Revision notes for AQA GCSE Chemistry Flame tests. Open the guide for explanations and worked examples. Written against the AQA GCSE Chemistry (8462) specification, so the content matches what's examinable rather than general Chemistry background.
Flame tests
What you'll learn
- How flame tests identify some metal ions in a sample.
- The five GCSE flame test colours you need to know.
- How to carry out a flame test safely and cleanly.
- Why mixtures of ions can make flame test results less reliable.
The basic idea: identifying ions
In this part of GCSE Chemistry, you use simple chemical tests to work out which ions are present in an unknown sample.
Definition
Ion
An ion is a particle with an electric charge. It forms when an atom or group of atoms gains or loses electrons.
A metal ion is usually a positive ion because metals tend to lose electrons. Positive ions are called cations.
Definition
Cation
A cation is a positively charged ion. In flame tests, the ions being identified are metal cations such as Li+, Na+ and Cu2+.
Flame tests are part of qualitative analysis.
Definition
Qualitative analysis
Qualitative analysis means finding out what substances are present, rather than measuring exactly how much is present.
What is a flame test?
A flame test is a test used to identify some metal cations by the colour they produce in a flame.
Definition
Flame test
A flame test is a qualitative test where a sample is placed in a Bunsen burner flame and the flame colour is observed to help identify certain metal ions.
The key point is that the metal ion is responsible for the characteristic flame colour. For example, sodium compounds produce a yellow flame because they contain sodium ions.
Key Idea
Core idea
Different metal cations can produce different flame colours, so the colour of the flame can be used as evidence for which metal ion is present.
The flame colours you must know
You only need to learn the flame colours for these metal ions in GCSE Chemistry.
| Metal ion in the compound | Flame colour |
|---|---|
| Lithium, Li+ | Crimson |
| Sodium, Na+ | Yellow |
| Potassium, K+ | Lilac |
| Calcium, Ca2+ | Orange-red |
| Copper, Cu2+ | Green |
Tip
Memory hook
A quick order to practise is: Li crimson, Na yellow, K lilac, Ca orange-red, Cu green. Say the ion and the colour together so they become paired in your memory.
Common Mistake
Learning extra colours instead of the required ones
You do not need to know flame colours for other metal ions for this specification. Focus on lithium, sodium, potassium, calcium and copper.
How to carry out a flame test
A typical school flame test uses a clean wire loop, often made from nichrome wire. Nichrome is a heat-resistant metal alloy used because it does not usually add a strong flame colour of its own.
A sample is the small amount of substance being tested. The sample might be a solid compound or a solution containing dissolved ions.
The wire loop must be cleaned before testing so that old chemicals do not affect the result. This unwanted carry-over is called contamination.
Definition
Contamination
Contamination means the sample or apparatus has unwanted substances on it, which can give misleading results.
Method
- Dip the wire loop into dilute hydrochloric acid.
- Hold the loop in a hot blue Bunsen flame until no flame colour is seen.
- Dip the clean loop into the sample.
- Place the loop at the edge of the blue flame.
- Observe and record the flame colour.
Dilute hydrochloric acid helps clean the wire loop and can help the metal compound form a chloride, which is often easier to vaporise in the flame. Vaporise means to change into a gas.
Tip
Why use a blue flame?
A blue Bunsen flame is used because it is hot and not strongly coloured. A yellow safety flame would make it much harder to see the flame test colour clearly.
Interpreting flame test results
Once you have the flame colour, compare it with the required GCSE list.
For example:
- Crimson flame → lithium ion present.
- Yellow flame → sodium ion present.
- Lilac flame → potassium ion present.
- Orange-red flame → calcium ion present.
- Green flame → copper ion present.
Notice that this is evidence for the cation, not the whole compound. A green flame tells you that copper ions are present, but it does not tell you the negative ion.
Example
Identifying metal ions from flame colours
A student tests three unknown samples. Sample A gives a lilac flame, sample B gives a green flame, and sample C gives an orange-red flame. Identify the metal ion in each sample.
-
Compare sample A with the flame colour list: lilac matches potassium, so sample A contains potassium ions.
-
Compare sample B with the flame colour list: green matches copper, so sample B contains copper ions.
-
Compare sample C with the flame colour list: orange-red matches calcium, so sample C contains calcium ions.
Why mixtures can be difficult
A flame test works best when the sample contains just one metal ion that produces a strong, clear colour.
If a sample contains a mixture of metal ions, one colour can hide another. This is called masking.
Definition
Masking
Masking happens when one flame colour is so strong that it hides another flame colour from a different ion in the same sample.
For example, sodium produces a very strong yellow flame. If sodium ions are present in a mixture, the yellow colour may make it difficult to see a lilac potassium flame.
Common Mistake
Mixtures can give misleading colours
If a sample contains more than one metal ion, the flame colour may not show every ion present. Flame tests are useful, but they are not always enough on their own for mixtures.
What flame tests can and cannot tell you
Flame tests are quick and useful, but they have limits.
They can help identify some metal cations from their flame colours.
They cannot identify every ion in a compound. They also do not identify negative ions, called anions.
Definition
Anion
An anion is a negatively charged ion. Flame tests in this topic are used for certain metal cations, not anions.
So if a compound gives a yellow flame, you can say it contains sodium ions. You cannot tell from the flame test alone whether the compound is sodium chloride, sodium sulfate, sodium nitrate, or another sodium compound.
Common Mistake
Naming the whole compound from the flame colour
Do not write “the compound is sodium chloride” just because the flame is yellow. The flame test only shows that sodium ions are present.
Recording results clearly
When you record a flame test, use the observation and the conclusion separately.
For example:
| Observation | Conclusion |
|---|---|
| Yellow flame | Sodium ions present |
| Green flame | Copper ions present |
| Crimson flame | Lithium ions present |
This helps avoid vague answers such as “it went sodium” or “it was copper coloured”. In exams, the colour and ion must match clearly.
Exam technique
In the exam
-
Learn the five required colour-ion pairs exactly: lithium crimson, sodium yellow, potassium lilac, calcium orange-red, copper green.
-
If asked what the test identifies, say metal ions or cations, not “metals” or “compounds”.
-
If a mixture is mentioned, remember that one flame colour can be masked by another, so the result may be less reliable.
Self review
Check yourself
- What metal ion gives a lilac flame in a flame test?
- Why must the wire loop be cleaned before testing a new sample?
- Why can a mixture of ions make flame test results harder to interpret?
Identification of ions by chemical and spectroscopic means (chemistry only)
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