A student carries out an experiment in which dilute hydrochloric acid is added to solid calcium carbonate. Explain the type of reaction that occurs and describe the role of the acid in terms of the ions present in solution.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Calcium carbonate is an ionic compound found in limestone. When dilute hydrochloric acid is added to calcium carbonate, a reaction takes place that produces a gas, a salt, and water. The ionic equation for the reaction is: CaCO₃ + 2H⁺ → Ca²⁺ + H₂O + CO₂.
Model answer (5 marks)
The reaction is an acid–base (neutralisation) reaction. The dilute HCl supplies H⁺ ions to the solution. The carbonate ion, CO₃²⁻, acts as the base and accepts the H⁺ ions. This protonation breaks the carbonate ion down, producing CO₂ gas. The remaining ions, Ca²⁺ and Cl⁻, combine to give the salt calcium chloride, and water is formed from the H⁺ and OH⁻ produced during the reaction. Thus the acid is neutralised, giving a salt, water and CO₂ gas.
Examiner tips
- Identify the reaction type first (acid–base).
- Show the role of H⁺ and CO₃²⁻ explicitly.
- Mention the products (CO₂, CaCl₂, H₂O).
- Use correct chemical symbols and charges.
Common mistakes
- Confusing the reaction as a decomposition instead of acid–base.
- Omitting the role of the carbonate ion as the base.
- Writing the salt incorrectly (e.g., CaCO₃ instead of CaCl₂).
Mark scheme (5 marks)
- The reaction is an acid-base (neutralisation) reaction
- The acid provides hydrogen ions (H⁺) in solution
- The carbonate ion (CO₃²⁻) acts as the base by accepting / reacting with the H⁺ ions
- Carbon dioxide gas is produced as the carbonate ion is broken down / decomposed by the acid
- A salt (calcium chloride) and water are also formed, showing the acid has been neutralised
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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