Leveling effect

In aqueous solution all very strong bases like Na+H–, Na+NH2–, Na+OC2H5– are levelled to the strength of OH– ion, for they react completely with H2O to produce OH–ions. In other words, they are stronger bases than the conjugate base OH-.
Na+H- +HOH = OH-  + NaH2+
So bases stronger than the conjugate base of the solution are levelled  (behave of equal strength ) and this effect is called leveling effect or solvent effect.

Similarly
  Since HF and HCl both are ~ 100% ionised in liquid NH3 to give ~100% NH4+ ions, these appear to be of equal strength and liquid NH3 acts as a levelling solvent for HF and HCl. In H2O, HF is only partially ionised, whereas HCl and HBr are ~ 100% ionised. So H2O is a differentiating solvent for HF and HBr/HCl. In other words, HF and H3O+ are of about equal strength acids. And HCl/HBr are  stronger  acids  than H3O+.

        HF+NH3=NH4+ +F-
         0               100
        HBr/HCl+NH3=NH4+  +Cl-
         0               100

        HF+H2O =H3O+  +F-
        45               55
       HBr/HCl+H2O =H3O+  +Cl-
         0                  100
     
 The solvents in which complete proton-transfer occurs are called levelling solvents?
  In other words, the solvent in which the solute is ~100% ionised, are called levelling solvents.
  Thus H2O is a differentiating solvent for HF and HCl, but for HCl and HBr it is a leveling solvent. Several mineral acids are partially ionised in glacial CH3COOH medium because CH3COOH is a poor proton-acceptor but rather a better proton donor. CH3COOH, therefore, acts as a differentiating solvent towards the mineral acids. But, for bases, CH3COOH act as a leveling solvent.

  CH3COOH+H+=CH3COOH2+  Poor
                                                 (weaker base)
  CH3COOH=CH3COO- +H        Good
                                                (stronger acid)
  HCl+CH3COOH = CH3COOH2+ +Cl-
    10                              90
  HBr
   15                                85 (suposed figures)
  HF
   20                                80

 NaOH+CH3COOH = CH3COONa+ H2O
     0                                  100
  KOH
     0                                   100
     This phenomenon viz the strength of all acids becomes equal to that of H3O+ion is called leveling effect of the solvent, and here water is called a leveling solvent for all these acids.