Why doesn't water burn?

[Liberal]

I'm sorry for asking a question that I'm sure is abysmally stupid, but if water is hydrogen and oxygen, why does it put out, rather than exacerbate, fire?

[Guy Propski]

This has been answered, both by Cecil and the TMs, but I'll tell you anyway. Water doesn't burn because it's already burned. Water is burned hydrogen. So it puts out fires for the same reason CO2 puts out fires.

[Liberal]

I still don't get it (burned hydrogen?), but thanks for your kind indulgence. I searched on the terms burn + water and got nothing that looked apropos.

[manhattan]

Originally posted by Libertarian
I searched on the terms burn + water and got nothing that looked apropos. Yes, unfortunately the column search engine has been acting up as of late. But by fortunate coincidence, I had that columned bookmarked for something else I was working on.

Here (http://www.straightdope.com/columns/970905.html) is Cecil Adams on why water is already burned.

[Liberal]

Thanks, Manny! Cecil is such a good explainer!

[stolichnaya]

"Burning" as we usually think of it is actually a rapid exothermic oxidation reaction. To really oversimplify it, oxygen combines chemically with another substance and energy is released.

Asking why water won't burn is sort of like asking why rust won't rust or asking why ice won't freeze. Water is already "full" of oxygen and theres no more room for it to be added through oxidation.


Water isn't really "burnt" as we normally think of the word, but it is the byproduct of the oxidation of hydrogen. If you hold a match to a hydrogen baloon in a sealed room, as the hydrogen burns, it's byproduct will be water vapor. So in this sense, water is "burnt hydrogen".

As to why it smothers flames, it merely deprives whatever's burning of the free gaseous oxygen it needs to continue to oxidate.

[Liberal]

So, a follow-up question, please.

Is burning a synonym for oxidation? Or is oxygen a catalyst for burning? Is there no other gas in which something can burn?

[lissener]

In a very limited sense, yes.

Not in the sense that the two words are interchangeable, though: the word "burn" won't suffice for all uses of the word "oxidize," and vice versa. But in the context of your question, yes. And in the sense that when a combustible substance is "burning," that process largely consists of the substance combining with oxygen.

But I'd never say my heart oxidizes for you.

[stolichnaya]

Is burning a synonym for oxidation? Not really a synonym, but what we typically think of as burning (wood burning, propane burning) is oxidation, yes.Or is oxygen a catalyst for burning?Oxygen isn't a catalyst per se, but it is almost always a necessary component.Is there no other gas in which something can burn?In the column that manhattan quoted above, Cecil mentions fluorine as a gas in which even water will burn, so the answer to your last question is "there are other gases". (I was going to try to answer that question yes or no but it made my head hurt.)

[wolfman]

It kind of depends on how scientifically you define burning. If you define burning to be an oxidative process, then it is onviously the same. But for everyday usage they are not quite the same. A log in a forest fire is both oxidizing and burning. Iron rusting is oxidizing, but not many people would commonly consider that to be burning since it is a slow process and give off no aprecible heat or light. The sun gives off tremendous amounts of heat and light, and is commonly referred to as burning, but it is not generated by an oxidative process.

[Dr. Lao]

Originally posted by Libertarian
So, a follow-up question, please.

Is burning a synonym for oxidation? Or is oxygen a catalyst for burning? Is there no other gas in which something can burn?
(1) No. Burning is an oxidative process, but not all oxidative processes are called burning. Redox chemical cells, for example. BTW, oxidation doesn't require oxygen (more on 3)

(2) No. A catalyst is something that increases the rate of a reaction without being consumed. O2 is consumed in the combustion process.

(3) Sure there are other gases. If a highly oxidized gas comes in contact with a reduced substance, rapid exothermic oxidation of the substance can occur (I would use this as my definition of burning). Fluorine gas is even more oxidized than O2. I imagine it will support the combustion of a great deal of substances even better than oxygen does.

[manhattan]

I'm sure Libertarian will correct me if I'm wrong, but I took his third question to be, "Are there reactions that we commonly understand to be "burning" or "combusting" but which do not involve oxidation?

I think that most people consider the energy released by uncontrolled fission or fusion to be "burning," at least in a lay sense. Are there other examples?

[Chronos]

I imagine that most folks would refer to any exothermic reaction which produces incandescent gasses as "burning", and I'm sure that there's some chemical reactions that would fall into that category without involving oxidation. For instance, I'm pretty sure that the explosion of nitroglycerine isn't oxidation (what's oxidizing what? There's only one reagent)-- Does it produce incandecent gasses (flames)?

[iampunha]

Okay . . . here's how I learned it in HS chem.

1. Hydrogen (hereafter known as H) in the presence of heat (hereafter known as ^) and Oxygen (hereafter known as O) and some sort of catalyst (a flame, perhaps) burns. The result is water vapor. This equation is best summed up as such:

2H2 + O2 ^——> 2H2O (gaseous)

NOTE: As I learned it, one puts the ^ over the arrow, thus indicating that the reactions occurs in the presence of heat. And I believe it's an exothermic reaction, though I may be wrong about that.

As for burning . . . some things burn in the presence of oxygen. I think most Earthly things require it. There are other ways to get heat, such as fusion and fission, though those require some amount of heat in the beginning (for the moment, let's forget cold fusion).

Then again, most things don't burn in the presence of oxygen and heat. They need a catalyst (eg, a flame).

[Dr. Lao]

I think that most people consider the energy released by uncontrolled fission or fusion to be "burning," at least in a lay sense. Are there other examples?
manhattan, I can't think of any chemical examples of something that would be considered in a layman sense burning, but does not involve redox chemistry. If somebody could give some examples, I could tell with it was a redox reaction or not. But I'm having trouble think of it from the other way around.
For instance, I'm pretty sure that the explosion of nitroglycerine isn't oxidation (what's oxidizing what? There's only one reagent)
When I first saw this I agreed with you Chronos. However when I looked at the reaction formula, I can say that it is definitely a reduction-oxidation reaction.
4[C3H5(ONO2)3] --> 12CO2 + 10H2O + 6N2 + O2 (from a Ask a scientist (http://www.newton.dep.anl.gov/askasci/chem99/chem99298.htm) page on the U.S. department of energy web site)
The carbon in nitroglycerine is in the +1 oxidation state. In the resulting carbon dioxide of the product the carbon is in the +4 oxidation state. Carbon is the principle reducing agent. In the reactant the nitrogen is +5, in the products it is neutral. Nitrogen is the principle oxidizing agent. One reason nitroglycerin is so explosive is that the oxidizing and reducing agents are on the same molecule.

[Chas.E]

Water DOES burn. You can burn snow. You can spray an aerosol of water vapor over an open flame and it will burn. But it takes a big, HOT flame to initate the burn.

[Andy]

Water is burned hydrogen. So it puts out fires for the same reason CO2 puts out fires.

They need a catalyst (eg, a flame).



Just a few points. Remember the fire triangle? CO2 tends to put out fires by displacement of the oxygen atmosphere sustaining the fire. Water tends to cool the fire so the reaction is no longer sustainable, although both probably operate in both modes to some extent.

Oxygen does not generally act as a catalyst (although in excited states it can). A flame or other heat sourse is not usually considered a catalyst. Defintions I use for a catalyst are either

i) A catalyst increases the rate at which equilibrium is reached in a reaction without affecting the position of equilibrium.
or
ii) A catalyst of a reaction is a substance whose order appears at a higher value in the rate equation that would be suggested by its stoicheiometry.

These probably aren't very useful definitions for everyday conversation, but they are more accurate. Forget the addage that catalysts are never consumed and have to be recoverable to be 'real' catalysts....high school lies I'm afraid.

The nitroglycerine reaction is probably best considered an internal disproportionation reaction, some parts of the molecule are oxidised, other parts are reduced.

Something to mull over: If combustion is combination with oxygen, why can't water (H2O) be oxidised in air ie burned, to give hydrogen peroxide (H2O2)?

[AWB]

Something to mull over: If combustion is combination with oxygen, why can't water (H2O) be oxidised in air ie burned, to give hydrogen peroxide (H2O2)?

SWAG: Because it takes energy to go that way. That is:

2H2O + O2 + energy --> 2H2O2

[AWB]

And before it's asked: why doesn't burning hydrogen leave H2O2? (H2 + O2 --> H2O2)

Because H2O has higher entropy, that is, it's at a lower energy state.

The second law of thermodynamics basically says that the entropy of the universe as a whole tends toward a maximum.

[Andy]

I'm not sure the entropic term is an especially important factor in determining the (Gibbs) free energy of water vs hydrogen peroxide. It's simply that water is thermodynamically more stable than hydogen peroxide, probably due to the weak O-O bond and O-O lone pair repulsion. Or something like that.

[Adolph Peewee]

the simple answer is that H2O is not a compound...therefore it cannot be turned into carbon!!

jeepers some ppl babble on!!

[Phobos]

and some make no sense :)

[G.B.H. Hornswoggler]

Because fire and water are elemental opposites. To burn, a substance must contain a minimus of fire in its makeup, which water, being a pure element, does not.

(If you want an answer from later than the sixteenth century, you could see those who responded before me, but I stand by my answer.)

[Lemur866]

Originally posted by Chas.E
Water DOES burn. You can burn snow. You can spray an aerosol of water vapor over an open flame and it will burn. But it takes a big, HOT flame to initate the burn.

Ummmm...Chas? Do you know the difference between boiling/evaporation and burning? When liquid water changes to water vapor, it is NOT burning. Think about this for a minute.

[Chas.E]

Originally posted by Lemur866
Originally posted by Chas.E
Water DOES burn. You can burn snow. You can spray an aerosol of water vapor over an open flame and it will burn. But it takes a big, HOT flame to initate the burn.

Ummmm...Chas? Do you know the difference between boiling/evaporation and burning? When liquid water changes to water vapor, it is NOT burning. Think about this for a minute.

Yes, I was a chem major in college (a long time ago). Do you know the difference between evaporation and sublimation?

Ask any winter camper what happens if you try to melt some snow in a pot over too high a heat. It will scorch and blacken.

Get a spray bottle and spray a mist of water over a hot gas barbecue. The flame has enough energy to knock the hydrogen and oxygen atoms out of the molecule, and it results in a huge flareup.

[Andy]

Chas.E said
You can spray an aerosol of water vapor over an open flame and it will burn. But it takes a big, HOT flame to initate the burn.


Does a long time ago, when you were in college, mean some time in the dark ages? Burning is reaction with oxygen. Water does not react with oxygen. A flame could cause ionisation of water molecules if sufficiently hot. If that is what you were really taught, I am stunned.

[RM Mentock]

Originally posted by Chas.E
Ask any winter camper what happens if you try to melt some snow in a pot over too high a heat. It will scorch and blacken.

Get a spray bottle and spray a mist of water over a hot gas barbecue. The flame has enough energy to knock the hydrogen and oxygen atoms out of the molecule, and it results in a huge flareup.

Good ones!

[Doctor Who]

I read an article in the Wall Street Journal a couple of years back about a group of arsonists who were using jet fuel to start fires. Apparently when the fuel burned it was so hot that when firefighters got there and sprayed water on the fire it would break the water down into Hydrogen and Oxygen and simply aggravate the situation. (I'm not sure if you can call that water burning.) Does anybody else remember this?

- Peter Wiggen

[lissener]

Originally posted by Chas.E
[BGet a spray bottle and spray a mist of water over a hot gas barbecue. The flame has enough energy to knock the hydrogen and oxygen atoms out of the molecule, and it results in a huge flareup. [/B]

Um, isn't this because the moisture causes the fat to splatter, mix with air, and combust? Isn't the water just acting as a mechanical catalyst?

[DrMatrix]

Chas.E said
You can spray an aerosol of water vapor over an open flame and it will burn. But it takes a big, HOT flame to initate the burn.
android209 responded with
If that is what you were really taught, I am stunned. I went to school in Louisiana. I am not at all surprised. android209, were your teachers always right? I had some excellent teachers and others that...Well, let's just say I'm not surprised.

Breaking water down into hydrogen and oxygen would absorb as much energy as would be released by burning hydrogen to produce water. So, I don't see how this would make it worse. Could it be by spreading the fire, not producing more energy.

[TNTruth]

I don't have a site handy, but regarding the burning jet fuel, this is why you don't use water on oil fires! The water just spreads the fire, and if conditions are right, it can cause the FUEL to vaporize and then produce a free air explosion.

Goggle on those three words to learn about homemade bombs.

The water doesn't separate and then recombine to form a larger fire. As DrMatrix said, that's a zero net energy reaction.

[Lemur866]

OK, the reason you don't use water to put out oil fires. Oil is lighter than water, it is also polar and won't mix with the water...so the oil floats on top of the water, still in contact with the oxygen, so it continues burning. The water can also cause the fire to spread, boiling water causing splatters of burning oil everywhere. But the water does not burn, it can boil, but it can't burn.

Chas: Sublimation is not the same as evaporation/boiling. Sublimation is the change from a solid directly into a gas, without passing through a liquid stage. CO2 works this way...solid CO2 doesn't melt, it changes directly into gaseous CO2. Ice will often sublimate in very dry climates. I grew up in Fairbanks Alaska, and the icicles on the houses would always sublimate away even though the temperatures were always below freezing, the snow pack would thin out, etc.

Now, it may be that a very very hot fire can dissociate 2H20 into 2H2 + O2...but that would *absorb* energy, not liberate it. It takes electrical energy to electrolyze water into hydrogen and oxygen gas, right? But anyway, splitting water isn't burning it, but the opposite of burning it, since the hydrogen you generate will burn. But then why doesn't the water put out the fire?

Well, if the fire is hot enough, the water will boil instantly and rise, and won't keep oxygen from the fire, which is why you usually put out fires with water. So what happens when you spray your campfire with water? It does release a bunch of energy, because you are transfering a lot of heat in the wood, rocks, metal, etc, into the air...you feel a burst of heat, just like when you pour water over hot rocks in a sauna. The total amount of heat is the same, but you've transfered it from the rocks to the air, where it is transfered to you.

And if your pot scorches when you try to melt snow...um, it isn't the *snow* that burns! It could be the *pot* burning, or it could be that unburned fuel gets deposited on the pot. But the snow doesn't burn, that's silly!

[Lemur866]

Ak! I mean, oil/jet fuel is *non-polar*! Water is polar, and the two won't mix. Sorry for the mix-up.

[wolfman]

Ask any winter camper what happens if you try to melt some snow in a pot over too high a heat. It will scorch and blacken

Chas, You have given me a great new excuse for when I screw up and burn things while cooking. "It wasn't my fault, the water must gotten charred somewhere"

[Dr. Lao]

Originally posted by Chas.E
Yes, I was a chem major in college (a long time ago). Do you know the difference between evaporation and sublimation?

Ask any winter camper what happens if you try to melt some snow in a pot over too high a heat. It will scorch and blacken.Organic substances turn black when they burn because incomplete combustion leaves some elemental carbon behind. If water burned it would not turn black because water doesn't contain carbon. Something organic in the snow is burning, not the snow itself.Get a spray bottle and spray a mist of water over a hot gas barbecue. The flame has enough energy to knock the hydrogen and oxygen atoms out of the molecule, and it results in a huge flareup.If you were a chem major in college you'll remember that energy is gained by forming bonds not breaking them. Knocking atoms out of a molecule always requires the input of energy. For there to be a release of energy some new compound must form that has more stable bonds in it than water does. I can't think of anything in a barbecue flame or in the atmosphere that would combined with oxygen and hydrogen in this way. If you do then I'd be interested in hearing about it. I suspect the flare-up is the result of air in the spray being injected into the fire.

[Lemur866]

The reason water won't burn in a nutshell:

Flourine has the highest electronegativity of any element, Oxygen is a close second. Therefore, if you form a bond with oxygen, you are in the lowest possible energy state, unless you have a bunch of flourine sitting around. But we don't have a bunch of flourine sitting around, due to quirks of nuclear fusion it is a much less common element in the universe than oxygen. You could "burn" water with flourine, but where are you going to get the flourine? Damn those pesky laws of physics!


OK, blackened snow...I'm thinking this could happen because snow is such a good insulator...not much heat gets transfered from the pot to the snow, so the pot blackens like an empty pot would. But this explanation strikes me as un-straight-dope-like...I really should get a pot of snow and try to melt it over a campfire and see what happens, but unfortunately it is August and there isn't any snow here....