What is a chemical reaction?

A chemical reaction is fundamentally defined as a process that involves the rearrangement of the molecular or ionic structure of a substance. This means that during a chemical reaction, bonds between atoms are broken and reformulated, allowing for the transformation of reactants into products with different chemical properties. The emphasis is on the structural change at the molecular level, leading to the creation of new substances as a result of this rearrangement.

This definition is precisely accurate because it captures the essence of what constitutes a chemical reaction, which includes processes such as combustion, oxidation, and synthesis. It is not restricted to specific conditions or outcomes, making it a broad and encompassing description.

In contrast, the other options either impose restrictions that do not universally apply to all chemical reactions or misunderstand the nature of such processes. For instance, a chemical reaction does not necessarily result in the formation of gas, nor does it require high temperatures to occur, as many reactions can happen at room temperature or even lower. Additionally, the notion that it creates new substances without changing existing ones is misleading, as it incorrectly suggests that the original substances remain unchanged; instead, they are transformed into entirely new entities.