Flame Tests T
Sections 5–8 (identifying ions and instrumental methods) are Chemistry only — they are not on the Combined Science (Trilogy) papers. They are also the home of Required practical 7.
Heat certain metal ions (cations) strongly in a flame and they glow a characteristic colour — a quick way to identify the metal. You need five ions and their colours, and nothing else: flame colours of other metals are not required.
- Clean a nichrome (or platinum) wire loop by dipping it in dilute acid and holding it in a blue Bunsen flame until it adds no colour — this removes any ions left from before, so you test only your sample.
- Dip the clean loop in the sample and hold it in the edge of the blue flame.
- Observe the colour. Don’t let the wire glow red-hot — that orange glow can be mistaken for a flame colour.
- A strong colour masks a weaker one. In a mixture of ions, an intense colour (sodium’s yellow especially) can hide the others, so you may not see them. This is exactly why flame emission spectroscopy (section 8) is better for mixtures — it can pick out every ion at once.
- A flame test identifies one cation. To fully identify a compound you still need a test for the anion (section 7).
- Calcium (orange-red) is the only one of the five that also forms a white hydroxide precipitate; the flame test is a handy way to tell Ca2+ from Mg2+, which look identical with sodium hydroxide.
🧪 Exam-style questions
What colour flame do sodium ions produce in a flame test? Tick (✓) one box.
A metal compound gives a green flame. Which metal ion does it contain? Tick (✓) one box.
A salt contains both sodium and potassium ions. Why is it difficult to identify the potassium from a flame test? Tick (✓) one box.
Two white solids both give a white precipitate with sodium hydroxide. One gives an orange-red flame; the other gives no flame colour. Which ion does the orange-red solid contain? Tick (✓) one box.
Testing Cations with Sodium Hydroxide T
Add sodium hydroxide solution to a solution of a metal salt and an insoluble metal hydroxide drops out as a precipitate. The colour of that precipitate — and whether it dissolves in excess — identifies the metal ion.
Three coloured precipitates (copper blue, iron(II) green, iron(III) brown) and three that are all white (aluminium, calcium, magnesium) — the whites need a second test to tell apart.
Add a few drops of NaOH first (slowly), then add it in excess:
- Copper(II), Cu2+ → blue precipitate.
- Iron(II), Fe2+ → green precipitate.
- Iron(III), Fe3+ → brown precipitate.
- Aluminium, Al3+ → white precipitate that dissolves in excess NaOH (giving a colourless solution).
- Calcium, Ca2+ → white precipitate, does not dissolve in excess.
- Magnesium, Mg2+ → white precipitate, does not dissolve in excess.
So of the three whites, only aluminium re-dissolves in excess. To separate the remaining two, use a flame test: calcium is orange-red, magnesium gives no colour.
You should be able to write balanced ionic equations for forming the insoluble hydroxides — it uses the same charge-balancing skill you met in C4. The metal ion’s charge fixes the number of OH–:
- Cu2+(aq) + 2OH–(aq) → Cu(OH)2(s)
- Fe2+(aq) + 2OH–(aq) → Fe(OH)2(s)
- Fe3+(aq) + 3OH–(aq) → Fe(OH)3(s)
- Al3+(aq) + 3OH–(aq) → Al(OH)3(s)
- Mg2+(aq) + 2OH–(aq) → Mg(OH)2(s)
- Ca2+(aq) + 2OH–(aq) → Ca(OH)2(s)
The pattern: a 2+ ion takes two OH–, a 3+ ion takes three — so the charges cancel and the formula is neutral.
- “Clear” when you mean “colourless”. A solution that loses its colour becomes colourless; “clear” only means see-through (copper sulfate solution is clear and blue). Examiners penalise “clear” here.
- Adding NaOH too fast. Add a few drops first — if you flood aluminium with excess straight away, the white precipitate forms and re-dissolves before you spot it.
- Over-writing the equation. You are not expected to write the equation for aluminium hydroxide dissolving (forming sodium aluminate) — only for forming the precipitates.
🧪 Exam-style questions
A few drops of sodium hydroxide solution are added to a metal salt solution and a brown precipitate forms. Which ion is present? Tick (✓) one box.
A white precipitate forms with sodium hydroxide solution, then dissolves when excess sodium hydroxide is added. Which ion is present? Tick (✓) one box.
Which is the correct balanced ionic equation for forming the iron(III) precipitate? Tick (✓) one box.
Two solutions both give a white precipitate with sodium hydroxide that does not dissolve in excess. What further test would tell them apart, and what result shows calcium? Tick (✓) one box.