In a typical fuel cell, such as the hydrogen-oxygen fuel cell, hydrogen ($H_2$) and oxygen ($O_2$) are used as reactants. They are bubbled through porous carbon electrodes into a concentrated aqueous sodium hydroxide solution. The overall reaction is the combustion of hydrogen to form water. Anode reaction: $2H_2(g) + 4OH^-(aq) \rightarrow 4H_2O(l) + 4e^-$ Cathode reaction: $O_2(g) + 2H_2O(l) + 4e^- \rightarrow 4OH^-(aq)$ Overall reaction: $2H_2(g) + O_2(g) \rightarrow 2H_2O(l)$ Therefore, the reactants (R) are $H_2(g)$ and $O_2(g)$, and the product (P) is $H_2O(l)$.