What does thermal decomposition signify?

Thermal decomposition refers to the process where a compound breaks down into simpler substances when subjected to heat. This process is typically irreversible, meaning that once the original compound has decomposed, it cannot be restored to its original form through the removal of heat or other means.

In the context of solid or liquid substances, thermal decomposition might involve the breaking of chemical bonds within the material, leading to the formation of gaseous products or different solid residues. For instance, heating calcium carbonate leads to the decomposition into calcium oxide and carbon dioxide gas. This illustrates that the original substance is fundamentally altered and cannot revert to its original state simply by cooling.

The other options do not accurately capture the nature of thermal decomposition. A reversible change in state pertains more to physical changes that do not alter the chemical composition of the substances involved, such as the transition between solid and liquid states. A physical change primarily in liquids would not appropriately describe thermal decomposition, as it encompasses chemical transformations that may occur in solids, liquids, or gases. Lastly, while gases can certainly relate to products of thermal decomposition, defining thermal decomposition as merely a chemical alteration of gases misses the primary focus on the original solid or liquid compound breaking down, thus simplifying a more complex process.