Vocabulary: Balancing Chemical Equations Vocabulary  Coefficient – a number that indicates how many of each atom or molecule there are. o For example, the coefficient “6” in 6H20 indicates there are six water molecules.  Combustion – a chemical reaction in which a fuel burns in a gas, usually oxygen. o Many combustion reactions involve a carbon-based fuel, such as methane (CH4), burning in oxygen gas (O2) to produce carbon dioxide (CO2) and water (H2O).  Compound – a pure substance composed o