Combustion and the Fire Triangle

Combustion reactions always involve fuel (a substance that burns) and an oxidizer (a substance that sustains combustion, also known as an oxidizing agent).

The general formula for combustion is as follows:  
|\text {Fuel} + O_{2} \rightarrow CO_{2(g)} + H_{2}O_{(g)}|

Here is the formula to illustrate the combustion of gasoline:
|2 \space C_{8}H_{18(l)} + 25\space O_{2(g)} \rightarrow 16 \space CO_{2(g)} + 18 \space H_{2}O_{(g)}|

The combustion triangle (the fire triangle) is a representation of the combination of the three key elements that form the basis of any combustion reaction.

For a combustion reaction to occur, three essential elements must be present simultaneously:

1. A fuel, a substance that burns such as wood or petroleum products.

2. An oxidizing agent, a substance that sustains combustion, mainly oxygen |\left(O_{2}\right)| .

3. A temperature high enough to achieve the ignition point (heat): this is the temperature that allows the provision of sufficient energy to initiate combustion. This is actually the activation energy that allows the reaction to take place. This value varies depending on the fuel used.

If fuel is removed, combustion stops. Should an engine run out of fuel, the combustion of fuel usually carried out by the engine ceases since there is no more fuel left to burn.

If the oxidizing agent is prevented from reaching the fire, combustion will cease. With a carbon dioxide fire extinguisher |\left(CO_{2}\right)|, a gas is projected onto the fire which expels oxygen, thus stopping combustion. The fire is then said to be smothered because it has no more oxygen.

If the temperature is lowered below the ignition temperature, combustion stops. When pouring water over a fire, the temperature drops below the ignition temperature; this prevents combustion from being sustained.

Rapid combustion is combustion that releases a lot of energy in the form of heat and light over a short period of time.

An exploding firework display is an example of rapid combustion because a large amount of heat is released in a short period of time.

The rapid combustion of gasoline in a car's engine enables it to move forward.

When a match is struck, enough heat is released for the match to produce a flame.

Spontaneous combustion is rapid combustion that reaches its ignition temperature without any external agent.

During a period of drought, heat ignites the wood, causing a forest fire.

Slow combustion is combustion that takes place over a long period of time. Energy is slowly released into the environment.

Fermentation is a slow combustion reaction. Bread and beer are two examples of fermentation reactions.

Decomposition, cellular respiration, and corrosion (rust) are also examples of slow combustion.