Chemical Tests for Gases, Metal Ions and Negative Ions
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Chemical Tests
| TERM | MEANING |
|---|---|
| Test | A procedure giving an observable result that identifies a substance. |
| Precipitate | An insoluble solid formed in a test, often coloured. |
| Flame test | Identifying a metal ion by the colour it gives a flame. |
| Limewater | Calcium hydroxide solution — turns milky with CO₂. |
| Damp litmus | Litmus paper used to test gases. |
Tests for Gases
| GAS | TEST & RESULT |
|---|---|
| Hydrogen | Lit splint → squeaky pop |
| Oxygen | Glowing splint → relights |
| Carbon dioxide | Limewater → turns milky |
| Ammonia | Damp red litmus → turns blue |
| Chlorine | Damp litmus → bleached white |
Flame Tests (Metal Ions)
| ION | FLAME COLOUR |
|---|---|
| Lithium, Li⁺ | Red |
| Sodium, Na⁺ | Yellow |
| Potassium, K⁺ | Lilac |
| Calcium, Ca²⁺ | Orange-red |
| Copper, Cu²⁺ | Blue-green |
Tests With Sodium Hydroxide
Add sodium hydroxide solution → a coloured precipitate identifies the metal ion:
- Cu²⁺ → blue
- Fe²⁺ → green
- Fe³⁺ → brown
- Ca²⁺ → white
- Al³⁺ → white, but dissolves in excess NaOH
- NH₄⁺ → ammonia gas on warming (damp red litmus turns blue).
Tests for Negative Ions
Carbonate (CO₃²⁻): add dilute acid → fizzes, CO₂ turns limewater milky.
Sulfate (SO₄²⁻): add dilute HCl then barium chloride → white precipitate.
Halides: add dilute nitric acid then silver nitrate → chloride white, bromide cream, iodide yellow.
The Why
Why we add acid before the sulfate/halide test: it removes carbonate ions that would otherwise give a false positive precipitate.
Why flame tests work: heat excites the metal ion's electrons; as they fall back they release light of a characteristic colour.
Sodium Carbonate
Carry out a flame test on the white powder. A yellow flame is produced. Add dilute hydrochloric acid to the powder. The gas produced is passed into limewater, which turns cloudy or milky, showing that carbon dioxide is present.
Potassium Carbonate
Carry out a flame test on the white powder. A lilac flame is produced. Add dilute hydrochloric acid to the powder. The gas produced is passed into limewater, which turns cloudy or milky, showing that carbon dioxide is present.
Copper Carbonate
Carry out a flame test on the white powder. A blue-green flame is produced. Add dilute hydrochloric acid to the powder. The gas produced is passed into limewater, which turns cloudy or milky, showing that carbon dioxide is present.
Calcium Carbonate
Carry out a flame test on the white powder. A brick-red flame is produced. Add dilute hydrochloric acid to the powder. The gas produced is passed into limewater, which turns cloudy or milky, showing that carbon dioxide is present.