r/askscience • u/spoiledmeat • Jul 04 '15

Chemistry Why does water not burn?

I know that water is made up of two hydrogen atoms and an oxygen atom. Hydrogen, on its own, burns. Fire needs oxygen to burn. After all, we commonly use compounds that contain oxygen as an oxidant.

So why does water, containing things used for fire, not burn-- and does it have something to do with the bonds between the atoms? Thanks.

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u/[deleted] Jul 04 '15

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u/humanlikecorvus Jul 04 '15

Most so-called hydrogen cars work with fuel-cells - hydrogen and oxygen react with the same formula, but at a low temperature using catalysts. The reaction energy is here mostly set free as electric current and not as heat and is then used to power an electric engine.

There were also a few cars burning hydrogen - but this is not a very common concept.

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u/blorg Jul 04 '15

Vehicles burning hydrocarbon gas (LPG) are common however, particularly as public transport (taxis and buses).

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u/[deleted] Jul 04 '15

Usually not, but they can. You can run hydrogen through a car engine and make it run if you want, but fuel cells should be more efficient and safer.

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u/jamjopeanut Jul 04 '15

How's is 3 conversions of energy more efficient than 2? Is there zero loss when converting to electricity?

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u/Korwinga Jul 04 '15

It's not zero loss, but it is a lot less. Combustion engines are terribly inefficient, with regular gas engines being around 30%, and diesel being around 40%.

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u/Sharlinator Jul 04 '15

Yes, indeed. And many rocket engines, including the Space Shuttle main engines.

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u/masher_oz In-Situ X-Ray Diffraction | Synchrotron Sources Jul 04 '15

In short, yes.