Which statement describes a characteristic of chemical reactivity?

The characteristic of chemical reactivity that is accurately described by the chosen statement involves transformations at the molecular level. When substances react chemically, they undergo changes that alter the bonds between atoms, resulting in the formation of new substances with different properties. This process occurs during chemical reactions, and it is fundamental to the way chemicals interact with each other.

In contrast to the selected answer, the other statements do not accurately reflect the nature of chemical reactivity. For instance, external conditions such as temperature, pressure, and the presence of catalysts can significantly influence how substances react, which is why the statement regarding the lack of influence from external conditions is incorrect. Additionally, while some chemical reactions may release heat, not all chemical reactions require heat to initiate; thus, the reasoning behind the necessity of heat is not applicable for all scenarios. Lastly, chemical reactivity inherently involves chemical changes rather than physical changes; physical changes do not alter the molecular structure of the substances involved.