Each electrode attracts ions which are of the opposite [[Electric charge|charge]]. Therefore, positively charged ions (called [[cation]]s) move towards the cathode, while negatively charged ions (termed [[anion]]s) move toward the anode. The energy required to separate the ions, and cause them to gather at the respective electrodes, is provided by an electrical power supply. At the probes, [[electron]]s are absorbed or released by the ions, forming a collection of the desired element or compound. For example, when [[water (molecule)|water]] is electrolyzed, [[hydrogen]] gas (H<sub>2</sub>) bubbles at the cathode, and [[oxygen]] gas (O<sub>2</sub>) rises at the anode. This effect was first discovered by [[William Nicholson (chemist)|William Nicholson]], an English chemist, in [[1800]].
iTheThe amount of electrical energy that must be added equals the change in [[Gibbs free energy]] of the reaction plus the losses in the system. The losses can (theoretically) be arbitrarily close to zero, so the maximum [[thermodynamics|thermodynamic]] efficiency equals the [[enthalpy]] change divided by the free energy change of the reaction. In most cases the electric input is larger than the enthalpy change of the reaction, so some energy is released in the form of heat. In some cases, for instance in the electrolysis of [[steam]] into hydrogen and oxygen at high temperature, the opposite is true. Heat is absorbed from the surroundings, and the [[heating value]] of the produced hydrogen is higher than the electric input. In this case the efficiency can be said to be greater than 100%. (It is worth noting that the maximum theoretic efficiency of a [[fuel cell]] is the inverse of that of electrolysis. It is thus impossible to create a [[perpetual motion]] machine by combining the two processes. See [[water fuel cell]] for an example of such an attempt.)
The following technologies are related to electrolysis:
