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AQA GCSE · Question 07.1 · Chemical analysis
This question is about sulfuric acid. Sulfuric acid contains sulfate ions.
Describe the test for the presence of sulfate ions in sulfuric acid. Give the result of the test.
This question is about sulfuric acid. Sulfuric acid contains sulfate ions.
Describe the test for the presence of sulfate ions in sulfuric acid. Give the result of the test.
How to approach this question
This question asks for the chemical test for sulfate ions (SO₄²⁻).
1. Recall the reagents used to test for sulfate ions. It involves a barium compound.
2. Remember that, like the halide test, an acid must be added first to remove interfering ions (in this case, sulfite ions). State which acid is used.
3. Describe the positive result (the colour and type of substance formed).
Full Answer
**Test:**
Add a few drops of dilute hydrochloric acid, followed by a few drops of barium chloride solution.
**Result:**
A white precipitate (of barium sulfate) is formed.
The standard test for sulfate ions (SO₄²⁻) in a solution involves the use of barium chloride (BaCl₂) solution.
1. First, dilute hydrochloric acid (HCl) is added. This is to acidify the solution and to react with and remove any sulfite ions (SO₃²⁻) that might be present, as they would also form a white precipitate with barium chloride and give a false positive result.
2. Next, barium chloride solution is added. If sulfate ions are present, they will react with the barium ions (Ba²⁺) to form barium sulfate (BaSO₄), which is a dense white precipitate.
The ionic equation for the reaction is: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s).
Common mistakes
✗ Forgetting to add hydrochloric acid first.
✗ Using the wrong acid (e.g., nitric acid which is for halides, or sulfuric acid which contains sulfate ions itself).
✗ Using the wrong barium compound (e.g., barium nitrate).
✗ Stating the wrong colour precipitate.
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