Identifying Metal IonsOxford AQA IGCSE Chemistry: Revision notes
Section 1
Why identify metal ions at all?
Many chemical tests exist to work out what unknown compounds are made of. One key part of qualitative analysis is finding out which metal ion is present in a sample. The simplest and quickest method for many metals is the flame test, which uses the fact that different metal ions produce different, characteristic flame colours when heated strongly.
Section 2
What is a flame test and why does it work?
In a flame test, a small sample of the unknown compound is held in a hot, non-luminous Bunsen flame. The heat energy excites electrons in the metal ions to higher energy levels; as these electrons fall back down, they release energy as light of a specific colour. Because each metal ion has its own characteristic set of energy levels, each metal produces its own distinctive flame colour.
Examiners want the flame colour named precisely - 'lilac' for potassium is not the same mark as 'purple'; use the exact spec word.
Section 3
The flame colours you must memorise
| Metal ion | Flame colour |
|---|---|
| Lithium | Crimson |
| Sodium | Yellow |
| Potassium | Lilac |
| Calcium | Red |
| Barium | Green |
Section 4
How is the required practical carried out?
- Dip a clean nichrome (or platinum) wire loop into dilute hydrochloric acid, then into a roaring Bunsen flame to clean it, until the flame shows no colour.
- Dip the clean loop into the solid or solution sample being tested.
- Hold the loop at the edge of a roaring (non-luminous) Bunsen flame.
- Observe and record the colour the flame turns.
- Clean the loop again before testing the next sample, to avoid cross-contamination.
A common error is not cleaning the wire loop between tests - leftover sodium contamination gives a strong yellow flame that can mask other, weaker colours.
Section 5
What are the limitations of flame tests?
Flame tests work well on samples containing a single metal ion. If a mixture of metal ions is present, one colour (especially the strong yellow of sodium) can hide the presence of others, so flame tests are less reliable for mixtures than for pure unknown compounds.
A sample that is a mixture of sodium chloride and potassium chloride usually just shows the strong yellow of sodium, hiding the lilac of potassium.
Must Know
- Flame tests identify metal ions from the colour they give a Bunsen flame.
- Lithium = crimson, Sodium = yellow, Potassium = lilac, Calcium = red, Barium = green.
- Clean the nichrome wire with acid and flame before and between each test.
- Flame tests are unreliable for mixtures because one colour can mask another (especially sodium's yellow).
That's the notes covered.
Carry on to the next subtopic.