In chemical reactions, what is a key characteristic?

In chemical reactions, a key characteristic is that the molecular structure changes to form new compounds. This process involves breaking the bonds in the reactants and forming new bonds in the products, which results in the production of substances with different chemical properties and structures than the original reactants.

During a chemical reaction, the rearrangement of atoms leads to the creation of new molecules, making it distinct from physical changes where the identity of the substances remains the same, and only their states or appearances might change. For example, when hydrogen gas reacts with oxygen gas, new water molecules are formed, which have different properties from either of the gaseous reactants.

The other options do not accurately describe this fundamental aspect of chemical reactions. The structure of the compounds does change (contrary to the first choice), energy changes can occur in various forms and may include both absorption and release (thus not limited to absorption alone), and reactions involve chemical changes and not merely alterations of physical states. Therefore, the transformation of molecular structure is central to understanding chemical reactions.