text: Another form for the first law is to write it with the primary variable as mass fraction (, given for example in kg/kg), then the equation changes to:
  where
  * the index  denotes the th species,
  *  is the diffusion flux vector of the th species (for example in mol/m-s),
  *  is the molar mass of the th species, and
  *  is the mixture density (for example in kg/m).
  The  is outside the gradient operator. This is because:
  where  is the partial density of the th species.
  Beyond this, in chemical systems other than ideal solutions or mixtures, the driving force for diffusion of each species is the gradient of chemical potential of this species. Then Fick's first law (one-dimensional case) can be written
  where
  * the index  denotes the th species
  *  is the concentration (mol/m)
  *  is the universal gas constant (J/K/mol)
  *  is the absolute temperature (K)
  *  is the chemical potential (J/mol).
  The driving force of Fick's law can be expressed as a fugacity difference:
  Fugacity  has Pa units.  is a partial pressure of component  in a vapor  or liquid  phase. At vapor liquid equilibrium the evaporation flux is zero because .