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Tests for cations: flame tests and ammonium ions

Edexcel IAL Chemistry · WCH13 Practical Skills in Chemistry I

Unit 3 questions test both the results of cation tests and how to carry them out. The chemistry is in the Unit 2 lessons; here the focus is on technique and on telling similar results apart.

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Flame test technique

1. Clean a nichrome (or platinum) wire by dipping it in concentrated hydrochloric acid and holding it in a hot flame until it gives no colour.

2. Dip the wire in the acid again, then in the solid sample.

3. Hold it in the edge of a non-luminous (blue) Bunsen flame and note the colour.

A wooden splint soaked in the solution is cheaper and easy to get, but the burning wood and its smoke can mask the colour (June 2025 mark scheme), because wood burns with its own yellow flame. Sodium is a common contaminant whose strong yellow can mask other colours.

The colours arise because electrons are excited to higher levels and give out light of particular frequencies as they fall back.

Flame colours

Ion	Flame colour
Li+	red (crimson)
Na+	yellow-orange
K+	lilac
Rb+	red-violet
Cs+	blue-violet
Ca2+	orange-red (brick red)
Sr2+	red (crimson or scarlet)
Ba2+	pale (apple) green
Mg2+	no colour

Learn the exact words: brick red is calcium, not strontium.

Ammonium ions and telling ions apart

Ammonium: warm the solid or solution with sodium hydroxide solution; ammonia is given off and turns damp red litmus blue. NH4+ + OH- -> NH3 + H2O.
When two flame colours are similar, use a precipitation test. Lithium and strontium both give red flames, but strontium sulfate is insoluble while lithium sulfate is soluble: add dilute sulfuric acid (or sodium sulfate solution) and only Sr2+ gives a white precipitate.
Group 2 hydroxides become more soluble down the group, and sulfates less soluble: Mg(OH)2 is a white precipitate with NaOH(aq); BaSO4 is a white precipitate with sulfate ions.

Worked example

Two colourless solutions are lithium chloride and strontium chloride. Both give red flames. How can you tell them apart?

Use solubility of sulfates: SrSO4 is insoluble, Li2SO4 is soluble.
Add a few drops of dilute sulfuric acid to each.
White precipitate: strontium chloride. No change: lithium chloride.

Answer: Add dilute H2SO4: a white precipitate shows Sr2+; no precipitate shows Li+.

Key idea: Clean the wire in conc HCl, use a blue flame. Li red, Na yellow, K lilac, Ca orange-red, Sr crimson, Ba green. NH4+: warm with NaOH, damp red litmus turns blue. Use sulfate precipitation to separate Sr2+ from Li+.

Check yourself

Which ion gives a lilac flame?
- Na+
- Li+
- Ba2+
- K+
Show answer
K+. Potassium gives lilac; sodium yellow; lithium red; barium green.
A solid gives an orange-red (brick red) flame. Which ion is present?
- Sr2+
- Ca2+
- Li+
- Na+
Show answer
Ca2+. Brick red is calcium. The October 2021 report saw many students give brick red for strontium.
Why is the flame test wire dipped in concentrated hydrochloric acid before use?
- To make the flame hotter
- To neutralise the sample
- To make the wire glow
- To clean off traces of other substances that would colour the flame
Show answer
To clean off traces of other substances that would colour the flame. Contaminants such as sodium would give their own colour.
Which ion gives a pale green flame?
- Ba2+
- Ca2+
- K+
- Mg2+
Show answer
Ba2+. Barium gives an apple-green flame.
What is the main disadvantage of using a wooden splint instead of a wire for a flame test?
- Splints are more expensive
- Splints cannot absorb solutions
- The burning wood gives a yellow flame that can mask the colour
- Splints only work for Group 2 ions
Show answer
The burning wood gives a yellow flame that can mask the colour. Wood contains sodium compounds and burns yellow.
Which reagent distinguishes strontium chloride solution from lithium chloride solution?
- Dilute sulfuric acid
- Dilute nitric acid
- Sodium chloride solution
- Distilled water
Show answer
Dilute sulfuric acid. Strontium sulfate precipitates; lithium sulfate stays dissolved. Only about 21% of candidates managed this in June 2025.
Describe the test for ammonium ions, including the result.
Show answer
Warm with sodium hydroxide solution; the gas given off (ammonia) turns damp red litmus paper blue.. Ammonia is the only common alkaline gas.

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Last updated 8 November 2026. Spotted a mistake? Tell us.