Flame tests for cations (HT)
Many metal ions produce characteristic colours when their compound is heated in a hot flame. This gives a quick way to identify the cation (positive metal ion) in a sample.
Method
- Dip a clean nichrome wire loop in concentrated HCl (cleans it).
- Hold in a non-luminous (blue) Bunsen flame until no colour shows.
- Dip the loop in the test compound (powder or paste).
- Hold in the edge of the flame.
- Observe the colour.
(Old AQA mark schemes still mention a platinum wire — nichrome is more common in schools.)
The five flame test colours (memorise)
| Cation | Flame colour |
|---|---|
| Lithium Li⁺ | Crimson red |
| Sodium Na⁺ | Yellow (intense, broad) |
| Potassium K⁺ | Lilac (best seen through cobalt-blue glass to filter Na contamination) |
| Calcium Ca²⁺ | Orange-red |
| Copper(II) Cu²⁺ | Green (sometimes blue-green) |
Mnemonic: "Lots of Naughty Kids Can Cook" — Li, Na, K, Ca, Cu.
Why colours occur
Heat excites electrons to higher energy levels; when they fall back, they emit photons of specific wavelengths characteristic of each element. Different cations have different energy gaps so different colours.
Limitations of flame tests
- Sodium contamination — small amounts of Na give a strong yellow that can mask K's lilac. Use cobalt-blue glass to filter.
- Mixture of ions — colours mix and become hard to identify.
- Some cations don't give a strong colour — flame tests miss many transition metal ions.
- Subjective — colour interpretation varies. Flame emission spectroscopy (C8.6) is more accurate.
✦Worked example
A student adds some compound to a flame and sees a lilac colour. Which cation is present?
Lilac → K⁺ (potassium ion).
⚠Common mistakes
- Confusing Li⁺ crimson with Ca²⁺ orange-red — Li is purer red; Ca has more orange.
- Using a flaming splint — must use a nichrome wire to introduce a small amount of compound.
- Not cleaning between tests — leftover ions give wrong colours.
- Reading the wrong colour for K — yellow Na often masks K; use cobalt-blue filter.
Links
Builds on C1.4 (electronic structure → energy levels). Sets up C8.5 (anion + cation tests with NaOH), C8.6 (instrumental methods).
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