What occurs during a chemical reaction?

During a chemical reaction, substances undergo a process where they combine and rearrange their atoms to form new substances with different properties. This transformation involves breaking the bonds of the reactants and forming new bonds to create products. For example, when hydrogen gas reacts with oxygen gas, they form water, a completely new substance.

This process is fundamental to chemical reactions and illustrates how matter can change form while retaining its mass, in accordance with the law of conservation of mass. It highlights that the initial substances are not simply altered; rather, new substances are produced, which can have very different characteristics and behaviors compared to the original materials.

The other choices incorrectly characterize the nature of chemical reactions. It's important to recognize that energy can be absorbed or released during a reaction, and matter cannot be created from nothing, adhering to the principle that mass is conserved. Additionally, chemical reactions involve changes in both chemical and physical properties, not just physical changes.