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THE LEVELLING EFFECT, LEVELLING AND DIFFERENTIATING SOLVENTS

The apparent strength of a protonic acid is dependent on the solvent in which the acid is dissolved. When all the acids in the acid chart which are stronger than H3O+ ion
(i.e., the acids above H3O+ acids) are added to H2O, they donate as proton to H2O to H3O+ ion and appear to have equal strength, since all these acids are levelled to the strength of H3O+ ion which is left in solution and is common to all such solutions.
This phenomenon viz. The strength of all the acids becomes equal to that of H3O+ion is called leveling effect of the solvent, water, and here water is called a leveling solvent for all these acids.

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.

The solvent 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.

Since HF and HCl both are ~ 100% ionised in liquid NH3 to give ~100% NH4+ ions, these appear to be of equal strength and liq. NH3 acts as a levelling solvent for HF and HCl. In H2O, HF is only partially ionised, whereas HCl and HBr are ~ 100% ionised.

Thus H2O is a differentiating solvent for HF, 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.

The weaker the acid, the stronger the conjugate base.

    It is very hard to discriminate between strong acids in water since they are fully deprotonated, so a mol of HI and a mol of HBr each act like one mol of H3O+. A solvent which is a weaker proton acceptor is needed to tell the difference.

The same logic applies to strong bases, one mole of any strong base can be treated as one mole of hydroxyl ions.

This is called the leveling effect - the limitation by the solvent of how strong acids or bases can be.

If two weak acids are put into a solvent which is a strong proton acceptor, such as ammonia, it is likewise hard to tell the difference between the two acids, since they will both act like a mol of ammonium ions. So the leveling effect changes with the solvent.
  Solvent  leveling
  HI  (l)  +  H2O(l)  = H3O+(aq)  +  I-(aq)
  HBr (l)  +  H2O(l)  = H3O+(aq)  +  Br-(aq) Because  HI(l)  and  HBr(l)  are  strong  acids,  both  transfer  their  protons essentially  completely  to  give  H3O+.  In  effect,  solutions  of  HI  and  HBr behave  as  though  they  are  solutions  of  H3O+ regardless  HI  is intrinsincally stronger  than  HBr.  Water  is  therefore  said  to  have  a leveling  effect  that  brings  all  strong  acids  down  to  the  acidity of  H3O+. To  distinguish  the  acidity  strengths  of  HBr and  HI,  one  has  to  use  a less  basic  solvent. Similar  situation  exists  for  strong  bases.  In  water,  strong  bases  behave as  solutions  of  OH-.  The  strengths  of  strong  bases  can  only  be distinguished  using  a  less  acidic  solvent.
Leveling Effect, which states that the strongest acid that can exist in aqueous solution is H3O+.