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Is oxygen really necessary for burning things?

What if there was no oxygen around? Would there still be a way to burn things?

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Yes, absolutely. Burning refers to oxidation accompanied by a flame and there are numerous reactions in which oxygen takes no part. Most of these are either with elementary halogens, or compounds thereof.

Here are some examples.

\begin{align} \ce{2K + F2 &-> 2KF}\\ \ce{2Fe_\text{wool} + 3Cl2 &-> 2FeCl3}\\ \ce{H2O + ClF3 &-> HF + HCl + OF2}\\ \ce{2H2O + ClF3 &-> 3HF + HCl + O2}\\ \ce{2Cl2 + 2OF2 &-> 4ClF + O2} \end{align}

So in fluorine chemistry, you don't oxygenise it; it flourises you.

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According to the dictionary, burning is to undergo rapid combustion or consume fuel in such a way as to give off heat, gasses, and, usually, light; be on fire:

Burning is an exothermic process of oxidation. Burning can happen in almost all oxidative environments under the right circumstances. Sparklers don't need air to burn because they contain oxidants such as $\ce{KNO3}$ or $\ce{KClO3}$. Another example is Iron and Chlorine. They react brightly creating a smoke of $\ce{FeCl3}$ or Ferric Chloride. This is represented by the following equation. $\ce{2Fe + 3Cl2->2FeCl3 + Heat}$

As written in the equation, heat is produced. This makes it an exothermic oxidative reaction. In other words, burning.

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Back in year ten, we were shown an experiment that was supposed to introduce us to the concept of redox chemistry, but it might as well serve as an example of how something can burn without oxygen:

A cylinder containing $\ce{CO2}$ was prepared, the top closed off with a removeable piece of glass. A bit of magnesium band was ignited and dropped into the carbon dioxide.

It continued burning with soot appearing on the walls of the cylinder.

The explanation of the above experiment was the following simple redox reaction:

$$\ce{2 Mg + CO2 -> 2 MgO + C}$$

This was obviously combustion in carbon dioxide atmosphere.

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    $\begingroup$ (+1) for magnesium, a metal that isn't choosy. Carbon dioxide, nitrogen, water? Nah, baby, I'll rather keep on burnin'. Even sand won't put the sucker out (unless there's a lot of it). $\endgroup$ – Linear Christmas Dec 15 '16 at 17:46
  • $\begingroup$ I think the question is asking about burning without any oxygen involved. The electropositivity of the magnesium simply pulled the oxygen out of the carbon dioxide leaving magnesium oxide and carbon $\endgroup$ – Arvin Singh Dec 17 '16 at 12:32
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    $\begingroup$ @ScienceAsap I read the question as ‘is oxygen gas required for burning?’ $\endgroup$ – Jan Dec 17 '16 at 19:41
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"Burning" is a rapid exothermic oxidation reaction, so you need some sort of oxidizer. Gaseous oxygen serves quite well, as well as oxygen bound up in various solids like nitrates (e.g. potassium nitrate in gunpowder).

If you don't want oxygen in any form, the two halogens fluorine and chlorine are also excellent oxidizers and can cause things to burn quite well.

Fluorine

https://www.youtube.com/watch?v=ca4JtA_EZzY

Chlorine

https://www.youtube.com/watch?v=aK85PZX2xNE

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