Expert: George Maxwell - 11/22/2007

Question
When something is burnt what happens to the oxygen that is fueling the combustion process? Some is turned into CO2 I imagine...does this happen to all or just some of the oxygen? If so, what happens to the rest of it?

Answer
Hi, and thanks for your question.

Yes, you've got the right idea with the "forming CO2" thing. "Fire" -as in the flames themselves- are just areas of hot fuel and oxygen gas reacting and giving out energy. The oxygen has to combine with the fuel in order for fire to occur. With most materials (wood, paper etc) the fuel is mostly carbon, so when the carbon and the oxygen have reacted, CO2 is the product. If another material burns, you're just forming a different oxide (e.g. if you burn magnesium, you're left with a white powder residue of magnesium oxide.

Any oxygen that isn't used in the combiustion process is simply carried away by the convection currents from the fire.

That's the theory - in the real world, there is often not enough oxygen to fully convert all of the carbon in burnig material to carbon dioxide. In this case, poisonouis carbon monoxide is formed (the carbon isn't as oxidised), and particles of unburnt fuel form in the smoke layer from the fire. This is the reason that firefigters have to be vey careful when entering a burning room where the door is closd: the closed door limits the supply of oxygen to the fuel, so large amounts of hot unburned fuel gas and carbon monoxide form. When the door is opened, the hot fuel gases meet oxygen and can burst back in to flame, often with explosve force.

Hope this helps - basic answer - the oxygen has to combine with the fuel for combustion to take place, so you form a fuel oxide. The most common fuel is carbon, so the most commonly produced oxide is carbon dioxide.