International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

Which reaction in the blast furnace illustrates decomposition?

1. C + O₂ -> CO₂

2. CaCO₃ -> CaO + CO₂

3. SiO₂ + CaO -> CaSiO₃

4. C + CO₂ -> 2CO

The correct answer is: CaCO₃ -> CaO + CO₂

The reaction that illustrates decomposition is represented by the breakdown of calcium carbonate (CaCO₃) into calcium oxide (CaO) and carbon dioxide (CO₂). In a decomposition reaction, a single compound is broken down into two or more simpler products. In this case, when calcium carbonate is heated, it undergoes the thermal decomposition process, resulting in the formation of calcium oxide and releasing carbon dioxide gas. This is a key reaction in various industrial processes, including the production of quicklime from limestone. The other reactions involve different chemical processes. The first reaction entails the combustion of carbon to form carbon dioxide, which is not decomposition. The third reaction represents a synthesis reaction where silicon dioxide reacts with calcium oxide to form calcium silicate, rather than breaking a compound down. The fourth reaction features a reduction process where carbon reacts with carbon dioxide to produce carbon monoxide, which also does not fit the decomposition criteria. Thus, the decomposition reaction is accurately depicted by the breakdown of calcium carbonate in the given choice.