Electrolysis

Electrolysis uses electrical energy to break down an ionic substance. It works when the substance contains ions that are free to move, such as when it is molten or dissolved in water. A substance that conducts electricity through moving ions and is decomposed by the current is called an electrolyte.

Two electrodes are placed in the electrolyte and connected to an electrical supply. The supply makes one electrode negative and the other positive. When current flows, ions move through the electrolyte towards the electrode with the opposite charge:

• Positive ions move towards the negative electrode, called the cathode. There they gain electrons and are discharged.
• Negative ions move towards the positive electrode, called the anode. There they lose electrons and are discharged.

Discharge means that ions gain or lose electrons at an electrode, producing particles that can form elements. For example, positive metal ions can gain electrons at the cathode to form metal atoms.

The electrical supply drives these changes: ions carry charge through the electrolyte, while electrons are supplied to or removed from the electrodes. The result is chemical decomposition of the electrolyte.

Electrolytes

An ionic compound conducts electricity only when its charged particles, or ions, can move. Mobile ions carry charge from place to place through the substance.

In a solid ionic compound, the ions are held in fixed positions in a lattice. Although the ions are charged, they cannot move through the solid, so it does not conduct electricity.

When the compound is melted or dissolved in water, its ions are no longer fixed in the lattice. They can move through the liquid or solution and carry charge, so the substance conducts electricity.

A liquid or solution that contains ions free to move and can therefore conduct electricity is called an electrolyte.