How are chemical reactions classified?

Chemists classify chemical reactions in a number of ways: by type of product, by types of reactants, by reaction outcome, and by reaction mechanism. Often a given reaction can be placed in two or even three categories, including gas-forming and precipitation reactions. Many reactions produce a gas such as carbon dioxide, hydrogen sulfide, ammonia, or sulfur dioxide. Cake batter rising is caused by a gas-forming reaction between an acid and baking soda (sodium hydrogen carbonate). Classification by types of reactants include acid-base reactions and oxidation-reduction reactions, which involve the transfer of one or more electrons from a reducing agent to an oxidizing agent. Examples of classification by reaction outcome include decomposition, polymerization, substitution, and elimination and addition reactions. Chain reactions and photolysis reactions are examples of classification by reaction mechanism, which provides details on how atoms are shuffled and reassembled in the formation of products.