What electrolysis is

1. Electrolysis uses electrical energy to break an ionic compound down into simpler substances, which are usually elements.
2. It is a chemical change, because new substances are formed at the electrodes and the original compound is used up.
3. Electrolysis only works when the ionic compound is molten or dissolved in water to make an aqueous solution.
4. The current used is always a direct current (d.c.), so that one electrode stays positive and the other stays negative for the whole process.
5. An ionic compound can be broken down this way because it is made of ions, which are particles that carry an electric charge.
6. Electrolysis is used on a large scale to extract reactive metals such as aluminium, to manufacture chlorine and hydrogen, and to coat objects with a thin layer of metal.

Molten and aqueous electrolytes

1. The substance that is broken down during electrolysis is called the electrolyte.
2. An electrolyte conducts electricity because it contains ions that are free to move.
3. In a solid ionic compound, the ions are held in fixed positions in a giant ionic lattice, so they cannot move and the solid does not conduct.
4. When the compound is heated until it melts, the lattice breaks down and the ions can slide past one another.
5. When the compound dissolves in water, the lattice also breaks apart and the ions spread out through the water, where they are free to move.
6. Solid sodium chloride therefore does not conduct electricity, but both molten sodium chloride and aqueous sodium chloride do.
7. A molten electrolyte must be kept above its melting point, and sodium chloride, for example, has to be heated to $801{}^{\circ }C$ before it melts.
8. An aqueous electrolyte usually needs no heating, which makes it easier and safer to use in the laboratory.
9. Substances made of covalent molecules, such as sugar, ethanol and pure water, contain no ions or almost none, so they are not electrolytes.
10. A metal conducts electricity through its delocalised electrons rather than through ions, so it carries a current without being broken down and is not an electrolyte.

The electrolytic cell

1. Electrolysis is carried out in an electrolytic cell, which is made of four main parts.
   1. Power supply: a d.c. source, such as a battery or a laboratory power pack, that drives the current around the circuit.
   2. Electrodes: two solid rods or plates of a conducting material that dip into the electrolyte.
   3. Electrolyte: the molten or aqueous ionic compound that fills the space between the electrodes.
   4. Connecting wires: these join each electrode to one terminal of the power supply.
2. The circuit is only complete when the current passes through the electrolyte from one electrode to the other.
3. The two electrodes must not touch, otherwise the current passes straight between them and the electrolyte is bypassed.
4. A lamp or an ammeter is often added to the circuit to show that a current is flowing.
5. Electrodes are usually made of graphite, a form of carbon, or of platinum, because both conduct electricity well.
6. Graphite and platinum are described as inert, because they do not react with the electrolyte or with the substances formed on them.
7. Graphite is cheap and has a very high melting point, so it is used in most school experiments and for molten electrolytes.
8. Platinum is extremely unreactive but expensive, so it is used mainly in small laboratory cells.
9. For a molten electrolyte, the solid compound is placed in a heat-resistant container such as a crucible and heated strongly until it melts around the electrodes.
10. For an aqueous electrolyte, the solution is held in a beaker or a small plastic cell, and any gases formed can be collected in test tubes placed over the electrodes.

Anode and cathode

1. The two electrodes are named according to the terminal of the power supply that each one is connected to.
2. The anode is the positive electrode, because it is connected to the positive terminal of the power supply.
3. The cathode is the negative electrode, because it is connected to the negative terminal of the power supply.
4. In a circuit diagram, the longer line of the cell symbol is the positive terminal, so the electrode wired to it is the anode.
5. The electrode wired to the shorter line of the cell symbol is the cathode.
6. Swapping the two wires at the power supply swaps the anode and the cathode, so always trace the wires rather than relying on where an electrode sits in the diagram.
7. The memory aid PANIC, which stands for Positive Anode Negative Is Cathode, helps you keep the two names the right way round.