What characterizes a redox reaction?

A redox reaction is characterized by the transfer of electrons between chemical species. In these reactions, one substance undergoes oxidation, losing electrons, while another substance undergoes reduction, gaining those electrons. This transfer of electrons is fundamental to the chemical changes taking place, leading to alterations in the oxidation states of the involved elements.

The first choice mentions the creation of new substances, which can happen in redox reactions, but it does not define the essential characteristic of such reactions, which is the electron transfer. The third option refers to acid-base reactions, which do not necessarily involve electron transfer and thus do not qualify as redox reactions. Lastly, the option regarding catalysts is also not correct, as redox reactions can occur without a catalyst; while catalysts can speed up reaction rates, they are not a defining feature of redox processes. The defining characteristic remains the movement of electrons, highlighting why the second option is the most accurate.