Describe a test to show the presence of sulfate ions in the solution. Give the result of the test if there are sulfate ions in the solution.
How to approach this question
1. **Test:** Recall the chemical test for sulfate ions (SO₄²⁻). What two chemicals are added to the sample solution, and in what order? (Hint: one is an acid, the other is a barium compound).
2. **Result:** If sulfate ions are present, they react with the barium ions. What is formed, and what does it look like?
Full Answer
Test: Add dilute hydrochloric acid, followed by barium chloride solution.
Result: A white precipitate forms.
The standard laboratory test for sulfate ions (SO₄²⁻) is as follows:
**Test:**
1. First, add a few drops of **dilute hydrochloric acid (HCl)** to the sample solution. This is to react with and remove any carbonate ions that might be present, as they would also form a white precipitate with barium ions and give a false positive result.
2. Then, add a few drops of **barium chloride solution (BaCl₂)**.
**Result:**
If sulfate ions are present, they will react with the barium ions from the barium chloride to form barium sulfate (BaSO₄). Barium sulfate is an insoluble solid, so a **white precipitate** will be observed.
The ionic equation for the reaction is: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)
Common mistakes
✗ Forgetting to add the acid first.
✗ Using the wrong acid (e.g., sulfuric acid, which contains sulfate ions itself).
✗ Using the wrong barium compound (e.g., barium sulfate).
✗ Giving the wrong colour for the precipitate.
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