‘Burning’ is an odd phenomenon. With the application of a small amount of heat, objects such as wood can be reduced to a blackened shadow of their former glory in a raging torrent of light, heat and smoke. How do things like wood burn though, and what even is fire?
The answer to this can be found in chemistry. All organic matter is predominantly made up of carbon, hydrogen, oxygen and nitrogen, which are all held together by covalent bonds (there are other types of bonds which come into play, but these are the most important).
These bonds are strong and stable in normal conditions, but as with all chemical bonds, the application of heat (such as from a match) will transfer energy to the bond, which makes it unstable (imagine the bond shaking and vibrating as it gets heated). As more heat is absorbed, the bonds will become weaker, and eventually will break.
The breaking of chemical bonds with heat like this is called pyrolysis, and this word is derived from the Greek words for ‘fire’ (pyro) and ‘to unbind’ (lysis).
In some circumstances, such as the burning of wood, the breaking of chemical bonds is an exothermic reaction, which means heat/ energy is released when they break. This release of heat can transfer to nearby bonds, which can give them enough energy to break and release more energy; and this is how fires can be self sufficient once the initial heat source, such as a match, is removed.
The pyrolysis of cellulose molecules (which make up a large amount of wood) into levoglucosan is shown below. This reaction is not exothermic.
The pyrolysis of cellulose to levoglucosan is the first stage in the burning of wood, and forms a tar like substance. Although this first reaction doesn’t release energy, the bonds in the levoglucosan molecule will also experience pyrolysis which is exothermic, and this produces smaller, more volatile compounds which form a gas. These small gaseous molecules can also undergo further pyrolysis, and release even more energy.
As these molecules are in a gaseous form, they will also readily react with the oxygen in the air to form compounds such as water and carbon dioxide. These molecules, amongst others, will make up the smoke.
A good fuel will be one which will undergo pyrolysis very with very little heat needed, and releases a lot of energy when the bonds are broken.
Now we understand the basics of how things burn, particularly wood, we can begin to answer the question of what fire is.
We know that the breaking of bonds is an exothermic reaction, which means energy is released, and this energy will be in the form of both heat and light. This heat from these bonds breaking is what we feel when we are close to fire, and the orange/ yellow colour of flames is the light released from these bonds.
Fire appears to take the shape of dancing flames because much of the bond breaking (and so energy releasing) is taking place in a gas (remember those small volatile compounds formed from the pyrolysis of levoglucosan which form a gas).
When fuel such as wood burns, chemical bonds are being broken by heat. These bonds release energy when they break, which transfers to surroundings as light and heat. This energy can transfer to other bonds and break them too – making the reaction self propagating as long as there is sufficient fuel.
The flames we see and feel when fuel burns is simply energy being released.
Image courtesy of liz west.
The energy required to break chemical bonds is less than the energy released when the bonds are broken. This results in a net energy released, which we see as fire and feel as heat.