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4.3.6Electrochemistry

Sulfate Ions — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 8 of 20 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Part 1 of 5Carrying Out the Test

4.3.6 Sulfate Ions

Acid is added first to remove carbonates, then barium ions are added to test for sulfate.

1
Acidify the sample by adding a few drops of dilute hydrochloric acid to the solution being tested.
2
Add a few drops of barium chloride solution, which provides barium ions to react with any sulfate.
3
Observe whether a white precipitate forms in the solution.
4
Record a white precipitate as the positive result, which confirms that sulfate ions are present.
5
Use a fresh test tube for each sample, so that a precipitate from one test cannot be mistaken for the result of another.

Part 2 of 5Chemistry of the Test

Precipitate
  • Barium sulfate, BaSO₄(s), is an insoluble white solid that forms when barium and sulfate ions meet.
Ionic equation
  • The ionic equation for the test is Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s).
Alternative reagent
  • Barium nitrate solution can be used instead of barium chloride, with the same white precipitate result.
Sulfate ion
  • The sulfate ion, SO₄²⁻, is a common anion found in salts such as copper(II) sulfate.
Positive result
  • A dense white precipitate that does not dissolve in dilute acid confirms that sulfate ions are present in the sample.

Part 3 of 5Why Acid Comes First

Why Acid Comes First
Carbonate interference
  • Carbonate ions would form a white barium carbonate precipitate, which would look like a positive sulfate result.
Hydrochloric acid use
  • Hydrochloric acid removes carbonates without forming a precipitate with barium ions, so it is the correct acid to use.
Result interpretation
  • A white precipitate after acidification is the correct evidence, because the carbonate interference has been removed.
Distinction
  • Silver nitrate is used for halides, not sulfates, so it cannot be used for this test.

Part 4 of 5Worked Example

Question

  • A solution is acidified with dilute hydrochloric acid and barium chloride is added, producing a white precipitate.
  • Identify the anion and write the ionic equation.

  1. A white precipitate with barium ions shows sulfate ions are present.
  2. The precipitate is barium sulfate, BaSO₄.
  3. Write the ionic equationBa²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s).

Answer

  • Sulfate, SO₄²⁻;
    • Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s).

Part 5 of 5Exam Checklist & Tips

4.3.6 – Key Points for the Exam

  • Sulfate ion The SO₄²⁻ ion, a common anion found in many salts.
  • Barium sulfate An insoluble white solid, BaSO₄, used as the positive result for sulfate.
  • Barium chloride A reagent solution that provides barium ions for the sulfate test.
  • Interference A substance or ion that gives a false result by forming an unwanted precipitate.

Common Mistakes

  • Using silver nitrate to test for sulfate, which is a test for halides.
  • Skipping the acid step, so that a carbonate could give a false white precipitate.
  • Writing the barium sulfate precipitate without its state symbol (s).
  • Stating that a white precipitate proves sulfate without acidifying first.

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