Flame Tests for Metal Cations
Flame tests are a method of qualitative analysis used to identify specific metal cations by observing the unique colour of light they emit when heated. For the ESAT, you must memorise the characteristic flame colours for five key metal ions.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Lithium (Li+) ions produce a crimson red flame.
- Sodium (Na+) ions give an intense yellow-orange flame.
- Potassium (K+) ions result in a lilac (pale purple) flame.
- Calcium (Ca2+) ions show a red-orange flame.
- Copper (Cu2+) ions burn with a green flame.
Definitions
- Flame Test
- A chemical procedure used to detect the presence of certain elements, primarily metal ions, based upon each element's characteristic emission spectrum which manifests as a specific flame colour.
- Cation
- A positively charged ion, typically a metal atom that has lost one or more electrons.
Worked example
An unknown chemical compound is heated in a Bunsen burner flame, which turns a distinct green colour. Which of the following cations is most likely present in the compound?
- 1
Identify the observed colour from the prompt:
a distinct green flame.
- 2
Recall the memorised list of flame test results for the required cations.
- 3
Match the observed green colour to the corresponding metal cation.
- 4
A green flame is the characteristic result for Copper (Cu2+) ions.
Answer: Cu2+
Common mistakes
- ×Confusing the two red-spectrum colours: Lithium gives a crimson red flame, while Calcium produces a red-orange flame. Be precise in recalling these distinct shades.
- ×Overlooking the faint lilac of Potassium: The lilac (pale purple) flame of Potassium can be weak and is easily masked by the much more intense yellow-orange flame from any contaminating Sodium ions.
- ×Forgetting that the test identifies the cation: The test reveals the metal ion (e.g., Na+), not the entire compound (e.g., NaCl). Questions may try to trick you with different anions attached to the correct metal.
No-calculator tips
- ✓This is a pure memory topic. Use mnemonics to link the elements and colours (e.g., associate the yellow of streetlights with Sodium).
- ✓Create colour-coded flashcards to test yourself, with the cation symbol on one side and the flame colour on the other.
- ✓Practice visualising the colours when you see the chemical symbols Li+, Na+, K+, Ca2+, and Cu2+.