As electrons fall back to lower levels, they emit light at specific wavelengths (colours).
Different metals have different energy level spacings → different coloured light.
IDENTIFYING MIXTURES:
Sodium gives a strong yellow colour that can mask other colours.
If sodium is present — its yellow can obscure potassium's lilac.
Spectroscopy (more precise) can separate colours in a mixture.
LIMITATIONS:
Some colours look similar (e.g. lithium crimson vs calcium orange-red can be confused).
Sodium contamination is common — masks other colours.
Flame tests only identify certain cations — cannot detect anions.
More precise identification requires instrumental methods.
RPCHEM 4: Identify metal ions using flame tests and other tests in this section.
Using Instrumental Analysis
FLAME EMISSION SPECTROSCOPY gives a more precise identification:
Sample atomised and passed through a flame.
Emitted light passed through a prism/diffraction grating.
Wavelength of light measured precisely.
Each metal gives a unique pattern of spectral lines.
ADVANTAGES over visual flame tests:
More precise — can identify multiple ions in a mixture.
Quantitative — can measure concentration, not just presence.
More sensitive — detects trace amounts.
More objective — doesn't rely on human colour perception.
This links to the 4.8.3.7 Flame emission spectroscopy section.
⚠️ Common Mistake
Sodium gives a strong YELLOW/ORANGE colour — not red. Lithium is CRIMSON/RED — don't confuse lithium and sodium. Potassium is LILAC — not blue or violet. Calcium is ORANGE-RED — can be confused with lithium if not observed carefully. Sodium contamination is the most common problem in flame tests.
📌 Key Note
Flame test colours: Li = crimson, Na = yellow/orange, K = lilac, Ca = orange-red, Cu = green. Method: clean nichrome wire, dip in sample, observe flame. Sodium masks other colours. Explained by electron excitation and emission. More precise: flame emission spectroscopy.
🎯 Matching Activity — Flame Test Colours
Match each metal ion to its flame test colour. — drag the symbols on the right to match the component names on the left.
Lithium (Li⁺)
Drop here
Sodium (Na⁺)
Drop here
Potassium (K⁺)
Drop here
Calcium (Ca²⁺)
Drop here
Copper (Cu²⁺)
Drop here
Green/blue-green flame
Orange-red flame
Crimson/red flame
Yellow/orange flame — even trace amounts give strong colour
Lilac/purple flame
🔬 Triple Science Only
Flame tests (4.8.3.1) is chemistry-only — not in Combined Science. RP Chemistry 4 includes flame tests as part of ion identification.
🧪 Required Practical
🔬 RP Chemistry 4 (chemistry-only) — Identify the ions in an unknown compound. Includes flame tests for metal cations.
Know the method, variables, equipment and how to analyse results.
🎯 Test Yourself
Question 1 of 2
1. A flame test on a solution gives a persistent yellow/orange colour. Which ion is most likely present?
2. Why might sodium contamination cause problems during a flame test for potassium?
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