Which statement best describes a combustion reaction?

• A. A reaction that forms a precipitate
• B. A reaction that releases energy by breaking molecules
• C. A reaction that absorbs energy and decreases temperature
• D. A reaction that involves oxidation of a fuel with oxygen

Answer: (D)

Combustion is a chemical reaction where a fuel reacts with an oxidizer, typically oxygen in the air, producing products such as carbon dioxide and water and releasing energy as heat (and often light). The best description is that it involves oxidation of a fuel with oxygen, which captures the essence of the process. The other statements miss key aspects: forming a precipitate is not a defining feature of combustion; energy release is a result of the overall exothermic reaction rather than just breaking molecules; and combustion does not absorb energy to lower temperature. For example, burning methane follows CH4 + 2 O2 → CO2 + 2 H2O + heat, illustrating oxidation with energy release.