What is happening in Thermite that allows Aluminum and Iron Oxide to burn so hot? Also, how is aluminum and iron able to burn like that considering they are metals?
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‘Burning’ can be more precisely called ‘rapid oxidation’. Or, it’s when lots of oxygen binds to something quickly. It also usually involves breaking bonds that were there previously, which is usually started by there being enough heat to start the bonds breaking, and then once that starts the energy released from the oxygen bonding creates enough heat to keep the reaction going. How much heat is dependent on how reactive the thing oxidizing is, but most metals burn as long as there’s enough oxygen and heat to keep the reaction going. In fact, one of the ways to identify unknown metals is to burn them and observe what color the flame is!
Despite how neutral we view it, Aluminum (Al) is actually really reactive with oxygen. Wikipedia lists this as a “high [chemical affinity](https://en.wikipedia.org/wiki/Chemical_affinity) to oxygen”, but you can think of it as Al will combine with oxygen faster and more aggressively than most other chemicals when given the chance. If you exposed pure Al to liquid oxygen, it would explode in fire! It’s just that Al usually reacts specially, quickly creating a film of Al oxide that protects the Al beneath it when it’s just atmospheric exposure. In ideal conditions, all Al would burn as hot (or hotter) as thermite because that’s the temperature Al burns at.
So the way thermite works is by taking a stable and concentrated source of non-air oxygen (iron oxide), finely mixing it with Al in a way to maximize the contact of their surface area, and then heating it enough that the oxygen starts to break away from the iron and instead reacts with the Al. This allows the Al and oxygen to react as fast as it can, which generates a huge amount of heat.
As a bonus, a byproduct is molten iron, which will have enough mass and heat to it to spread the heat created from the burning Al onto whatever is underneath the thermite (and what gets splashed by the bubbling mixture will get some scorching too).
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