The levelling effect of the solvent

In the balance:

HA + H2O      H3O+ A

We can glimpse a "competition" for the possession of the proton. The two species that are vying for the proton are H2O and  A. If the challenge is won by H2O, the HA acid will be a strong acid.

competizione per il protone

Therefore, this is what happens when we have to do with hydrohalic acids such as HCl or common oxyacids as HNO3 or H2SO4.

For example:                                                         HNO+ H2O    H3O + NO3

In an aqueous solution of nitric acid, the species HNO is practically non-existent! The only true acid in the solution is H3O. H3O  Is the species capable of neutralizing the bases, certainly not HNO3

This information has cuffs really very important:

1) H3O+  is certainly the acidic species stronger in solution..

2)All the strong acids, apparently, are equal in solution. It is not possible to distinguish the strength of HNO3 and  HCl from aqueous solution of these acids. Both are completely dissociated.

This second flap is known as levelling effect of the solvent.

The speech is equally valid for the strong bases. Those that commonly consider the bases stronger are the hydroxides:

NaOH  Na+ OH

hydroxides of alkali metals are soluble in water and dissociate completely in Na+ and OH- .. Also in this case, is not NaOH base in solution! The only basic species in solution is OH.

OHis the strongest base that we can find in aqueous solution.