Combustion is a chemical reaction.  In a chemical reaction, molecules are broken up and their atoms are rearranged to form different kinds of molecules.
 
All chemical reactions are either endothermic or exothermic.  Use your knowledge of roots and prefixes to make an educated guess about what these words mean.  First, study the root "therm."  In what other words have you seen this root?  What has it meant?  Now, what is an exothermic reaction?  An endothermic reaction?
 
Combustion is the term for a rapid oxidation process, commonly called burning.  All combustion therefore requires oxygen, which is why a candle flame inside a closed jar goes out when it uses up all the available oxygen.  Much of our energy, including most energy to power vehicles, comes from the combustion of hydrocarbon fuels, because combustion is an exothermic process.

The complete combustion of methane is represented by the molecular equation:

CH₄ + 2O₂ --> CO₂ + 2H₂O

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