What occurs during a chemical reaction?

During a chemical reaction, one or more substances undergo a transformation to become different substances. This process involves breaking and forming chemical bonds, resulting in new products with distinct properties from the original reactants. The transformation can include changes in composition, structure, and energy. For example, when hydrogen gas reacts with oxygen gas, they form water, which is a different substance from either gas initially.

Other options do not accurately describe what happens in a chemical reaction. The first option incorrectly suggests that substances can be combined without alteration, which does not reflect the fundamental nature of a chemical reaction where reactants are converted into products. The third option implies that heat is always necessary for a reaction, but many reactions can occur at room temperature or without any heat input. The fourth option mentions only a change in physical state, which is characteristic of physical changes rather than chemical reactions, where new substances are formed.