Flame tests: some metal ions give a flame their own colour
Definition
Flame test
A test that identifies certain metal ions by the distinctive colour they give to a Bunsen flame.
Definition
Metal ion
A positively charged particle formed when a metal atom loses one or more electrons.
- A flame test identifies some metal ions by the colour they give to a flame.
- The metal ion in a compound is what produces the colour, so the colour tells you which metal is present.
- Only certain metals give a clear colour; many others, including most transition metals, do not.
- If a sample contains a mixture of ions, a strong colour can hide a weaker one.
Practical
Identifying metal ions by flame colour
- Clean a nichrome wire by dipping it in dilute hydrochloric acid and holding it in the flame until it burns with no colour.
- Dip the clean wire into the solid or solution being tested.
- Hold the tip at the edge of a roaring blue Bunsen flame and note the colour.
- Clean the wire again between samples so a leftover ion does not give a false colour.
- Record each flame colour and match it to the known metal ions.
The five flame colours to learn
- Lithium (Li+\text{Li}^+Li+): crimson red.
- Sodium (Na+\text{Na}^+Na+): yellow.
- Potassium (K+\text{K}^+K+): lilac.
- Calcium (Ca2+\text{Ca}^{2+}Ca2+): orange-red.
- Copper (Cu2+\text{Cu}^{2+}Cu2+): green.
Exam technique
- Learn the ion and its colour as a pair, and give the specific colour, such as lilac, not just purple.
- If asked, explain that a leftover ion on a dirty wire, or a strong sodium colour, can mask another metal.
Getting a clear result and common slips
- Use a clean wire and a blue flame, because a yellow, dirty flame hides the true colour.
- The strong yellow of sodium easily masks the colours of other metals in the same sample.
- A flame test only identifies the metal ion, not the negative ion in the compound.
Self review
- What does a flame test identify?
- Give the flame colours for lithium, sodium, potassium, calcium and copper ions.
- Why must the wire be cleaned between samples?
- Why can a flame test fail to detect a metal in a mixture of ions?
