Combustion

Combustion is the burning of a compound in oxygen, releasing heat. For carbon compounds it is the reaction that makes alkanes and alcohols such useful fuels. Malay: pembakaran · Chinese: 燃烧.

There are two kinds. In complete combustion, plenty of oxygen is available and the compound burns fully to carbon dioxide and water; the flame is clean and blue and gives out the most heat. In incomplete combustion, the oxygen supply is limited, so as well as water the reaction gives the poisonous gas carbon monoxide and unburnt carbon as soot; the flame is yellow, sooty and less hot. The larger a hydrocarbon molecule, the higher its proportion of carbon and so the more likely it is to burn with a sooty flame. This is why the small alkanes near the top of a homologous series make the cleanest fuels, while larger ones tend to smoke unless the air supply is generous.

Example. Methane burning fully gives carbon dioxide and water: CH₄ + 2O₂ → CO₂ + 2H₂O. With a restricted air supply the same fuel can instead give carbon monoxide, CO, and soot, which is why a blocked gas heater is dangerous.

Confusion to avoid. Do not treat complete and incomplete combustion as the same reaction. Complete combustion needs excess oxygen and gives carbon dioxide only; incomplete combustion happens in limited oxygen and gives carbon monoxide and soot. In an answer, always state the oxygen condition before naming the products.

Combustion is the reaction that links the alkanes and alcohols to their role as fuels and explains why controlling the air supply matters, a practical thread that runs through Carbon Compounds in SPM Chemistry.