… You’ll find smoke around a fire!

Everybody ‘knows’ that when you burn something it makes smoke, but what is smoke? And how is it made? For that matter, what is fire? What happens when you burn something? Hopefully by the time I finish this I will have answered all of these questions.

Firstly, I will look at what fire actually is. What we experience as fire is actually the energy, in the form of light and heat, given off by the combustion of a fuel. The heat of the fire is such that it causes incandescence (the release of EM radiation, including light, from a hot object as a result of its temperature) in the gases given off, which we see as flames. Incandescence can also be seen in the glowing embers, or heated metals, with hotter objects giving of brighter light. This concept is used in lightbulbs, with the metal filament heated until it gives off light. Fire, or combustion, is actually a chemical reaction. To be precise, it is an exothermic oxidation reaction. This means that a substance reacts with oxygen, and gives off heat as one of the by-products.

The chemical equation for combustion is Fuel + Oxygen = By-product(s) + Heat. The fuel and by-products change depending on what is being burned, but I will be giving examples using the more common fuels. As the vast majority of living things on earth are Carbon based lifeforms, any fuel from natural sources, including oil, coal, gas and wood, will contain high levels of Carbon. This Carbon reacts with oxygen to make Carbon Dioxide. Coal is almost pure carbon, while oil and gas are Hydrocarbons, meaning they contain Hydrogen molecules bonded to the Carbon, while wood is made mainly of Cellulose, a sugar, which contains Carbon, Hydrogen and Oxygen.

The chemical reactions for the combustion of common fuels are written below. I have chosen to write them as balanced chemical equations, meaning they have an equal number of molecules on either side, however if you wish to simplify them, simply take away the large numbers before each molecule. Just remember, C = Carbon, O = Oxygen, H = Hydrogen, CO2 = Carbon Dioxide and H2O = Water.

For coal, the equation is C + O2 = CO2 + Heat. This is the reaction for pure coal, however in reality the coal contains small amounts of Sulphur, Nitrogen and other materials, resulting in Sulphur and Nitrogen Oxides being included in the by-products. For petrol (Isooctane), the equation is 2 C8H18 + 25 O8 = 16 CO2 + 18 H2O. Once again, this is for the pure fuel, in reality petrol contains Sulphur, Nitrogen and other chemicals, meaning it also gives off Sulphates and Nitrates among the by-products. Butane, which is the gas used in cigarette lighters, burns with the equation 2 C4H10 + 13 O2 = 10 H2O + 8 CO2. As Butane is a more refined product, it contains almost no contaminants.  With wood, or Cellulose, the equation is C6H10O3 + 6 O2 = 6 CO2 + 5 H2O.

As you can see from the equations, the molecules move around and form bonds with other elements. This is because when exposed to Oxygen, the atomic bonds between the atoms in the fuel break, through either gaining or losing an electron. This is important because occasionally the Carbon atoms, rather than bonding with the Oxygen to form Carbon Dioxide, bond with each other to create small, solid particles of Carbon which we know as soot. This is important because it is these soot particles, along with the Carbon Dioxide and the steam formed when the water by-product boils, form what we call smoke.

This proves, once and for all, that the saying is true. There really is no smoke without an exothermic oxidation reaction! I hope you have enjoyed this quick exploration of the chemistry of fire, and as always please ask any questions!!


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