Electrolysis

Electrolysis uses an electric current to split an ionic compound into the elements it is made from. The liquid being split is called the electrolyte; it conducts because its ions are free to move. Two rods called electrodes dip in and are joined to a battery: the cathode is the negative one, the anode is the positive one.

A solid ionic compound cannot be electrolysed because its ions are locked in a fixed lattice and cannot move. Melting it, or dissolving it in water, frees the ions so they can drift to the electrodes and carry the current.

Positive ions are pulled to the cathode, where they gain electrons (reduction) and become metal atoms or hydrogen gas. Negative ions are pulled to the anode, where they lose electrons (oxidation) and become a non-metal. Use OIL RIG: Oxidation Is Loss, Reduction Is Gain.

For a molten compound only its own two ions are present, so you get the metal at the cathode and the non-metal at the anode. In water there are also H+ and OH- ions, which compete; the ion that is discharged depends on reactivity, concentration and the electrode used.