Chemical analysis — section overview
Section C8 covers how chemists identify unknown substances using various tests and analytical techniques.
Pure substances and mixtures
A pure substance contains only one element or compound — it has a sharp, definite melting/boiling point. A mixture has a melting point range (impure → lower and broader melting point than the pure substance).
Flame tests (identifying metal ions)
| Flame colour | Metal ion |
|---|---|
| Yellow/orange | Na⁺ |
| Lilac/purple | K⁺ |
| Green | Cu²⁺ |
| Brick red | Ca²⁺ |
| Crimson red | Li⁺ |
Method: clean wire loop → dip in sample → hold in roaring Bunsen flame.
Precipitation reactions — identifying metal ions
Add NaOH solution:
- Fe²⁺ → green precipitate (iron(II) hydroxide)
- Fe³⁺ → brown/rust precipitate (iron(III) hydroxide)
- Cu²⁺ → blue precipitate (copper(II) hydroxide)
- Al³⁺ → white precipitate, dissolves in excess NaOH → aluminium hydroxide (amphoteric)
Identifying non-metal ions
Carbonate (CO₃²⁻): add dilute acid → CO₂ gas produced → limewater turns milky.
Halide ions (Cl⁻, Br⁻, I⁻): add dilute HNO₃ then AgNO₃:
- Cl⁻ → white precipitate (AgCl)
- Br⁻ → cream precipitate (AgBr)
- I⁻ → yellow precipitate (AgI)
Sulfate (SO₄²⁻): add dilute HCl then BaCl₂ → white precipitate of BaSO₄.
Chromatography
Used to separate and identify components of a mixture.
Rf value = distance moved by spot / distance moved by solvent front
A pure substance produces ONE spot; a mixture produces multiple spots.
Instrumental analysis (brief AQA overview)
- Flame emission spectroscopy (FES): measures wavelengths of light emitted; used in hospitals to check ion concentrations
- Gas chromatography (GC): separates mixtures of gases; retention time identifies substances
- Mass spectrometry (MS): identifies molecular masses; often combined with GC
Common exam mistakes in C8
- Rf = spot distance / solvent front distance — it is always ≤ 1; if you get > 1 you've divided the wrong way
- Flame tests — sodium masks other colours — yellow from Na⁺ is very intense; impure samples may show yellow even if other metals present
- Limewater and CO₂ — limewater turns MILKY (not clear or blue)
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