Electrolysis Definition

Electrolysis is a process by which electric current is passed through a substance to effect a chemical change. The chemical change is one in which the substance loses or gains an electron (oxidation or reduction). The process is carried out in an electrolytic cell, an apparatus consisting of positive and negative electrodes held apart and dipped into a solution containing positively and negatively charged ions. The substance to be transformed may form the electrode, may constitute the solution, or may be dissolved in the solution.

Positive and Negative Electrodes

When the electrodes are connected to a battery (or another source of electrical energy), one becomes positively charged and one becomes negatively charged. Electric current (i.e., electrons) enters through the negatively charged electrode (cathode); components of the solution travel to this electrode, combine with the electrons, and are transformed (reduced). The products can be neutral elements or new molecules. Components of the solution also travel to the other electrode (anode), give up their electrons, and are transformed (oxidized) to neutral elements or new molecules.

Ingredients or Materials

• Electrolyte: a watery solution or a salt melting which gives the ions a possibility to transfer between two electrodes.
• Direct current: the current that flows in one direction only.
• Electrodes: positive and negative electrodes held apart and dipped into a solution containing positively and negatively charged ions.
• External power source: a battery or another source of electrical energy.

Products of Electrolysis

The desired products of electrolysis are often in a different physical state from the electrolyte and can be removed by:

• Evaporation
• Filtration
• Decantation

To predict the reactions of the electrolysis, the “electrochemical electromotive series” is used. This table shows the elements ordered by their standard potentials (E0). The standard potential shows the capability, in regard to hydrogen ions, to give up electrons. In this table, the standard potential of hydrogen is 0. The other elements have a positive or a negative standard potential.

Electrolysis is used extensively in metallurgical processes, such as in extraction (electrowinning) or purification (electrorefining) of metals from ores or compounds and in deposition of metals from solution (electroplating).