Qualitative Analysis: Tests for Ions Notes
Edexcel GCSE Chemistry: Revision notes
Key facts
- A test for an ion must be unique: only that ion gives the positive result.
- Flame tests: Li⁺ red, Na⁺ yellow, K⁺ lilac, Ca²⁺ orange-red, Cu²⁺ blue-green.
- Sodium hydroxide gives coloured precipitates; aluminium hydroxide dissolves in excess, calcium hydroxide does not.
- Carbonates give CO₂ with acid; sulfates give a white precipitate with barium chloride; halides give coloured precipitates with silver nitrate.
- Instrumental methods are more sensitive, accurate and fast.
A unique test
A unique test gives a positive result for one ion only, so the result identifies it.
A positive result must only ever come from that ion. If two ions gave the same result, the test could not tell you which was present.
If a result is ambiguous, a second confirmatory test is needed.
- 1
Add the test reagent
to the unknown sample
- 2
Positive result
this should come from one ion only
- 3
Identify the ion
only that ion could give this result
- 4
Ambiguous result
carry out a second confirmatory test
- A test for an ionpositive for that ion only
Why must a test for an ion be unique?
Flame tests
Heated in a flame, metal ions in a solid give a characteristic colour.
A flame test identifies certain metal ions in solid compounds. Dip a clean wire loop in dilute hydrochloric acid, pick up a sample, and hold it in a roaring blue Bunsen flame. Record the flame colour.
- 1
Clean the loop
dip a wire loop in dilute hydrochloric acid
- 2
Pick up the sample
dip the loop in the solid
- 3
Heat it
hold it in a roaring blue Bunsen flame
- 4
Record the colour
for example lilac for potassium, yellow for sodium
Group 1 ions
- Lithium, : red
- Sodium, : yellow
- Potassium, : lilac
Other ions
- Calcium, : orange-red
- Copper, : blue-green
Which ion gives a lilac flame?
Sodium hydroxide test
Sodium hydroxide makes insoluble hydroxides with many metal ions; the colour and behaviour in excess identify the ion.
Adding sodium hydroxide solution to a metal ion makes a coloured precipitate, an insoluble hydroxide. Aluminium and calcium both give white, so add excess sodium hydroxide. Aluminium hydroxide dissolves, but calcium hydroxide does not.
Ammonium ions give no precipitate. Warming releases ammonia, an alkaline gas that turns damp red litmus paper blue.
- 1
Add sodium hydroxide
to a solution of the metal ion
- 2
Precipitate forms
note its colour: blue Cu²⁺, green Fe²⁺, orange-brown Fe³⁺, white Al³⁺ or Ca²⁺
- 3
White precipitate
add excess: Al³⁺ dissolves, Ca²⁺ does not
- 4
No precipitate
warm it: ammonia turns damp red litmus blue, so NH₄⁺
White precipitate
- Al³⁺: dissolves in excess
- Ca²⁺: insoluble in excess
Coloured precipitate
- Cu²⁺: blue
- Fe²⁺: green
- Fe³⁺: orange-brown
No precipitate
- NH₄⁺: ammonia gas on warming
A white precipitate forms with sodium hydroxide and dissolves in excess. Which ion is present?
Carbonates, sulfates and halides
Each negative ion has its own test: acid for carbonates, barium chloride for sulfates and silver nitrate for halides.
Carbonate ions give carbon dioxide with dilute acid, which turns limewater cloudy.
For sulfate and halide tests, first add dilute acid to remove carbonate ions, which would give a false precipitate. Use hydrochloric acid for sulfates and nitric acid for halides.
| Carbonate | Sulfate | Halide | |
|---|---|---|---|
| Add | Dilute acid | Dilute HCl, then barium chloride | Dilute nitric acid, then silver nitrate |
| Positive result | Gas turns limewater cloudy | White precipitate | Cl⁻ white, Br⁻ cream, I⁻ yellow |
Carbonate
- Add:
- Dilute acid
- Positive result:
- Gas turns limewater cloudy
Sulfate
- Add:
- Dilute HCl, then barium chloride
- Positive result:
- White precipitate
Halide
- Add:
- Dilute nitric acid, then silver nitrate
- Positive result:
- Cl⁻ white, Br⁻ cream, I⁻ yellow
- 1
Acidify
add dilute hydrochloric acid to remove carbonate ions
- 2
Add barium chloride
solution
- 3
Observe
white precipitate of barium sulfate
A cream precipitate forms with silver nitrate. Which halide ion is present?
Unknown salts and instruments
Combine a cation test and an anion test; instrumental methods improve on manual tests.
This is a core practical. Use a flame test or sodium hydroxide test for the metal ion and the anion tests, then combine the results. A white precipitate that dissolves in excess sodium hydroxide, plus a white precipitate with acidified barium chloride, identifies aluminium sulfate.
Flame photometry is an instrumental method. It finds metal ions and their concentration by comparing readings with a calibration curve.
- 1
Test the cation
flame test or sodium hydroxide
- 2
Test the anion
carbonate, sulfate or halide test
- 3
Combine results
name the salt
| Manual tests | Instrumental methods | |
|---|---|---|
| Sensitivity | Less sensitive | More sensitive |
| Accuracy | Less accurate | More accurate |
| Speed | Slower | Faster |
Manual tests
- Sensitivity:
- Less sensitive
- Accuracy:
- Less accurate
- Speed:
- Slower
Instrumental methods
- Sensitivity:
- More sensitive
- Accuracy:
- More accurate
- Speed:
- Faster
Give one advantage of instrumental methods over manual tests.
Try an exam question
A solution contains aluminium ions and sulfate ions. Describe the tests you would do to show that both ions are present.
[4 marks]
- [1]Add sodium hydroxide solution: a white precipitate forms, showing aluminium ions.
- [1]Add excess sodium hydroxide: the precipitate dissolves, which distinguishes aluminium from calcium.
- [1]Add dilute hydrochloric acid, then barium chloride solution.
- [1]A white precipitate forms, showing sulfate ions.
That's the notes covered.
Carry on to the next subtopic.