What characterizes an oxidation-reduction reaction?

An oxidation-reduction reaction, commonly known as a redox reaction, is fundamentally characterized by the transfer of electrons between substances. In these reactions, one substance gets oxidized, meaning it loses electrons, while another substance gets reduced, gaining those electrons. This electron transfer is crucial because it alters the oxidation states of the reactants involved, leading to chemical changes.

Understanding redox reactions is essential in various contexts, including combustion, respiration, and corrosion processes. The relationship between the oxidized and reduced states directly influences the properties and reactivity of the substances involved. Therefore, the focus on electron transfer is what defines these specific types of chemical reactions.

Other choices don't fit the definition of redox reactions. The transfer of thermal energy refers to heat and is not involved in the electron transfer central to redox. The formation of new compounds can occur in redox reactions, but it is not a defining characteristic, as many reactions can produce new compounds without involving electron transfer. The change of state from solid to liquid pertains to physical changes in matter rather than a chemical reaction, which is a different concept altogether. Thus, it is the transfer of electrons that is the hallmark of oxidation-reduction reactions.