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Part One-Burning Down a Tree
Friday, May 01, 2015

  Why is it that you can’t go out and burn down a tree with a single match? Even if the tree is completely dead and bone dry, no one would walk out with a single match and hold it to the trunk expecting the tree to catch on fire. Why not? You have oxygen, a combustible material (dry wood), and a flame. These are the three elements we all learned were necessary for combustion, and yet we know, intuitively, that the single match approach will not work unless we get very, very lucky.

 So what are we missing?

  While the chemistry of wood combustion is an extremely complex phenomenon, we can cut through almost all of the details and get down to the nitty-gritty of combustion. Stripped of all the details, we have Oxygen reacting with the wood to generate CO­2 and heat. But that sort of begs another question; why do we need to add heat?

  There is plenty of O2 out there all the time. Increasing heat raises the energy of the elements being heated, but heat is just another way of measuring the average speed of a bunch of molecules. When we say that a gas is hotter, we are essentially saying that the molecules are whizzing around faster and banging into each other with more force. That’s why gases expand as they get hotter. That’s why hot-air balloons work. That’s why closed tanks build pressure and explode in a fire.

  Solids are a little different because the molecules are locked in place, but molecules are always vibrating like plucked guitar strings. The hotter they are, the more vigorous the vibration.

  But temperature is just an average. In any given volume of gas, there will be some very cold, slow molecules and a few very hot, fast molecules. Occasionally one of these very hot molecules is going to bang into our dry tree with enough force to react with the wood. Bam! We’ve got CO2 plus heat and… nothing. Trees do not spontaneously combust.

 Let’s take a little trip into reaction kinetics. Bear with me, it’s not as bad as it sounds. A reaction kinetic is usually depicted as a ball going up a hill to a certain point (the top of our kinetic hill) at which the reaction occurs and the ball rolls down the other side to release the products of the reaction and, in the case of combustion, excess heat. The thing that pushed the ball up the hill, in the case of combustion, is heat. But if you push the ball nearly to the top, nothing happens. The molecule is warmer, but no reaction occurs and no excess heat is generated.

Reaction-Kinetics

 So think about that for a minute. You had some heat which pushed the ball partway up the hill but not far enough to generate more heat. If a rogue Oxygen molecule comes crashing into our dry tree and succeeds in combusting one molecule of wood, heat is generated, but not enough heat to matter. The heat is absorbed by the surrounding wood. The ball goes up the hill a bit for a few surrounding molecules, but it doesn’t go up far enough to tip over the edge and promote another reaction.

 Hang in there. We are almost home.

 That’s why you can’t burn down a tree with a single match. The match will cause some reactions, but the heat generated by those reactions will be pulled away (absorbed) by the relatively cold wood under the surface. Your fire will go out.

  Think about how you start fires (any Boy Scouts or Girl Scouts out there?). You either use a chemical accelerant: diesel, kerosene, etc., or you use thin stuff with a lot of surface area; like leaves or very small twigs. The leaves and small twigs don’t have enough cold molecules under their surfaces to carry away the heat of combustion. The leaves burn completely and you add small twigs, you add larger twigs, small branches, larger branches, logs, trees, forests, Northern California.

  If there is enough heat available, even a full-grown, live tree will burn.

  So you can either start with a chemical accelerant, you can start with leaves and twigs, or you could just heat up the tree. If the tree was 400° to start with, trust me, you would have no problem burning it down with just one match. All of the molecules in the tree will have had their kinetic balls pushed almost to the top of their kinetic hills. But for those of you who may want to try the “hot tree” method, I would advise running away quickly after throwing the lit match. Good luck.

  And what, you are wondering, does all of this have to do with urban riots? Read Part Two for the answer.


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