What does a "redox couple" consist of?

A "redox couple" consists of an oxidized species and its corresponding reduced form. In electrochemistry, redox reactions involve the transfer of electrons between two species, leading to one being oxidized (losing electrons) and the other being reduced (gaining electrons). This pair of species, where one is an oxidation product and the other its reduction product, forms the redox couple.

For example, in the redox couple involving hydrogen ions (H⁺) and hydrogen gas (H₂), H⁺ is the oxidized form and H₂ is the reduced form. The relationship between these two forms is crucial in understanding various electrochemical processes, such as those occurring in batteries and fuel cells.

The other options do not accurately describe a redox couple. A metal and its oxide refer to a specific type of reaction but do not encompass the broader definition of a redox couple. Similarly, an acid and its conjugate base, as well as a gas and its liquid state, relate to different chemical concepts and do not involve the transfer of electrons that defines redox processes.