Thermite welding uses the strong reducing power of aluminium to displace iron from its oxide. The reaction is highly exothermic.
Fe₂O₃(s) + 2Al(s) → 2Fe(l) + Al₂O₃(s) + Heat
Reduction: Fe₂O₃ loses oxygen to become Fe.
Oxidation: Al gains oxygen to become Al₂O₃.
Result: Molten iron (approx 2500°C) fills the gap between the rails.
Why it glows: above about 525°C a solid emits visible light. Colour shifts red → orange → yellow → white as the temperature climbs.
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What this experiment shows
The thermite reaction is the standard demonstration that a more reactive metal will take oxygen away from the oxide of a less reactive one. Aluminium is above iron in the reactivity series, so a mixture of aluminium powder and iron(III) oxide, once started, reduces the iron oxide to molten iron and leaves aluminium oxide behind. The reaction is strongly exothermic - hot enough to melt the iron it makes, which is why it is used to weld railway track in the field rather than in a workshop.
Fe2O3 + 2Al → 2Fe + Al2O3
What you need
Iron(III) oxide and aluminium powder, dry and finely divided
A refractory crucible standing in dry sand
Magnesium ribbon fuse, or a strip of magnesium in a bed of barium peroxide
Tongs, safety screen and a face shield
How it is done
Mix the dry iron(III) oxide and aluminium powder in the ratio the equation gives.
Stand the crucible in sand, well away from anything that can catch.
Push a short magnesium fuse into the mixture and light it from a distance.
Stand back behind the screen. The reaction runs itself once it has started.
Let everything cool before you go near it. Molten iron stays dangerous for a long time.
What you should see
A brilliant white flare and a shower of sparks.
Molten iron collects at the bottom of the crucible and sets into a grey bead.
A grey-white powder, aluminium oxide, is left on top of it.
The crucible glows: the energy released is far larger than the energy put in to start it.
Where marks are lost
Calling aluminium the oxidising agent. It is the reducing agent - it is oxidised, and it reduces the iron(III) oxide.
Saying the reaction is exothermic because you had to heat it. The magnesium only supplies the activation energy; the energy that comes out is much larger.
Writing the equation unbalanced. Two aluminium atoms are needed for one Fe2O3.
Forgetting to name the products. Marks are for iron and aluminium oxide, not "a metal and a powder".
Questions students ask
Why does aluminium displace iron?
Aluminium is higher in the reactivity series, so it holds oxygen more strongly than iron does. In a competition for the oxygen, aluminium wins, and the iron is left as the free metal.
Is the thermite reaction redox?
Yes. Aluminium loses electrons and is oxidised to Al3+; iron(III) is reduced to iron. Oxidation and reduction always happen together.
Why is it used to weld rails?
It makes its own molten iron on the spot, with no mains power and no furnace, so a rail joint can be welded anywhere on the track.
Written for Cambridge IGCSE Chemistry 0620 and CBSE Class 9-12 by
Ajay Shekhawat, founder of MrChemCoach. Run the simulator above,
then check yourself against the questions.