Section 1 of 6The idea of the test
Barium ions meet sulfate ions
- Barium ions meet sulfate ions Barium ions, Ba2+, react with sulfate ions, SO42-, to form barium sulfate, which is insoluble in water.
- Because it is insoluble, the barium sulfate appears as a white solid suspended in the liquid: a precipitate.
Choice of reagent
- Choice of reagent The usual reagent is barium chloride solution, BaCl2(aq).
- Barium nitrate solution works equally well.
Why acid first
- Why acid first Barium ions also form a white precipitate with carbonate ions.
- Adding acid first destroys any carbonate present, so it cannot cause a false positive.
Section 2 of 6Method
1
Place a small amount of the solution to be tested in a test tube.
2
Add a few drops of dilute hydrochloric acid and mix. If the sample fizzes, wait until it stops.
3
Add a few drops of barium chloride solution.
4
A white precipitate means sulfate ions are present. If the solution stays clear, there are no sulfate ions.
Section 3 of 6Equations
| Type | Equation |
|---|---|
| Ionic equation | Ba2+(aq) + SO42-(aq) → BaSO4(s) |
| Example full equation | BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaCl(aq) |
| Why acid removes carbonate | CO32-(aq) + 2H+(aq) → CO2(g) + H2O(l) |
Section 4 of 6Reasoning through the result
1Acidify
- Acidify Hydrochloric acid reacts with any carbonate and releases carbon dioxide.
2Add barium ions
- Add barium ions The barium chloride supplies Ba2+ ions.
3Precipitate forms
- Precipitate forms If sulfate is present, insoluble barium sulfate forms.
4Conclusion
- Conclusion A white precipitate that is not removed by acid confirms sulfate ions.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Sulfate ion | The negative ion SO42-, found in compounds such as sodium sulfate. |
| Barium chloride | BaCl2, the soluble salt used to supply barium ions. |
| Precipitate | An insoluble solid formed when a reaction takes place in solution. |
| Ionic equation | An equation showing only the ions that actually take part in the reaction. |
| False positive | A result that suggests an ion is present when it is not. |
Section 6 of 6Exam technique
1
Adding barium chloride without acid first. Carbonate ions would also give a white precipitate, so the result would be unreliable.
2
Saying the precipitate is barium chloride. It is barium sulfate, BaSO4.
3
Forgetting that barium nitrate can be used instead of barium chloride.
4
Describing the positive result as 'cloudy white gas'. It is a white solid precipitate, not a gas.