OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2023: Question 11

✅ Correct Answer

Row D is correct because adding dilute hydrochloric acid to aqueous potassium carbonate produces effervescence (due to carbon dioxide gas evolution), and adding dilute hydrochloric acid to aqueous silver nitrate produces a white precipitate of silver chloride.

💡 Key Knowledge

• Carbonate test: Addition of any dilute acid to a carbonate (or hydrogencarbonate) produces effervescence. The gas turns limewater cloudy: CO₃²⁻(aq) + 2H⁺(aq) → CO₂(g) + H₂O(l)
• Silver nitrate test: Aqueous silver nitrate reacts with chloride ions (introduced by hydrochloric acid) to form an insoluble white precipitate: Ag⁺(aq) + Cl⁻(aq) → AgCl(s)

🧠 Exam Technique

Break the question down into two independent mini-tasks. First, analyse what happens when HCl(aq) meets K₂CO₃(aq) . Then, analyse what happens when HCl(aq) meets AgNO₃(aq) . Match your findings to the options to secure the single mark quickly.

❌ Common Errors

• Forgetting that hydrochloric acid itself contains chloride ions ( Cl⁻ ), leading students to mistakenly think nothing happens when mixed with silver nitrate.
• Confusing the order of tests—remember that when testing for halides with silver nitrate, nitric acid is normally used instead of hydrochloric acid to remove interfering carbonate/sulfite impurities! However, adding HCl directly to silver nitrate *will* precipitate AgCl.