The International Union of Chemical and Applied Chemistry (IUPAC) defines a catalyst as a substance that increases the rate of a reaction without altering the overall standard Gibbs free energy change. This effect is called catalysis, and reactions involving catalysts are called catalytic reactions.
The composition, chemical properties, and mass of a catalyst remain unchanged before and after the reaction; its relationship with the reaction system is like that of a lock and key, exhibiting high selectivity (or specificity). A catalyst does not catalyze all chemical reactions; some chemical reactions do not require a single catalyst.
