AFOQT Practice Test

The correct answer identifies that a chemical change is characterized by the formation of new substances with different properties. During a chemical change, the original substances undergo a transformation at the molecular level, resulting in the creation of new compounds or materials that have distinct characteristics compared to the components that initiated the reaction. This shift in composition and properties is a fundamental aspect of chemical processes, such as rust forming on metal or food cooking.

In contrast to this, the options pertaining to mass, original properties, and reversibility do not accurately define a chemical change. While mass conservation holds true in a closed system during a reaction, it does not illuminate the essence of a chemical transformation. The original properties of the substance do not remain unchanged; rather, they alter significantly. Additionally, while some chemical changes may be reversible, this is not a defining trait of all chemical reactions, as many are irreversible. Thus, the focus on the formation of new substances with distinct properties captures the essence of what constitutes a chemical change.