Combustion is a chemical reaction in which a substance reacts rapidly with an oxidising agent, usually oxygen, accompanied by the release of energy, often as heat and sometimes as visible light. In ordinary everyday language, combustion is commonly called burning. Typical fuels include hydrocarbons such as methane, propane and petrol, as well as hydrogen, carbon, wood, coal and many other organic materials.
Complete combustion of a hydrocarbon generally produces carbon dioxide and water, whereas incomplete combustion can produce carbon monoxide, soot and other partially oxidised substances. Combustion is therefore not simply the presence of flames. A flame is a visible region of hot reacting gases, while combustion is the underlying chemical process. Some combustion reactions occur without a visible flame, particularly in solid materials, catalytic surfaces and certain slow oxidation processes.
Combustion is fundamentally an oxidation process involving the transfer or rearrangement of electrons and the formation of chemically more stable products. In a hydrocarbon fuel, carbon and hydrogen atoms are initially bonded to one another and to other atoms within the fuel molecules. During combustion, these bonds are broken and new bonds form with oxygen. For methane, the ideal complete combustion equation is CH4 plus 2O2 producing CO2 plus 2H2O. The atoms are conserved, but their arrangement changes.
The products have lower chemical potential energy than the reactants under the relevant conditions, and the difference is released primarily as thermal energy. Combustion can therefore be understood simultaneously at the molecular level as a rearrangement of chemical bonds and at the macroscopic level as a process that releases heat and often produces hot gases.
Oxygen is the most familiar oxidising agent in combustion because atmospheric air contains approximately 21 percent oxygen by volume. However, combustion does not fundamentally require atmospheric oxygen. Other oxidising substances can support combustion under appropriate conditions, including fluorine, chlorine compounds and chemically bound oxidisers used in propellants. The broader concept is therefore oxidation involving a sufficiently energetic reaction with an oxidiser rather than merely a reaction with air.
In ordinary fires, oxygen from the surrounding atmosphere diffuses toward the fuel and reacts with volatile fuel molecules or decomposition products at high temperature. The rate of combustion depends strongly on the availability of oxygen, fuel concentration, temperature, surface area and mixing. This is why restricting oxygen can extinguish a fire even though the fuel itself remains present.
The combustion of a solid fuel such as wood involves several stages rather than a single simple reaction. When heated, the solid material can undergo pyrolysis, a process in which heat causes thermal decomposition and produces gases, vapours, liquids and a carbon rich residue called char. Many of the volatile products can then mix with oxygen and burn in the gas phase, producing the visible flame. Char can undergo further surface oxidation and glowing combustion.
Liquids such as petrol also commonly burn through vapour produced above the liquid surface rather than through the bulk liquid itself. Gaseous fuels can mix directly with oxygen and undergo combustion in a flame. These distinctions are important in fire science because the physical state, volatility, surface area and transport of heat and oxygen determine how a material ignites and continues burning.
Combustion can be complete or incomplete depending on the supply of oxygen, temperature, mixing and reaction conditions. Complete combustion of methane produces carbon dioxide and water when sufficient oxygen is available and the reaction proceeds effectively. Incomplete combustion can occur when oxygen is limited or when mixing is poor, producing carbon monoxide, unburned hydrocarbons, soot and other products. Carbon monoxide is particularly important because it is toxic and can accumulate during poorly ventilated combustion.
Soot consists largely of carbonaceous particulate material produced through complex high temperature chemistry. Real flames can contain hundreds or thousands of intermediate chemical species even when the overall reaction is represented by a simple equation. Combustion chemistry is therefore much more complex than the simplified equations commonly used in introductory chemistry.
A fire requires three fundamental elements traditionally represented by the fire triangle: fuel, an oxidiser and sufficient heat to initiate and sustain the reaction. Modern fire science sometimes uses the fire tetrahedron, which adds a chemical chain reaction as a fourth component. Removing any essential component can stop combustion. Cooling reduces temperature below the level required to sustain the reaction, smothering reduces oxygen availability, and removing or isolating the fuel prevents continued reaction.
Fire extinguishers exploit these principles in different ways. Water primarily removes heat and can also help isolate combustible material, while carbon dioxide can displace oxygen in suitable enclosed environments. Foam can separate fuel from oxygen, and specialised extinguishing agents interrupt chemical processes or absorb heat depending on their formulation.
Combustion is central to human energy use and has shaped technology and civilisation. Burning wood provided early sources of heat and cooking energy, while coal combustion powered much of the Industrial Revolution. Internal combustion engines use controlled combustion to convert chemical energy into mechanical work. Gas turbines and boilers also rely on combustion to produce high temperature gases or steam. Industrial furnaces use combustion for metal processing, ceramics, glass production and chemical manufacturing.
However, combustion also produces pollutants and greenhouse gases. Fossil fuel combustion releases carbon dioxide accumulated over geological timescales and can produce nitrogen oxides, particulate matter, carbon monoxide and other pollutants. The environmental consequences therefore depend on the fuel, combustion conditions, emission controls and scale of use.
Combustion should be distinguished from slower forms of oxidation. Rusting of iron is an oxidation process but is not ordinarily classified as combustion because it proceeds extremely slowly and does not produce the characteristic rapid release of energy associated with burning. Respiration is also an oxidation process and releases energy from chemical compounds, but biological respiration proceeds through controlled enzyme mediated pathways rather than through the rapid high temperature chemistry of a flame.
Combustion can be spontaneous when a substance reaches conditions at which its oxidation becomes self sustaining, or it can require an ignition source. Autoignition temperature, flash point and fire point are related but distinct concepts used to describe ignition behaviour. Understanding these distinctions is important when analysing fuels, fires, engines and industrial chemical processes.
