electrolysis
/ ih-lek-TROL-iss-iss /
Imagine pushing a reaction uphill. Water happily stays as water; it has no urge to split into hydrogen and oxygen. But pass a strong enough electric current through it and you can force the split anyway — bubbles of gas rising from each electrode. Electrolysis is using electricity to drive a chemical reaction that would not happen on its own.
In an electrolysis setup, an outside power source (a battery or charger) acts as a pump for electrons. It pulls electrons off one electrode and crams them onto the other, forcing oxidation at one and reduction at the other even though the reaction is not spontaneous. The substance being decomposed must be able to conduct ions — usually a molten salt or a solution — so that ions can drift to the electrodes and react there.
Electrolysis is the workhorse of heavy industry. Aluminium is won from its ore this way; chlorine and sodium hydroxide are made by electrolyzing brine; metals are purified and objects electroplated. The price is energy: because you are pushing the reaction uphill, electrolysis always consumes electrical work rather than producing it.
Pass a current through salty water and bubbles appear: hydrogen gas collects at one electrode and chlorine at the other, while sodium hydroxide builds up in the solution. None of this happens without the current.
Electrolysis of brine: a current forces a split that would never happen alone.
How much product you get is fixed by Faraday's laws: it is proportional to the charge passed. Doubling the current or the time roughly doubles the amount deposited or released.