Flame Test
Summary: A flame test is a qualitative analysis technique used to identify metal cations based on the characteristic colour they produce when heated in a Bunsen burner flame. Each metal ion gives a distinct flame colour: lithium (red), sodium (yellow), potassium (lilac), calcium (orange-red), copper (blue-green), barium (apple green). Tags: igcse chemistry definition practical-skills qualitative-analysis Created: 2026-07-14 Last Updated: 2026-07-23
A flame test is a simple, rapid qualitative test used to identify the presence of certain metal cations in a sample. When a metal compound is heated in a hot Bunsen burner flame, the metal ions emit light of characteristic colours. Each metal produces a distinctive flame colour that can be used for identification.
How to Perform a Flame Test
- Clean a nichrome or platinum wire by dipping in concentrated HCl and heating in the blue flame until no colour is produced
- Dip the clean wire into the sample (solid or solution)
- Hold the wire in the hottest part of the Bunsen burner flame (the blue cone)
- Observe the colour produced
- Record the colour and identify the cation
Why clean with HCl? The acid removes any oxide layer from the wire and converts the sample to a volatile chloride, which vaporises more readily in the flame.
Characteristic Flame Colours
| Cation | Flame Colour | Notes |
|---|---|---|
| Li⁺ (Lithium) | Crimson red | Deep red |
| Na⁺ (Sodium) | Yellow | Very intense, persistent — even trace amounts are visible |
| K⁺ (Potassium) | Lilac | Pale purple — can be masked by Na⁺ yellow; use cobalt blue glass to filter |
| Ca²⁺ (Calcium) | Orange-red (brick red) | |
| Ba²⁺ (Barium) | Apple green | Pale yellow-green |
| Cu²⁺ (Copper) | Blue-green | Green with a blue tinge |
The Cobalt Blue Glass Trick
Sodium’s yellow flame is so intense that even trace impurities can mask other colours. Potassium’s lilac is especially hard to see if sodium is present.
Using a cobalt blue glass filter:
- Absorbs the yellow light from sodium
- Allows the lilac light from potassium to pass through
- Essential for confirming potassium in the presence of sodium
Why Do Metals Produce Colours?
- Heat energy excites the metal’s electrons to higher energy levels
- When electrons fall back to their original level, they release energy as light
- The energy gap between levels is different for each metal → different wavelength/colour
Key Facts
- Flame tests identify metal cations by characteristic flame colours
- Clean wire with HCl and heat before use
- Li⁺ = red; Na⁺ = yellow; K⁺ = lilac; Ca²⁺ = orange-red; Cu²⁺ = blue-green; Ba²⁺ = apple green
- Use cobalt blue glass to see potassium’s lilac in the presence of sodium
- Only identifies the cation — does NOT identify the anion
- Simple, fast, but can be masked by sodium contamination
See Also
This is a key term definition page — part of the IGCSE Chemistry (0620/0971) keyword glossary.