  We show that the Hall scale in a weakly ionized plasma depends on the
fractional ionization of the medium and, Hall MHD description becomes important
whenever the ion-neutral collision frequency is comparable to the ion-gyration
frequency, or, the ion-neutral collisional mean free path is smaller than the
ion gyro-radius. Wave properties of a weakly-ionized plasma also depends on the
fractional ionization and plasma Hall parameters, and whistler mode is the most
dominant mode in such a medium. Thus Hall MHD description will be important in
astrophysical disks, dark molecular clouds, neutron star crusts, and, solar and
planetary atmosphere.
