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Specifically wood and paper and other hydrocarbons, but not necessarily excluding other things that burn. We all know wet wood is very hard to ignite. Wetness here refers to water.

But why is that the case? At the chemical level, what is it about water that makes it so effective at inhibiting the reaction of hydrocarbons with oxygen? I guess this also ties in to questions like "Why can't you strike an ordinary match underwater?"

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    $\begingroup$ Many hydrocarbons are less dense than water, float on water's surface as seen so often and still may burn (e.g., after oil spills). But the evaporation of water is a significant heat sink, disfavouring continuation of combustion. And a «blanket» of finely dispersed water droplets may shield the flame from oxygen often required to continue combustion, too. $\endgroup$ – Buttonwood Sep 28 '19 at 23:20
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    $\begingroup$ In general, you can make many things burn all right under conditions which intuitively seem impossible, you just have to find a better oxidizer or a way to improve kinetics. Even water itself burns all right in a flow of fluorine gas: $$\ce{2 H2O + 2 F2 -> 4 HF + O2}$$ $\endgroup$ – andselisk Sep 29 '19 at 2:22
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To have combustion, you need three things: fuel, oxidizer, and heat. To ignite a piece of wood, you need to raise its temperature to the combustion temperature, where a significant amount of the solid hydrocarbons can vaporize, and then ignite. However, if this wood is wet, the energy input will distribute among the water and solid hydrocarbon. Water is a great heat sink (large heat capacity), so it can absorb a large amount of energy. Thereby, the wet piece of wood is inhibited from achieving the combustion temperature until a large amount of the water has been evaporated off.

One analogous example is heating a full and open, aluminum can of soda (sugar water) with a butane torch. The aluminum won't melt, initially, since the water in the can absorbs the energy. However, if you boil off the water, then the aluminum can will melt after a certain amount of heating.

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Burning (=oxidizing with oxygen from the air) organic material generates heat, because the reaction is exothermic, CO2 and H2O being energetically more stable than the organic material before combustion. Of course, despite of not being in thermodynamic equilibrium, we all don't ignite spontaneously, as there is an energetic barrier preventing this. In order to overcome this energetic barrier, we would have to put in some energy first, usually in the form of heat.

In the end, if we want to burn something, we want to create a chain reaction. Create enough energy (heat) to overcome the energy barrier for the oxidation of organic matter with oxygen from the air. Then, if more matter oxidizes, more heat is created, facilitating the oxidation of even more matter, and so on. Water on the other hand serves as a very efficient heat sink, as it is one of the most (energetically) expensive materials to warm up. And evaporating water needs a lot of energy, it really likes it liquid state because of the hydrogen bonds between its molecules.

So if you hold your lighter on a piece of wet wood, most of the energy will be taken up by heating or evaporating water, and almost nothing will go into facilitating the oxidation of the wood.

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  • $\begingroup$ Perhaps your A is getting diwnvoted because your emphasis on activation energy though correct neglects the pyrolysis /evaporation of organic matter (no one knows what exactly :)) . I did up vote because I find much downvotes not fair. Perhaps it was your style. The answer could have been better but this is rather general. $\endgroup$ – Alchimista Sep 29 '19 at 9:49

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