Balancing numerous chemical reactions often involves exchanges of electrons between different species. These reactions are called oxidation-reduction reactions, oxido-reduction reactions, or redox reactions. They can take place in a homogeneous phase, specifically, within a solution. They can also take place in a heterogeneous medium, on the surface of electrodes. In the first case, we simply speak of redox reactions (it is understood in solution). In the second, we speak of electrochemical reactions.