text: all reactions are going both ways.  for instance, H2 + O2--->H2O ( I know it's not balanced) also goes from H2O---->H2 + O2.  A 100% yield is never acheived because the reaction reaches a state of equilibrium where the reactants are going from H2 + O2--->H2O just as fast as they are going from H2O--->H2 + O2.\n\nAn entire semester of inorganic chemistry is devoted to the concept of equalibrium.