OCR A-Level Chemistry AS Breadth in chemistry (01), June 2024: Question 10

✅ Correct Answer: B

The correct sequence is carbonate, sulfate, halide.

💡 Key Knowledge

• 1. Carbonate test first: Add dilute acid (e.g., HNO₃). Fizzing indicates CO₃²⁻. This test produces no false positives with the reagents used for the other two tests. (Note: Never use HCl if you are testing for halides afterwards!)
• 2. Sulfate test second: Add aqueous barium ions ( Ba²⁺ , usually BaCl ₂ or Ba(NO₃)₂ ). A white precipitate of BaSO₄ confirms sulfate. Carbonates also give a white precipitate with barium, which is why carbonates must be tested and removed first!
• 3. Halide test last: Add aqueous silver nitrate ( AgNO₃ ), followed by aqueous ammonia. Sulfates and carbonates do not interfere here, but both would give precipitates with silver ions, which is why halides must be tested last.

🧠 Exam Technique

When tackling multi-ion identification questions, always look for ion interference rules:

• Ask yourself: Does reagent X precipitate unwanted ions Y or Z?
• Barium precipitates both sulfates and carbonates. Therefore, carbonates must be eliminated before adding barium.

❌ Common Errors

• Selecting option D (sulfate first) leads to a false positive for sulfates if carbonates are present, because BaCO₃ is also a white solid.
• Failing to realise that the order of reagents is strictly dependent on avoiding cross-precipitation.