Chemical reaction:
A chemical reaction is a reaction in which reactants get converted into products. Chemical reaction involves breaking of old bonds and making of new bonds. In the process of reaction, mass and energy is conserved.
Chemical reactions can also occur in the living system. Some chemical reactions occur within fraction of seconds, whereas some may take very long. Some reactions are spontaneous, whereas some are non spontaneous. Some are reversible, whereas some are irreversible. It is the activation energy on which the rate of reaction depends. Catalysts are the substances that reduce the activation energy barrier and speed up the rate of reaction. In the living system, role of catalyst is performed by enzymes. Like the catalyst, enzymes are also obtained in unaltered form at the end of reaction.