Resistance to oxidation follows the reactivity series

1. A metal reacts with the oxygen in air to form its oxide, which is an oxidation.
2. A metal high in the reactivity series is oxidised quickly, because its atoms lose electrons readily.
3. Potassium and sodium tarnish within seconds of being cut, so they are stored under oil.
4. Iron and zinc are oxidised slowly in moist air over weeks and months.
5. Gold and platinum are not oxidised in air at all, which is why they keep their shine.

Aluminium behaves better than its position suggests

1. Aluminium sits above zinc and iron, so it would be expected to corrode faster than either.
2. In practice aluminium objects last for years without visibly corroding.
3. The metal reacts at once with air to form a thin layer of aluminium oxide.
4. That layer sticks firmly to the surface and seals the metal underneath from further attack.
5. The oxide layer, not low reactivity, is what makes aluminium useful outdoors.

Recycling returns a used metal to service

1. A used metal object is collected, sorted, melted and cast into new stock.
2. The metal is an element, so it is not used up and can be recycled many times over.
3. Recycling skips mining, transport and reduction of the ore entirely.
4. Recycling aluminium uses only a small fraction of the energy that electrolysis of its ore demands.
5. Each tonne recycled leaves a tonne of ore in the ground for later use.

The case for recycling is economic as well as environmental

1. Less energy is bought, so the running cost per tonne of metal is usually lower.
2. Less ore is mined, which limits habitat damage and the waste rock left behind.
3. Fewer fossil fuels are burned, so less carbon dioxide reaches the atmosphere.
4. Less metal is sent to landfill, where it takes up space and can leach into the ground.
5. A reserve of ore is a finite raw material, so using less of it protects future supply.

Judging the evidence on recycling a particular metal

1. The saving is largest where extraction is most expensive, which is why aluminium is the clearest case.
2. For a cheaply extracted metal the saving is smaller, though it still exists.
3. The quality of the recycled metal matters, since some scrap is only fit for lower-grade uses.
4. Distance counts, because transporting scrap a long way uses fuel that eats into the saving.
5. A sound judgement compares energy, cost and supply together rather than any one of them alone.