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Magnesium powder burns extremely well and reaches temperatures of 2500°C. However, attempts to extinguish such a magnesium fire with conventional water (e.g. from a garden hose) only make it worse: the flame grows astronomically and the whole thing gets even hotter. Why is this?

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Possibly a better home chemistry.stackexchange.com ? –  Qmechanic Jul 30 '12 at 21:32
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Certainly as good on chemistry. If you want it migrated flag for a mod and we'll ask if they want it. –  dmckee Jul 30 '12 at 21:40
    
I don’t mind either way. I posted it on physics because I assumed it has more to do with pressure (expansion of hot water) than with chemical reaction. –  Timwi Jul 30 '12 at 23:16

4 Answers 4

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Magnesium reacts with water to produce hydrogen and a lot of heat. Metallic magnesium reacts only slowly, but magnesium vapour, produced when Mg burns, reacts extremely quickly due to the high temperature and efficient mixing, and produces heat very rapidly. Hence the explosion when water is added to burning magnesium.

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The total energy released per gram of magnesium stays the same:

$$Mg+O_2 -->MgO_2$$ is identical in overall effect to the pair of reactions $$Mg+2H_2O-->MgO_2+2H_2$$ $$2H_2+O_2-->2H_2O$$

Water is consumed and produced in exactly the same amounts; hydrogen is produced and consumed in exactly the same amounts

Liquid water makes more oxygen available to the magnesium; the hydrogen gas can mix with the oxygen in the air more quickly.

You could get the same effect by grinding the magnesium into a fine powder and blowing it into the air.

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magnesium burns hot enough to separate water molecules and because hydrogen is flammable and oxygen is what fire needs to get bigger your hose becomes a nice flame thrower :)

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Magnesium burns so hot that when Water is added the water is instantly converted to hydrogen and oxygen (both flammable gases). It is the gases that explode.

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