What happens to the molecules of a substance when it is charring?

Answer 1

Normally, when substance burns, the molecules undergo a rapid oxidation, which excites the molecules greatly. The heat that is input into the substance, and the oxidation process, causes the molecules to move around very quickly. As the molecules return to their lower energy resting state, they shed the excess energy in the form of heat. When the goal is to char, instead of burn, the molecules are exposed to the same heat as for burning, but they are denied oxygen. Since the materials do not have access to oxygen, they do not burn (rapidly oxidize). All of the molecules still get excited, and move and bump into each other, but the fuel is not consumed, as that requires oxygen. Once the molecules of the substance get hot enough, any molecules of water or other liquids change state to a gas and are driven out of the substance being heated, because of the expansion of the gas. Charring is most commonly used to dry wood completely, without using up the wood's potential energy as fuel.