Oxidation and reduction
Oxidation is loss of electrons; reduction is gain of electrons -- remembered by the mnemonic OIL RIG (Oxidation Is Loss, Reduction Is Gain). Every redox reaction involves both happening together: one species is oxidized while another is reduced.
Example 1: In the reaction , identify what is oxidized and what is reduced.
Zinc goes from 0 to +2, losing 2 electrons -- it is oxidized. Copper goes from +2 to 0, gaining 2 electrons -- it is reduced.
Oxidation numbers
Oxidation numbers track how "oxidized" an atom is in a compound. Free elements have an oxidation number of 0. Simple ions have an oxidation number equal to their charge. In compounds, oxygen is usually -2 and hydrogen is usually +1; the oxidation numbers in a neutral compound must add up to 0 (or to the ion's overall charge).
Example 2: Find the oxidation number of sulfur in sulfuric acid, .
Hydrogen contributes ; oxygen contributes . The compound is neutral, so sulfur's oxidation number satisfies , giving .
Electrolysis
Electrolysis passes an electric current through a molten ionic compound or a solution containing ions, splitting it apart. Positive ions (cations) move to the negative electrode (cathode) and gain electrons (reduction); negative ions (anions) move to the positive electrode (anode) and lose electrons (oxidation). This is how reactive metals like aluminium are extracted from their molten ores, and how compounds like water can be split into hydrogen and oxygen.