNO3 | Cream precipitate. |
| Iodide (I-) | Acidify with dilute HNO3, add aq. AgNO3 | Yellow precipitate. |
| Nitrate (NO3-) | Add aq. NaOH, then Al foil; warm carefully | Ammonia gas produced (turns damp red litmus blue). |
| Sulfate (SO42-) | Acidify with dilute HNO3, add aq. Ba(NO3)2 | White precipitate. |
| Sulfite (SO32-) | Add acidified aq. KMnO4 | Purple ? Colourless. |
Tests for Aqueous Cations (Positive Ions)
| Cation | With Aqueous NaOH | With Aqueous Ammonia |
|---|---|---|
| Aluminium (Al3+) | White ppt., soluble in excess (colourless) | White ppt., insoluble in excess. |
| Ammonium (NH4+) | Ammonia produced on warming | —. |
| Calcium (Ca2+) | White ppt., insoluble in excess | No ppt. or very slight white ppt.. |
| Chromium(III) (Cr3+) | Green ppt., soluble in excess (dark green) | Green ppt., insoluble in excess. |
| Copper(II) (Cu2+) | Light blue ppt., insoluble in excess | Light blue ppt., soluble in excess (dark blue). |
| Iron(II) (Fe2+) | Green ppt., insoluble in excess | Green ppt., insoluble in excess. |
| Iron(III) (Fe3+) | Red-brown ppt., insoluble in excess | Red-brown ppt., insoluble in excess. |
| Zinc (Zn2+) | White ppt., soluble in excess (colourless) | White ppt., soluble in excess (colourless). |
- Ammonia (NH3): Turns damp red litmus paper blue.
- Carbon Dioxide (CO2): Turns limewater cloudy/milky.
- Chlorine (Cl2): Bleaches damp litmus paper white.
- Hydrogen (H2): 'Pops' with a lighted splint.
- Oxygen (O2): Relights a glowing splint.
- Sulfur Dioxide (SO2): Turns acidified aqueous KMnO4 from purple to colourless.
Exam Traps
- CO2 turns limewater cloudy — do not use litmus or a splint test for carbon dioxide.
Flame Tests for Cations
- Lithium (Li+): Red.
- Sodium (Na+): Yellow.
- Potassium (K+): Lilac.
- Calcium (Ca2+): Orange-red.
- Barium (Ba2+): Light green / Yellow-green.
- Copper(II) (Cu2+): Blue-green.
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