What is Combustion?
A chemical reaction in which a fuel reacts rapidly with oxygen, releasing energy in the form of heat and light.
Combustion is one of the most important and familiar chemical reactions in everyday life. Every time you see a flame — whether from a candle, a gas cooker, a campfire, or a car engine — you are witnessing combustion in action.
For combustion to occur, three things must be present at the same time:
- Fuel — the substance that burns (e.g. wood, petrol, natural gas, wax)
- Oxygen — usually from the air (about 21% of air is oxygen)
- Heat (activation energy) — enough energy to start the reaction
This trio is often called the fire triangle. Remove any one of the three, and the combustion reaction stops. This is the scientific principle behind fire extinguishers!
Think of the fire triangle like a three-legged stool. Remove one leg and the whole stool falls over. Remove fuel, oxygen, or heat, and the fire goes out.
In MYP, combustion is a perfect example of the key concept Change and the related concept Transformation — atoms are rearranged, chemical bonds are broken and formed, and chemical energy is transformed into heat and light energy. This connects directly to our unit theme: Action Triggers Reaction. The activation energy (heat) is the "action" that triggers the combustion reaction.
Fuels and Their Origins
A substance that stores chemical energy and releases it as heat (and light) during combustion.
Fuels can come from many different sources. In chemistry, we classify them broadly into two groups:
Fossil Fuels (non-renewable)
- Formed from the remains of ancient organisms over millions of years under pressure and heat
- Examples: coal, crude oil (petroleum), natural gas (mainly methane, CH₄)
- These are the world's most widely used energy sources, but they are finite — once used, they cannot be replaced on a human timescale
Biofuels (renewable)
- Made from recently living biological material (biomass)
- Examples: wood, ethanol from sugar cane, biodiesel from plant oils
- Biofuels are often described as carbon neutral: the CO₂ released when they burn was only recently absorbed from the atmosphere by the plants that produced them, so there is no net addition of carbon to the atmosphere — in theory.
A process or fuel is carbon neutral if the amount of CO₂ it releases is balanced by an equivalent amount of CO₂ absorbed, resulting in no net increase in atmospheric carbon dioxide.
The 'carbon neutral' label for biofuels is a simplification. In practice, growing, harvesting, and transporting biofuels also requires energy (often from fossil fuels), so biofuels do contribute some net CO₂. This is an important nuance for Criterion D discussions about trade-offs.
Most fuels used in daily life — petrol, diesel, kerosene, liquefied petroleum gas (LPG) — are hydrocarbons: compounds made of hydrogen and carbon atoms only.
Hydrogen (H₂) is sometimes described as a future fuel. It produces only water when it burns, making it very clean. However, producing and storing hydrogen safely remains a major challenge.
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