A chemical reaction is called product-favored if there are more products than reactants after the reaction is completed. Product-favored reactions are often called spontaneous reactions, but the word spontaneous implies that a reaction happens as soon as the reactants are mixed. That is not always the case. For example, the combustion of methane:

CH₄ (g) + 2 O₂ (g) CO₂ (g) + 2 H₂O (g)

(seen in bunsen burner flames and in gas-heated homes) is product-favored since when a mixture of methane and oxygen is sparked or lit with a flame, a reaction occurs which converts virtually all the methane and oxygen to carbon dioxide and water. The reaction is not spontaneous, however, since the methane-oxygen mixture is stable indefinitely until a spark or flame initiates the reaction.