What is the primary characteristic of a chemical change?

The primary characteristic of a chemical change is that energy is absorbed or released during the process. This energy change often results in the formation of new substances, altering the chemical composition of the original materials. For example, in a combustion reaction, energy is released in the form of heat and light, while in endothermic reactions, energy is absorbed.

Chemical changes are typically permanent because they produce different products with distinct physical and chemical properties compared to the starting materials. This distinguishes chemical changes from physical changes, which may involve a change in state or appearance without altering the substance itself.

While the other options touch on various aspects of changes in matter, they do not accurately define a chemical change. Substances typically do not remain the same in a chemical change, as this involves a change to their molecular structure. Similarly, chemical changes are not temporary; once a reaction occurs, the new substances produced cannot revert to their original state without further chemical reactions. Lastly, while phase changes can occur alongside chemical changes, they are not the defining feature of a chemical transformation. Thus, the release or absorption of energy is what fundamentally characterizes a chemical change.