A Level Chemistry OCR Practice Exam

What is thermal decomposition?

1. The formation of new substances without heat

2. The process of a chemical substance breaking down with heat

3. The process of combining substances through heat

4. The creation of larger molecules from smaller ones

The correct answer is: The process of a chemical substance breaking down with heat

Thermal decomposition is accurately defined as the process in which a chemical substance breaks down into simpler substances or elements when subjected to heat. This breakdown typically occurs as a result of the absorption of thermal energy, which disrupts the chemical bonds within the substance, leading to its decomposition. In the context of this question, the focus on heat is crucial. The process is characterized by the requirement of sufficient thermal energy to instigate this decomposition reaction. For example, when calcium carbonate (CaCO₃) is heated, it decomposes into calcium oxide (CaO) and carbon dioxide (CO₂). This reaction exemplifies thermal decomposition as it involves a single compound breaking down due to heat. The other definitions do not accurately represent thermal decomposition: the first option discusses the formation of new substances without heat, which is not related to the concept. The third option relates more to synthesis reactions, where substances combine rather than break down. The fourth option addresses polymerization or a process that leads to larger molecules, which again, diverges from the concept of decomposition. Thus, the choice that correctly describes thermal decomposition is the one that emphasizes the breakdown of a substance due to heat input.