How Electrolysis Works

Click through each step to understand the basics.

What Is Electrolysis?
Electrolysis is a process that uses externally supplied electrical energy to force a chemical reaction that wouldn't happen naturally on its own, breaking a compound down into its component substances. Since it converts electrical energy into chemical energy, it can be seen as the exact opposite of a battery, which converts chemical energy into electrical energy.
Electrolysis of Water
Pure water doesn't conduct electricity well on its own, but once a small amount of electrolyte is dissolved in it and a direct current is applied, hydrogen gas is produced at the cathode and oxygen gas at the anode. The volume ratio of hydrogen to oxygen produced comes out to roughly 2:1, matching the chemical formula H₂O.
How Electroplating Works
Electroplating passes current through a solution containing dissolved ions of the metal to be plated, connecting the object to be plated as the cathode so metal ions are reduced and deposited onto its surface as a thin metal layer. Plating jewelry with gold or silver, or chrome-plating steel products, both use this principle.
Aluminum Refining and Other Industrial Uses
Aluminum is mass-produced by electrolyzing molten aluminum oxide to obtain pure aluminum metal, a process that requires a large amount of electrical energy — which is why aluminum smelters are typically built where electricity is cheap. Electrolysis is also widely used in copper refining and in producing highly reactive metals like sodium and magnesium.
What Electrolysis Requires
Electrolysis requires an electrolyte in which ions can move freely, two electrodes (an anode and a cathode) through which current flows, and a direct-current power source to continuously supply current. Unlike a battery, electrolysis doesn't happen spontaneously, so an external power source is essential.
The Opposite Relationship With Batteries
A battery converts the energy from a spontaneous redox reaction into electrical energy, while electrolysis uses input electrical energy to force a non-spontaneous reaction to occur. In fact, feeding the hydrogen obtained from electrolyzing water into a fuel cell can generate electricity again, meaning the two processes can work together as a way to store and later release energy.
Electrolyzing Saltwater and the Chlor-Alkali Process
Electrolyzing saltwater (an aqueous sodium chloride solution) produces chlorine gas at the anode and hydrogen gas plus sodium hydroxide (caustic soda) at the cathode — this is called the chlor-alkali process. The chlorine and sodium hydroxide produced this way are used as raw materials across many industries, including bleach, disinfectants, soap, and paper manufacturing.

Why Electrolysis Is Worth Understanding

Electrolysis is a technology that uses electrical energy to force chemical reactions that don't happen easily on their own, and it's used throughout modern industry, from hydrogen production to metal plating and aluminum refining. If a battery converts chemical energy into electricity, electrolysis handles the conversion in the opposite direction.

Electrolysis Is Also a Redox Reaction

Oxidation and reduction happen simultaneously at the two electrodes during electrolysis — a related topic worth exploring separately, covering how oxidation and reduction are explained through the movement of electrons.

Frequently Asked Questions

Is pure water hard to electrolyze?

Yes — pure water contains very few ions, so it barely conducts electricity and electrolyzes very slowly. That's why, in practice, a small amount of electrolyte like sulfuric acid or sodium hydroxide is dissolved in the water first, so the ions can carry the current well before electrolysis is carried out.

Are electrolysis and batteries complete opposites?

In terms of principle, they run in opposite directions, but a single device can sometimes do both. A rechargeable battery acts like a battery when in use, converting chemical energy to electricity, and acts like an electrolysis cell when charging, using electrical energy to reverse the chemical reaction.