(a) The protic acids which have very high value of the acid dissociation or ionization constant, ionize/dissociate almost completely in aqueous solution at ordinary dilutions. They are called strong acids.Example: HCl, H2SO4, and HNO3.
(b) Acids, which dissociate in aqueous solution to a smaller extent at ordinary dilution, to give low concentration of H+ ions in solution, are called weak acids.Example: CH3COOH, (COOH)2, H3PO4, HCN, HF, etc. The Ka or K1 values of such acids are much smaller than one.
(c) On dilution, the ionization of an acid increases. So, concentration of H+ ions also increases on dilution. Therefore, strength of the acid increases with dilution. It is for this reason that the acid strengths are compared at equal concentrations.
(d) For di- and tri- protic acids, first dissociation is stronger than the second, which in turn is much stronger than the third dissociation i.e., K1 » K2 » K3. This is because the removal of a proton from a negatively charged species is more difficult than from a neutral molecule.