Combustion

Summary: A rapid exothermic reaction between a substance and oxygen, producing oxides. Complete combustion produces CO₂ and H₂O. Tags: igcse chemistry definition reactions Created: 2026-07-14 Last Updated: 2026-07-14

Combustion is a rapid, exothermic chemical reaction between a fuel and an oxidising agent, almost always oxygen, that releases energy in the form of heat and light. When a hydrocarbon undergoes complete combustion in an excess supply of oxygen, the products are exclusively carbon dioxide and water vapour, and the maximum possible amount of energy is released (e.g., CH₄ + 2O₂ → CO₂ + 2H₂O). In contrast, incomplete combustion occurs when the oxygen supply is limited or insufficient, producing carbon monoxide (CO), a colourless, odourless, and highly toxic gas, and/or elemental carbon in the form of soot — both of which are hazardous and indicate lower energy efficiency. Carbon monoxide is particularly dangerous because it binds irreversibly to haemoglobin in red blood cells, preventing oxygen transport and causing asphyxiation, which is why gas boilers and appliances must be regularly serviced and rooms adequately ventilated. The combustion of fossil fuels such as coal, petroleum, and natural gas is the dominant source of global energy for electricity generation, heating, and transport, but it is also the primary anthropogenic contributor to rising CO₂ levels and climate change. IGCSE exam questions frequently require students to write balanced symbol equations for both complete and incomplete combustion of given alkanes (methane, ethane, propane) and to explain the dangers associated with carbon monoxide poisoning and soot production.

Key term — IGCSE Chemistry (0620/0971) glossary.