Most metals are found in the Earth's crust as compounds

1. A rock is worth mining only when it holds enough of a metal compound to make extraction pay.
2. Most metals are found combined with other elements, very often as an oxide or a carbonate.
3. Gold and platinum are so unreactive that they occur as the uncombined elements.
4. An unreactive metal resists combining with oxygen, which is why it survives as the metal itself.
5. Obtaining a metal from a compound therefore means removing whatever it is joined to.

Extraction is a reduction, because the metal loses oxygen

1. Gain of oxygen is called oxidation and loss of oxygen is called reduction.
2. Heating a metal oxide with a substance that takes its oxygen reduces the ore: $$metal\; oxide+reducing\; agent\to metal+oxide\; of\; the\; reducing\; agent$$
3. The substance that takes the oxygen is itself oxidised in the same reaction.
4. The two descriptions of reduction agree: an oxide loses oxygen because the metal ions in it gain electrons.
5. Extracting a metal from a compound in its ore is a reduction of that metal.

Metals below carbon are reduced by heating with carbon

1. Carbon sits in the reactivity series between aluminium and zinc, placed there by experiment.
2. Carbon takes oxygen from the oxide of any metal below it, because those metals hold their oxygen less strongly.
3. Iron oxide is reduced by carbon on this basis: $$iron\; oxide+carbon\to iron+carbon\; dioxide$$
4. Zinc, iron, tin, lead and copper are all extracted this way.
5. Heating with carbon is cheap, since carbon comes from coke and the heat is supplied by burning fuel.

Metals above carbon are extracted by electrolysis

1. Potassium, sodium, calcium, magnesium and aluminium all sit above carbon, so carbon cannot reduce their ores.
2. Passing a direct current through the molten compound decomposes it and releases the metal.
3. Aluminium is obtained from aluminium oxide, which melts far too high to be liquefied on its own.
4. The oxide is instead dissolved in molten cryolite, which lowers the working temperature and cuts the energy used.
5. Aluminium forms at the negative electrode and oxygen is released at the positive electrode.

Reactivity and cost together decide the method

1. The position of the metal in the reactivity series decides whether carbon is strong enough to reduce its ore.
2. Where carbon works it is always chosen, because the process costs far less to run.
3. Electrolysis is reserved for the metals that nothing cheaper will reduce.
4. The price of a metal reflects this, so aluminium costs more to produce than iron of the same mass.
5. A metal found uncombined needs no reduction at all, only separation from the rock around it.