🔹 Definition of Ions
- Ions are atoms or groups of atoms that carry an electric charge due to the loss or gain of electrons.
- Cations are positively charged ions (formed by loss of electrons).
- Anions are negatively charged ions (formed by the gain of electrons).
🔹 Tests for Cations (Using Aqueous Sodium Hydroxide or Aqueous Ammonia)
| Cation | NaOH (aq) | NH₃ (aq) |
|---|---|---|
| Al³⁺ (Aluminium) | White ppt, soluble in excess to give colourless solution | White ppt, insoluble in excess |
| Zn²⁺ (Zinc) | White ppt, soluble in excess forming colourless solution | White ppt, soluble in excess |
| Ca²⁺ (Calcium) | White ppt, insoluble in excess | No ppt or very slight |
| NH₄⁺ (Ammonium) | No ppt, but on heating gives ammonia gas (turns damp red litmus blue) | – |
| Cu²⁺ (Copper (II)) | Light blue ppt, insoluble in excess | Light blue ppt, soluble in excess to form deep blue solution |
| Fe²⁺ (Iron (II)) | Green ppt, insoluble in excess | Green ppt, insoluble in excess |
| Fe³⁺ (Iron (III)) | Reddish-brown ppt, insoluble in excess | Reddish-brown ppt, insoluble in excess |
| Cr³⁺ (Chromium (III)) | Grey-green ppt, soluble in excess forming green solution | Grey-green ppt, insoluble in excess |
🔹 Tests for Anions (Using Reagents Like AgNO₃, BaCl₂, Dilute HCl or HNO₃)
| Anion | Test | Observation |
|---|---|---|
| Carbonate (CO₃²⁻) | Add dilute HCl | Effervescence, CO₂ gas produced (test with limewater) |
| Chloride (Cl⁻) | Add dilute nitric acid, then silver nitrate | White ppt (AgCl) |
| Bromide (Br⁻) | Add dilute nitric acid, then silver nitrate | Cream ppt (AgBr) |
| Iodide (I⁻) | Add dilute nitric acid, then silver nitrate | Yellow ppt (AgI) |
| Nitrate (NO₃⁻) | Add NaOH and aluminium foil, then warm | Ammonia gas given off (test with red litmus) |
| Sulphate (SO₄²⁻) | Add dilute HCl, then barium chloride | White ppt (BaSO₄) |
| Sulphite (SO₃²⁻) | Add dilute HCl and warm | Gas produced (SO₂), turns acidified potassium manganate(VII) from purple to colourless |
🔹 Gas Tests
| Gas | Test | Observation |
|---|---|---|
| Hydrogen (H₂) | Insert a lighted splint | Burns with a ‘pop’ sound |
| Oxygen (O₂) | Insert a glowing splint | Splint relights |
| Chlorine (Cl₂) | Damp blue litmus paper | Paper turns red then bleaches white |
| Carbon dioxide (CO₂) | Bubble through limewater | Limewater turns milky |
| Ammonia (NH₃) | Damp red litmus paper | Turns blue |
| Sulfur dioxide (SO₂) | Acidified potassium manganate(VII) | Turns purple to colourless |
🔹 Flame Tests for Metal Cations
| Metal Ion | Flame Colour |
|---|---|
| Lithium (Li⁺) | Crimson red |
| Calcium (Ca²⁺) | Brick red |
| Barium (Ba²⁺) | Apple green / Pale green |
| Copper (Cu²⁺) | Blue-green |
| Sodium (Na⁺) | Yellow |
| Potassium (K⁺) | Lilac / Light purple |
✅ Tips for Practicals:
- Always acidify with dilute nitric acid when testing halide ions to avoid confusing ppt from carbonates.
- When using flame tests, use a clean platinum or nichrome wire.
- Test carbonate and nitrate ions separately before testing for halides or sulphates.
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