A process that results in the interconversion of @CT01038@. Chemical reactions may be @E02035@ or @S05970@. (It should be noted that this definition includes experimentally observable interconversions of conformers.) Detectable chemical reactions normally involve sets of @M03986@ as indicated by this definition, but it is often conceptually convenient to use the term also for changes involving single molecular entities (i.e. 'microscopic chemical events').