2.7.3Inorganic Chemistry
Tests for Cations — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
5 min read 7 parts Lesson 38 of 52
Part 1 of 5Why sodium hydroxide works
Cations are positive ions
- Cations are positive ions A cation is a positively charged ion.
- Most metal ions in solution are cations, and many of them form coloured, insoluble hydroxides.
- Because the colours differ from metal to metal, the colour of the solid that appears is a reliable fingerprint.
What a precipitate is
- What a precipitate is A precipitate is an insoluble solid formed when two solutions are mixed.
- It shows as a cloudy suspension or as flakes that slowly settle.
- In these tests the precipitate is the metal hydroxide, written with the state symbol (s).
When nothing appears
- When nothing appears If no precipitate forms, the metal hydroxide may be soluble, or there may be too little of the metal ion in the sample.
- A very faint cloudiness or slight colour change can still count as a positive result.
Part 2 of 5Carrying out the test
1
Place a small sample of the unknown solution in a test tube.
2
Add a few drops of dilute sodium hydroxide solution, NaOH(aq), and shake gently.
3
Look closely for any precipitate and record its exact colour.
4
Keep adding a little more sodium hydroxide, because some precipitates change or dissolve when more alkali is added.
5
Match the colour of the precipitate to the table and write the ionic equation for the reaction.
Part 3 of 5Results for the metal ions you must know
| Cation | Ionic equation | Observation |
|---|---|---|
| Copper(II), Cu2+ | Cu2+(aq) + 2OH–(aq) → Cu(OH)2(s) | Light blue precipitate |
| Iron(II), Fe2+ | Fe2+(aq) + 2OH–(aq) → Fe(OH)2(s) | Pale green precipitate |
| Iron(III), Fe3+ | Fe3+(aq) + 3OH–(aq) → Fe(OH)3(s) | Orange-brown precipitate |
Part 4 of 5Testing for ammonium ions
1Add alkali
- Add alkali Add sodium hydroxide solution to the sample containing NH4+ ions.
2Warm gently
- Warm gently Heating the mixture helps the reaction go and drives off the gas formed.
3Ammonia is released
- Ammonia is released NH4+(aq) + OH–(aq) → NH3(g) + H2O(l)
4Test the gas
- Test the gas Hold damp red litmus paper at the mouth of the tube.
- Ammonia dissolves in the moisture to make an alkaline solution, so the paper turns blue.
Part 5 of 5Making a clear conclusion
Name the ion, not just the colour
- Name the ion, not just the colour A full answer states the reagent, the observation and the ion identified, for example: a light blue precipitate with sodium hydroxide shows copper(II) ions are present.
Telling the two irons apart
- Telling the two irons apart Both iron ions give a hydroxide, but the green one contains Fe2+ and the orange-brown one contains Fe3+.
- The charge on the metal ion decides the colour.
Why ammonium is different
- Why ammonium is different Ammonium ions give no precipitate.
- The evidence is a gas, so you need the extra step of testing the gas with litmus paper.
Key terms
| Term | Meaning |
|---|---|
| Cation | A positively charged ion, such as Cu2+ or NH4+. |
| Precipitate | An insoluble solid that forms when two solutions react. |
| Hydroxide ion | OH–, the ion in alkalis such as sodium hydroxide that combines with metal ions to give a precipitate. |
| Ionic equation | An equation showing only the ions that take part in a reaction, leaving out the spectator ions. |
| Damp red litmus paper | Indicator paper that turns blue in alkaline ammonia gas. |
Avoid these mistakes
1
Writing just 'blue' for copper(II) hydroxide. Say 'light blue precipitate' to be sure of the mark.
2
Mixing up the iron ions. Iron(II) gives a pale green precipitate and iron(III) gives an orange-brown one.
3
Forgetting to warm the mixture when testing for ammonium ions, or forgetting that the litmus paper must be damp.
4
Dropping the state symbols in ionic equations. The ions are (aq) and the hydroxide formed is (s).
5
Rejecting a faint cloudiness. A small amount of the ion can give only a slight precipitate, which is still a positive result.
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