If the substance being oxidized is in direct contact with the substance being reduced, what will be produced?

a. mass

b. heat energy

c. friction energy

d. potential energy

The correct option is D. When an oxidant and a reductant are in direct contact, a potential energy is generated whose value is determined by the potential differences between the valence electrons in atoms of the participating elements. The oxidant is the species that gains electrons and is reduced in the process while the reductant is the specie that loses electrons and is oxidized in the process.