Fire is a chemical reaction that converts a fuel and oxygen into carbon dioxide and water. It is an exothermic reaction, in other words, one that produces heat. This is due to the fact that the chemical bonds in the oxygen molecule are fairly weak and the new bonds produced are more stable – thus there is a net production of energy.
Fire’s basic combustion equation is: fuel + oxygen —> carbon dioxide + water, a line many of us had drummed into us by school teachers. However, combustion reactions do not proceed directly from oxygen to carbon dioxide. Instead, a welter of intermediate molecules are involved along the way. Sometimes incomplete combustion occurs, and these intermediate molecules are produced in unusually large quantities. For example, if a flame does not get enough oxygen it can produce carbon monoxide instead of carbon dioxide.
Flames occur when two gases react, producing both heat and light. When different gases react, they produce different amounts of energy, causing some flames to burn at a higher temperature than others. The flame of a household candle, for instance, will reach between 800°C and 1000°C . A flame can be made to burn at a higher temperature by adjusting the reaction, for example using pure oxygen instead of air. Burning a mix of oxygen and acetylene, or oxy-acetylene, will produce a flame that burns at over 3000°C and can be used to cut, melt and weld metals.
We still don’t fully understand each step in the process or all the intermediate molecules involved – and of course these vary depending on the fuel too. This is one reason why we have begun studying fire in space; it is easier to study the finer points of combustion without the complicating effects of gravity.
