Can Water be flammable?

peazz

Hi all,

im aware the moon, orbiting saturn, titan has such an atomsphere and is cold enough for methane to exist as a gas, liquid and a solid mass, this sparked a question in my head...

would there be any moon/planet where conditions would make water flammable? i know this is most prob a dumb question, but here on earth methane as you all know is highly flammable. its just something i wondered could exist at all..

some thoughts please guys

EDIT: I just found my answer, Water is already burnt so there fore would not be flammable :)

Chronos

Water is flammable at temperatures high enough to decompose it into hydrogen and oxygen [e.g. a metal fire].

russ_watters

No, water is not flammable. Water is a waste product of combustion. In a metal fire, as the name implies, it is the metal burning, not the water. The water provides the oxidizer: http://en.wikipedia.org/wiki/Magnesium

Magnesium's combustion reaction is so energetic that it can afford to give up some of its energy to "unburn" the water and steal the oxygen while keeping the overall reaction exothermic.

twofish-quant

In fact, you can burn water if you make it react with fluorine which is more reactive than oxygen.

http://answers.yahoo.com/question/in...0222114AA6iAcT

mgb_phys

Not sure if that counts as burning.

But if you want a really fun oxidiser try Chlorine Tri-Flouride http://en.wikipedia.org/wiki/Chlorine_trifluoride, it burns asbestos!

russ_watters

That's exactly the same phenomena as what I mentioned for magnesium: the activation heat splits the water into hydrogen and oxygen and then the flourine burns with the oxygen. I wouldn't call that water burning, since burning is one element combining with oxygen in an exothermic reaction. At best I would call that water providing the oxidizer for flourine to burn with.

onomatomanic

I haven't done any chemistry since high school, but IIRC fluorine is a halogen, so the reaction should produce HF and O2 rather than H2 and F2O, right?

As to when to call something flammable, the answer depends on whether flammable is used as a technical or as a colloquial term, I'd say. I seem to remember my chem teacher using "combustible" and something really awkward like "combustion-promotive" when he wanted to be extra-precise about distinguishing between oxidizable and oxidizing substances.

*shrug*

russ_watters

You're right - my chemistry isn't that strong and I misread where they listed the reaction in that link: H2O + F2 = 2HF + 1/2O2

I'm not sure what you call that reaction, then. For technical vs colloquial, I'm not sure what the OP was really thinking, but then s/he probably wasn't either!

The M man

If you think thermodynamics, combustion is the complete decomposition of a substance into Carbon Dioxide,Water and metal oxides/Hydroxides,etc.

Thus, as water is an end product, you can't really call it combustible.Like the guys said, reacting water with substances like sodium,magnesium, or even yellow phosphorous can't really be called burning it.

Also, even though methane can be found in solid,liquid and gaseous states on the moon, it is still combustible. Change in state doesn't guarantee a change in properties of the substance.

mgb_phys

No combustion is the (rapid) oxidation of a substance.

Carbon combusts into carbon dioxide, CO2 can then be reduced back to C and O2 by a more powerful oxidizer but that isn't combustion.

DaveC426913

So, if burning is defined as oxidation, does that mean you could turn water into peroxide (H2O2)?

mgb_phys

And would O₂ -> O₃ or O₃ -> O₂ be 'burning' ?

onomatomanic

Again, my memory is a bit hazy, but I think oxidation in this context doesn't necessarily involve actual oxygen-atoms, but is used as a generic term for a certain type of reactions/chemical bondings.

mgb_phys

yes oxidation is basicaly adding electrons, something more reactive than oxygen (like fluorine) can out-oxidize oxygen.

Ruphus

The kind of reaction you're discussing here is a displacement reaction, a highly reactive element kicking out a less reactive element from a compound and taking its place (like the famous thermit reaction). Oxygen tends to react strongly with many other substances because it needs a couple of electrons to fill its outer shell and is very good at getting them, to the point that the only element that is better at it is fluorine.

When fluorine reacts with water, rapid oxidation (combustion) of the water will occur and the water will burn with a very hot flame, producing oxygen and hydrogen fluoride. If someone was watching and didn't know what substances were being used then they would probably assume that some very flammable fuel was being burned in plenty of oxygen. The fact that hot oxygen gas and even some ozone are being given off as the water goes up in flames can make the whole thing even more spectacular.

russ_watters

You forgot the other component of what "burning" is: it is exothermic. (also, typically "rapid", which would tend to exclude rust)

onomatomanic

To keep using this thread as a chemistry refresher: Aren't all exothermic reactions rapid? My thinking is that once favourable conditions exist for the reaction to begin (e.g. once activation energy is supplied), any exothermic reaction should be self-sustaining until one of the reagents is depleted. Or is there more to it?