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?
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.
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.
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 is Cecil Adams on why water is already burned.
Thanks, Manny! Cecil is such a good explainer!
“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.
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?
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.
Not really a synonym, but what we typically think of as burning (wood burning, propane burning) is oxidation, yes.
Oxygen isn’t a catalyst per se, but it is almost always a necessary component.
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.)
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.
(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.
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?
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)?
Okay . . . here’s how I learned it in HS chem.
- 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:
2H[sub]2[/sub] + O[sub]2[/sub] ^——> 2H[sub]2[/sub]O (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).
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.
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 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.
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.
Just a few points. Remember the fire triangle? CO[sub]2[/sub] 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 (H[sub]2[/sub]O) be oxidised in air ie burned, to give hydrogen peroxide (H[sub]2[/sub]O[sub]2[/sub])?
*Something to mull over: If combustion is combination with oxygen, why can’t water (H[sub]2[/sub]O) be oxidised in air ie burned, to give hydrogen peroxide (H[sub]2[/sub]O[sub]2[/sub])? *
SWAG: Because it takes energy to go that way. That is:
2H[sub]2[/sub]O + O[sub]2[/sub] + energy --> 2H[sub]2[/sub]O[sub]2[/sub]
And before it’s asked: why doesn’t burning hydrogen leave H[sub]2[/sub]O[sub]2[/sub]? (H[sub]2[/sub] + O[sub]2[/sub] --> H[sub]2[/sub]O[sub]2[/sub])
Because H[sub]2[/sub]O 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.
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.