Calcium carbonate (CaCO₃) decomposes when heated to form calcium oxide (CaO) and carbon dioxide (CO₂). Explain, using ideas about elements, compounds, and mixtures, why calcium carbonate is a compound and why the products of this reaction are not a mixture.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Calcium carbonate is a white solid found in limestone. When strongly heated, it breaks down into two substances: calcium oxide and carbon dioxide.
Model answer (5 marks)
Calcium carbonate is a compound because it is made up of calcium, carbon and oxygen atoms that are chemically bonded together. The atoms are present in a fixed ratio of 1:1:3, so the composition is constant. A mixture would contain two or more substances that are not chemically bonded.
The reaction gives calcium oxide and carbon dioxide. Each of these is a separate compound with its own formula (CaO and CO₂) and its own chemical bonds. They are not chemically bonded to each other, so they are not a single compound or a mixture. They could be collected together, but each product is still a distinct compound, not a mixture.
The reaction gives calcium oxide and carbon dioxide. Each of these is a separate compound with its own formula (CaO and CO₂) and its own chemical bonds. They are not chemically bonded to each other, so they are not a single compound or a mixture. They could be collected together, but each product is still a distinct compound, not a mixture.
Examiner tips
- Use the definition of a compound – atoms chemically bonded in a fixed ratio.
- State the fixed 1:1:3 ratio for CaCO₃.
- Explain that CaO and CO₂ are separate compounds, not bonded together.
- Mention that they are not a mixture because they are distinct substances.
Common mistakes
- Saying CaCO₃ is a mixture instead of a compound.
- Claiming CaO and CO₂ are chemically bonded to each other.
- Forgetting to mention the fixed ratio of elements in CaCO₃.
Mark scheme (5 marks)
- Calcium carbonate is a compound because it contains two or more elements chemically combined / bonded together
- The elements in calcium carbonate are present in fixed proportions / a fixed ratio
- A mixture contains two or more substances that are NOT chemically combined / bonded
- Calcium oxide and carbon dioxide are separate / distinct compounds (each with their own chemical formula/bonds)
- The products (calcium oxide and carbon dioxide) are not in fixed proportions to each other / are not chemically bonded to each other, so they could form a mixture if collected together, BUT each individual product is itself a compound not a mixture
Key terms in this question
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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