Why is water not flammable?

[neorichieb1971]

I don't do science, even though I like things scientific.

I was thinking the other day that if water is H2O shouldn't it be more flammable than anything else? Two parts Hydrogen and one part oxygen. In my book that would look like a fireball waiting to happen. But instead it puts fires out.

So I did some research (bored at work). Found out the following -

Objects, such as wood have a flash point temperature at which it starts to put off vapors or gases. It is these gases that burn. As fire continues it heats up more of the object so it keeps producing gases and the fire continues. Water does not smother a fire by keeping the air away from it. Water strikes the object and cools it to below its flash point so it produces no more gases that burn and the fire will extinguish. The steam produced by the water can help put it out to a small extent. The hot steam can cause serious injury to a firefighter just as the fire can.

Thats interesting, I've never thought of it scientifically before. But that makes sense.

But this is the really interesting part -

Water is burned hydrogen (H2O) and can't burn twice. Water retards fire by cooling the burning material and by separating the burning object from its oxygen supply either by wetting the object or by forming a blanket of steam around it.

So what is required to make water flamable?

Potassium can do this, and the heat generated is sufficient to ignite the hydrogen left behind (which then reacts with oxygen in the air to make water again!).

Sounds like cars should have more potassium in them.


Water is a weird substance.

In theory if we ever entered a post apocalyptic world, more hydrogen would burn making more water in the process. So doesn't it sort of make sense that life on this planet will exist in some form or other for eternity as long as Earth acts as a life container?


Sorry for dribbling on.. its one of those Sundays at work

[rancor]

Wasn't there a river in Detroit or Chicago that caught on fire once?

It was because of the pollution, but still - it would be weird to see a river of flames.

[Davey]

It was the Cuyahoga River in Cleveland. I wasn't alive back then (1969), but I believe that was one of the things that started the "Cleveland sucks let's all make fun of it lol" era.

[Sasupoika]

As said, water is already product of burning hydrogen ( O2 + 2 H2 -> 2 H2O ). Heat breaks bond between hydrogen molecules and allows them to bind with oxygen to more favourable form ( H20 ). Water molecules are not flammable because they are in octet-state, which is the energy-efficient and stable. You could say that they are satisfied, each atom sharing their electrons through covalent bond so that each of the atoms gets electron configuration of 'something-s2-p6'.

Bleh, my chemistry might be bit rusty. Please correct me if I am wrong.

[brentsg]

This thread makes me miss my aerothermal chemistry class.



Not really

[maxlords]

I think water can become flammable at high enough temperatures....it wouldn't be in liquid form by that point though.

As a followup question though....what happens to hydrogen and oxygen once they burn? They don't just turn back into hydrogen and oxygen.....they become energy right?

[Ex-Cyber]

As a followup question though....what happens to hydrogen and oxygen once they burn? They don't just turn back into hydrogen and oxygen.....they become energy right?

You start with two hydrogen molecules and one oxygen molecule (each of which contains two atoms) and end with two molecules of water (each of which contains two hydrogen atoms and one oxygen atom). You have two oxygen atoms and four hydrogen atoms both before and after the reaction. The reaction releases energy, but the atoms aren't converted into energy. Theoretically, there is a tiny decrease in mass associated with this reaction, but practically the difference is too small to measure in chemical (as opposed to nuclear) reactions.

A rough analogy would be that you have a spring mounted to the ceiling with a weight hanging from it by an alligator clip. A small amount of work will open the alligator clip, causing the spring to snap upward and the weight to fall downward. The result is still that you still have a weight and a spring, but now they're in a more stable configuration with less potential energy. If you don't lift up the weight and stretch out the spring again, you don't get another snap/fall reaction, even if you apply many times the work needed to open the alligator clip.

[ColonelFatso]

Yeah, the energy released (or absorbed) in a standard chemical reaction is the difference between the energy of the old chemical bonds and the energy of the new ones. The H2O configuration is less energetic and more stable than the H2 configuration (I forget almost everything, but I seem to remember it's something about the electron-sharing properties of diatomic gases), so energy would need to be put in (see: electrolysis) to seperate the water into its constituent elements again.

[popawell]

Many metals if they are provided sufficient surface area or a certain amount of heat will make water 'burn'. I have been around metals and casting practices enough to know why you do not add water to a magnesium fire or thermite reactions.

[maxlords]

As a followup question though....what happens to hydrogen and oxygen once they burn? They don't just turn back into hydrogen and oxygen.....they become energy right?

You start with two hydrogen molecules and one oxygen molecule (each of which contains two atoms) and end with two molecules of water (each of which contains two hydrogen atoms and one oxygen atom). You have two oxygen atoms and four hydrogen atoms both before and after the reaction. The reaction releases energy, but the atoms aren't converted into energy. Theoretically, there is a tiny decrease in mass associated with this reaction, but practically the difference is too small to measure in chemical (as opposed to nuclear) reactions.

A rough analogy would be that you have a spring mounted to the ceiling with a weight hanging from it by an alligator clip. A small amount of work will open the alligator clip, causing the spring to snap upward and the weight to fall downward. The result is still that you still have a weight and a spring, but now they're in a more stable configuration with less potential energy. If you don't lift up the weight and stretch out the spring again, you don't get another snap/fall reaction, even if you apply many times the work needed to open the alligator clip.

TY...it's been far too long since my chemistry classes

[Ixmucane2]

Many metals if they are provided sufficient surface area or a certain amount of heat will make water 'burn'. I have been around metals and casting practices enough to know why you do not add water to a magnesium fire or thermite reactions.

But this is not burning water, it's some exothermal reaction that consumes water. For example magnesium burns to magnesium oxide, appropriating the oxygen from water and releasing hydrogen.
In a sufficiently unwise setup, this hydrogen might be hot and concentrated enough to burn (producing new water) before dispersing.

According to Wikipedia, magnesium can similarly burn in carbon dioxide, and even in nitrogen (forming nitride). I don't want to try.

[Sumez]

Water is a weird substance.

Awesome quote.