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As the title states, why should $\ce{CO2}$ fire extinguishers containing carbon dioxide not be used on fires with magnesium metal?

I understand that $\ce{CO2}$ extinguishers work by displacing oxygen, a crucial component of a fire. Does this somehow not work when magnesium is involved, or is there another reason?

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The reaction of magnesium metal (Mg) with carbon dioxide is extremely exothermic (heat evolving). In other words, carbon dioxide makes magnesium metal burn.

$$\ce{2Mg(s) + CO2(g) -> 2MgO(s) + C(s)}$$

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It's not just carbon dioxide. Just about any gas that contains oxygen enables magnesium to continue to burn because magnesium is strongly reducing enough to pull oxygen out of the gas molecules, displacing the other element. The reaction with carbon dioxide given by MaxW is typical. Water (vapor) also contains oxygen and thus water also fails to extinguish burning magnesium; in fact water vapor would lead to displacing hydrogen and creating an explosion risk. Burning magnesium also keeps going in nitrogen, forming a nitride.

It's not just magnesium, either. Other active metals such as calcium have similar properties. Special fire extinguishers, labeled Class D and containing dry powders that displace oxygen without generating other combustion reactions, are to be used to put metal fires out.

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