Water contains hydrogen and oxygen. Why doesn’t it burn?

Dear Cecil:

Why doesn't water burn? It's made of hydrogen, which is flammable, and oxygen, a necessary component of flame. Yet every time I put this question to someone who knows about chemistry their eyes roll back in their head and they nearly pass out, and when they come to they give some explanation that is so complicated and incomprehensible I have to lock them in the trunk and drive them around town for a while to make them shut up. I'm appealing to your brilliance to help me live a more settled life.

Cecil replies:

Cecil replies:

Sean, this kind of behavior is just not nice. At least it never works for me. Besides, there’s an easy answer to your question. Water doesn’t burn because it’s already burnt.

Oh, sure, it doesn’t look burnt. Nonetheless, it’s one of the chief products of combustion. Light a candle, gas jet, whatever, and what do you get? Mainly carbon dioxide and water. We started off with a hydrocarbon and the hydrogen oxidized. The result is water, a substance far more stable and thus less flammable than an unburnt mixture of hydrogen and oxygen.

Still, if you try hard enough you can get even water to burn. Try torching the stuff in the presence of fluorine gas. You get a nice hot flame that produces oxygen and hydrogen fluoride, which are more stable than water plus fluorine. That’s about as simple as I can make it, pal. Hope it brings you inner peace.

Send questions to Cecil via cecil@straightdope.com.

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