How are oxidation-reduction (redox) reactions defined?

Oxidation-reduction (redox) reactions are characterized by the transfer of electrons between substances. In these reactions, one substance undergoes oxidation by losing electrons, while another substance undergoes reduction by gaining those electrons. This transfer process is fundamental to redox chemistry and is crucial for various biological and chemical processes, including cellular respiration and combustion.

In redox reactions, the oxidation state of the reacting species changes, reflecting the electron transfer. For example, in a reaction where iron oxidizes to form iron oxide, iron loses electrons and increases its oxidation state, while oxygen gains electrons and decreases its oxidation state. This interplay of oxidation and reduction is a defining feature of redox reactions.

Other options, such as changes in temperature, specific reaction conditions like acidity, or the production of gas, do not inherently define redox reactions. They may occur in some redox processes, but they are not essential characteristics of all redox reactions. Thus, the defining characteristic of redox reactions is indeed the transfer of electrons between substances.