Tests for gases and ions
Tests for common gases
| Gas | Test | Positive result |
|---|---|---|
| Hydrogen (H₂) | Lit splint at the mouth of the tube | "Squeaky pop" |
| Oxygen (O₂) | Glowing splint inserted | Splint relights |
| Carbon dioxide (CO₂) | Bubble through limewater | Cloudy / milky |
| Chlorine (Cl₂) | Damp blue litmus paper | Bleached white (red first, then white) |
You must learn the test, the gas's identity, and the named indicator.
Flame tests for cations
Dip a clean platinum/nichrome wire (cleaned in HCl) into the salt and hold in a blue Bunsen flame:
| Cation | Flame colour |
|---|---|
| Lithium (Li⁺) | Crimson red |
| Sodium (Na⁺) | Yellow / orange |
| Potassium (K⁺) | Lilac |
| Calcium (Ca²⁺) | Orange-red |
| Copper (Cu²⁺) | Green |
Limitation: only works for one cation at a time; sodium contamination masks others.
Hydroxide precipitates (for transition-metal cations)
Add sodium hydroxide solution dropwise:
| Cation | Precipitate colour |
|---|---|
| Cu²⁺ | Blue |
| Fe²⁺ | Green (turns brown in air) |
| Fe³⁺ | Brown / rusty |
Tests for anions
- Carbonate (CO₃²⁻) — add dilute acid → fizz, gas turns limewater milky → CO₂ → carbonate confirmed.
- Sulfate (SO₄²⁻) — add dilute HCl then barium chloride → white precipitate (BaSO₄).
- Halides (Cl⁻, Br⁻, I⁻) — add dilute nitric acid then silver nitrate:
- Cl⁻ → white ppt (AgCl)
- Br⁻ → cream ppt (AgBr)
- I⁻ → yellow ppt (AgI)
OCR PAG C4
Identify unknown salts using these reagents. Mark schemes always require the observation ("white precipitate"), not just "positive test".
OCR exam tip
When asked to "describe the test", structure each answer as reagent → observation → conclusion. Three steps, three marks ready to drop.
AI-generated · claude-opus-4-7 · v3-ocr-combined-science-leaves