Flame Colours and Cation Tests

Cation identification is tested on Paper 6 (Alternative to Practical) and in Paper 2/4 theory questions on the Cambridge 0620 syllabus. You need two sets of tests: flame tests for Group I and II cations, and sodium hydroxide precipitation tests for transition metal and ammonium ions.

Flame tests

Procedure

1. Clean a nichrome wire loop by dipping it in concentrated hydrochloric acid and holding it in a roaring Bunsen flame until no colour is seen
2. Dip the clean loop into the solid sample or a solution of the salt
3. Hold the loop at the edge of a roaring (blue) Bunsen flame
4. Observe and record the flame colour
5. Clean the loop between tests to avoid contamination

Required flame colours

Cation	Ion	Flame colour
Lithium	Li⁺	Crimson / red
Sodium	Na⁺	Yellow / orange
Potassium	K⁺	Lilac / purple
Calcium	Ca²⁺	Orange-red
Copper	Cu²⁺	Blue-green / green

Sodium contamination is the most common practical problem. Sodium is present in trace amounts almost everywhere, producing a persistent yellow flame that masks other colours. Viewing through blue cobalt glass filters out the yellow sodium colour, making potassium’s lilac flame visible.

Why flame tests work

When heated, electrons in the metal ion absorb energy and are promoted to higher energy levels. When they fall back to their original energy level, they emit light of a specific wavelength (colour) characteristic of that element.

Sodium hydroxide (NaOH) solution tests

Adding dilute sodium hydroxide solution to a solution containing metal cations produces an insoluble metal hydroxide precipitate. The colour identifies the cation.

Procedure

1. Place about 2 cm³ of the test solution in a test tube
2. Add dilute NaOH solution dropwise
3. Note the colour of any precipitate formed
4. Continue adding NaOH in excess (to see if the precipitate dissolves)

Results table

Cation	Precipitate colour	Formula	Effect of excess NaOH
Aluminium, Al³⁺	White	Al(OH)₃	Dissolves — precipitate disappears (amphoteric)
Calcium, Ca²⁺	White	Ca(OH)₂	Does not dissolve
Magnesium, Mg²⁺	White	Mg(OH)₂	Does not dissolve
Copper(II), Cu²⁺	Blue	Cu(OH)₂	Does not dissolve
Iron(II), Fe²⁺	Green	Fe(OH)₂	Does not dissolve
Iron(III), Fe³⁺	Brown / rust-brown	Fe(OH)₃	Does not dissolve
Zinc, Zn²⁺	White	Zn(OH)₂	Dissolves — precipitate disappears (amphoteric)
Chromium(III), Cr³⁺	Green	Cr(OH)₃	Dissolves (amphoteric)

Distinguishing white precipitates

Three cations give white precipitates with NaOH: Al³⁺, Ca²⁺/Mg²⁺, and Zn²⁺. To tell them apart:

• Al³⁺ and Zn²⁺: the white precipitate dissolves in excess NaOH (these hydroxides are amphoteric)
• Ca²⁺ and Mg²⁺: the white precipitate does NOT dissolve in excess NaOH
• To distinguish Ca²⁺ from Mg²⁺: use a flame test (calcium = orange-red, magnesium gives no flame colour)
• To distinguish Al³⁺ from Zn²⁺: add ammonia solution — both give white precipitates, but only Zn(OH)₂ dissolves in excess ammonia

Testing for ammonium ions (NH₄⁺)

Ammonium is not a metal cation, but its test uses NaOH:

1. Add NaOH solution to the test substance
2. Warm gently
3. Hold damp red litmus paper over the mouth of the test tube
4. If NH₄⁺ is present, ammonia gas is produced — the damp red litmus turns blue

NH₄⁺ + OH⁻ → NH₃ + H₂O

Ionic equations for precipitation

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)

Worked exam question

A solution gives a green precipitate when sodium hydroxide is added. The precipitate does not dissolve in excess NaOH. A flame test on the original solid shows no characteristic colour. Identify the cation. [2]

The green precipitate indicates Fe²⁺ or possibly Cr³⁺ [1]. Since the precipitate does not dissolve in excess NaOH, it is Fe²⁺ (iron(II)) — Cr(OH)₃ would dissolve in excess [1].

Common exam mistakes

1. Confusing iron(II) (green precipitate) with iron(III) (brown precipitate). Remember: Fe(II) = green, Fe(III) = brown.
2. Not testing with excess NaOH — this step is essential for distinguishing Al³⁺ and Zn²⁺ (dissolves) from Ca²⁺ and Mg²⁺ (does not dissolve).
3. Writing “orange” for sodium’s flame colour without accepting “yellow” — the standard answer is yellow or yellow-orange.
4. Forgetting to clean the nichrome wire between flame tests, leading to sodium contamination masking the true colour.