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
Yellow/orange flame — even trace amounts give strong colour
Orange-red flame
Crimson/red flame
Green/blue-green flame
Lilac/purple flame
⭐ Higher Tier Only
Explain flame test colours in terms of electron excitation and emission at characteristic wavelengths. Relate the wavelength of emitted light to the energy gap between electron levels. Explain why flame emission spectroscopy is more quantitative and precise than visual flame tests.
🔬 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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