# If it was heated up enough, could water-soaked wood in a vacuum catch on fire? [closed]

Obviously you can set a damp piece of wood on fire- not exactly revolutionary. I specifically mean if the wood was in a perfect vacuum. It doesn't matter where the heat comes from-just say the amount of heat produced from the fire magically appears at the wood

You can't create fire in a perfect vaccum but could you make it hot enough so that the water molecules seperate into $\rm H_2$ and $\rm O_2$ so that you can create fire by completing the fire triangle with the oxygen.

It has come to my attention that simplification is required

We'll assume that the 'wood' is a $1\ \mathrm{cm} \times 1\ \mathrm{cm} \times 1\ \mathrm{cm}$ block of carbon in a perfect vacuum by which the volume is $2\ \mathrm{cm} \times 2\ \mathrm{cm} \times 2\ \mathrm{cm}$. The amount of water is the same as carbon (because if this idea is correct then there would be a 100% combustion I think) and the vacuum chamber is made from tungsten.

## closed as off-topic by sammy gerbil, ZeroTheHero, Kyle Kanos, John Rennie, Jon CusterSep 11 '18 at 13:06

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• I personally have soaked a piece of wood and thrown it in the fireplace; it burned. The fire dries the wood then burns it. So, yes. – InertialObserver Sep 10 '18 at 17:33
• I'm voting to close this question as off-topic because it is asking about Chemistry rather than Physics. – sammy gerbil Sep 10 '18 at 18:06
• – yolo Sep 10 '18 at 19:06

at the very high temperature of 3000 °C more than half of the water molecules are decomposed

So, with enough heat, you could produce oxygen from water through thermal decomposition. I suspect at this temperature that the wood will have also decomposed into simpler molecules. Assuming that everything is contained, some of the oxygen could combine with what is left of the wood. As @BowlOfRed pointed out, this process can hardly be considered combustion in the normal sense.

This post from the chemistry stack exchange calculates that 50% of carbon dioxide is dissociated at ~6200K, far hotter than is necessary to dissociate water. I'm not a chemist so I can't tell you what proportion of water would end up as carbon dioxide, but at 3000 °C CO2 would be more favorable than water.

• I question whether or not oxygen would combine with the wood under these conditions. If the temperature is high enough to decompose water, that temperature will surely decompose wood. Again, if the temperature is high enough to decompose water, oxygen is being prevented from recombining with hydrogen. Why would you expect oxygen to combine with carbon (from the wood) under these circumstances? – David White Sep 10 '18 at 19:37
• @DavidWhite please see my edit above. CO2 dissociates at a much higher temperature than I expected, so I believe a significant portion of the water would be converted to CO2. – Robert Stiffler Sep 10 '18 at 19:50
• Before reaching that temperature- would the wood melt? – yolo Sep 10 '18 at 19:55
• @RobertStiffler, thanks, and note that I gave you an upvote for your excellent addition to your post. – David White Sep 10 '18 at 20:13
• @yolo, Re, "would the wood melt?" Wood does not melt. It decomposes. Wood is mostly made of cellulose, and at temperatures significantly lower that what you've been talking about, cellulose will decompose into water vapor, and solid, amorphous carbon. What Robert Stiffler is suggesting here is that, at 3000 C, y0u can expect at least some of the water vapor and some of the carbon to react with each other to form hydrogen and carbon dioxide. – user205719 Sep 10 '18 at 21:12

Burning or oxidation is the process of releasing energy by allowing molecules in the fuel to recombine with oxygen to form water and carbon dioxide. In other words, water is one of the end products.

That means you're always going to need more energy to split them than you get back from the recombination. As such, I wouldn't call this process "burning".

well what happens with the wood?

If you heat wood without oxygen, you'll drive off almost all of the volatile compounds leaving behind mostly carbon items (charcoal). That will certainly happen here and is probably most of the story until you start to get near 3000K. I suspect that once you get hot enough to dissociate the water, the oxygen will start to combine with the carbon and you'll get a complex equilibrium of carbon chains, CO, CO$_2$, H, H$_2$, O, O$_2$, H$_2$O, and a few bits of other things. Above 3000K, the normal end products (CO$_2$ and H$_2$O) aren't so stable. Instead of entirely removing the (-CH-) species, they will break apart and may recombine with the remaining carbon chains (basically driving the reaction in reverse to a certain extent).

If you continue heating above 4000K, then the molecules are even more unstable and you get a much higher fraction of individual atoms running around. Even the carbon will be evaporating at that point.

• well what happens with the wood? – yolo Sep 10 '18 at 17:59