What is a critical characteristic of thermal decomposition?

Thermal decomposition is fundamentally characterized by an irreversible change in chemical composition. During this process, a compound breaks down into simpler substances when subjected to heat, leading to a transformation that cannot easily revert to the original compound without a specific set of conditions or reactions.

This transformation results in distinct products that differ from the starting material in both structure and chemical properties. For example, when calcium carbonate is heated, it decomposes into calcium oxide and carbon dioxide gas. This reaction showcases the nature of thermal decomposition as it cannot simply be reversed by cooling or restoring the original conditions; the original compound has chemically changed.

Other options do not accurately represent the nature of thermal decomposition. A reversible change back to the original material does not align with the concept of thermal decomposition, as this process is inherently linked to irreversible changes. While high oxygen levels may play a role in combustion, they are not necessary conditions for thermal decomposition, which can occur in the absence of oxygen. Regarding the concept of thermal stability, it refers more to a material's ability to withstand heat without decomposing, rather than being a defining characteristic of thermal decomposition itself. Thus, the correct response emphasizes the key aspect of this process in its irreversible nature.