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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
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

1 Answer

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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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