Science and Technology

Combustion and the Fire Triangle

Concept sheet | Science and Technology

Secondary

Top of page
The Fire Triangle
The Three Types of Combustion
Rapid Combustion
Spontaneous Combustion
Slow Combustion
See Also

Definition

Combustion is an oxidation reaction that releases energy.

The chemical equation for combustion usually takes the following form.

||\text{Fuel}+\text{Oxidizing agent}\rightarrow\text{Products}+\text{Energy}||

Note that the fuel and oxidizing agent are on the reactant side, and the energy is on the product side.

Example

The combustion of glucose during cellular respiration takes place according to the following chemical equation.

||\text{C}_6\text{H}_{12}\text{O}_{6\ \text{(aq)}}+6\ \text{O}_{2\ \text{(g)}}\rightarrow 6\ \text{CO}_{2\ \text{(g)}}+6\ \text{H}_2\text{O}_{\text{(l)}}+\text{Energy}||

The combustion of methane |(\text{CH}_4),| the main constituent in natural gas, takes place according to the following chemical equation.

||\text{CH}_{4\ \text{(g)}}+2\ \text{O}_{2\ \text{(g)}}\rightarrow\text{CO}_{2\ \text{(g)}}+2\ \text{H}_2\text{O}_{\text{(l)}}+\text{Energy}||

The Fire Triangle

Tip

The fire triangle helps us remember the 3 elements required for combustion to take place. These elements are :

A fuel
An oxidizing agent
The reaching of the ignition temperature

If one of the elements is no longer present, combustion stops.

A triangle with a fire in the center. Each vertex of the triangle represents an element of the fire triangle: fuel, oxidizing agent, ignition temperature.

Definitions

Fuel is the substance that burns. Examples include wood, paper, and fossil fuels (oil, coal, natural gas, etc.).
The oxidizing agent is the substance that sustains the combustion. The most common oxidizer is oxygen |(\text{O}_2),| because it is present in the air.
The ignition temperature is the minimum temperature the fuel and oxidizing agent must reach to initiate combustion. This temperature is specific to each substance.

Why Does Damp Wood Burn Less Well than Dry Wood?

The Three Types of Combustion

Combustion reactions can be classified into 3 types based on how combustion is initiated and the rate at which energy is released.

Rapid Combustion
Spontaneous Combustion
Slow Combustion

Rapid Combustion

Rapid combustion releases energy rapidly in the form of light and heat. Rapid combustion is usually triggered by a flame or spark.

Examples

Here are a few examples of rapid combustion.

Wood burning in a campfire

guys_who_shoot, Shutterstock.com

Propane burning in a BBQ

REC Stock Footage, Shutterstock.com

Combustion of materials caused by a power failure

serhii.suravikin, Shutterstock.com

Spontaneous Combustion

Spontaneous combustion releases energy rapidly, just like rapid combustion, but is initiated without the aid of a flame or spark.

Spontaneous combustion occurs when the heat of the surrounding environment allows a substance to reach its ignition temperature.

Examples

Here are a few examples of situations that can lead to spontaneous combustion.

In periods of drought, the risk of forest fires is higher. The oxygen |(\text{O}_2)| present in the air and the ambient heat can spontaneously ignite trees.
The heat released by the organic matter in hay can start a barn fire.
Potassium |(\text{K})| must be stored in oil because its reaction with water |(\text{H}_2\text{O})| produces hydrogen |(\text{H}_2)| and generates a lot of heat. This heat is sufficient for the |\text{H}_2| to spontaneously ignite on contact with |\text{O}_2| in the air.