1. Answer first: phosphoric acid is not a "very strong" acid-it is a medium-strength acid
Phosphoric acid (H₃PO₄) is not a "very strong" acid. In chemical classification it is a medium-strength acid (a triprotic acid): it is markedly stronger than common weak acids such as carbonic acid and acetic acid, but clearly weaker than strong acids such as hydrochloric, sulfuric and nitric acid. The statement "phosphoric acid is a very strong acid" is therefore inaccurate and should be denied.
2. Why phosphoric acid is not a strong acid
Strong acids are defined by complete dissociation in water; phosphoric acid dissociates only partially.
A strong acid is one that dissociates completely (or almost completely) into hydrogen ions in aqueous solution, such as hydrochloric acid, nitric acid, and sulfuric acid (whose first step dissociates completely). Phosphoric acid is a triprotic acid that dissociates in three reversible steps; a large proportion of undissociated H₃PO₄ molecules remains in solution, so it does not meet the "complete dissociation" criterion.
Its dissociation constants show limited ionization.
The three stepwise dissociation constants of phosphoric acid are pKa1 ≈ 2.15, pKa2 ≈ 7.20 and pKa3 ≈ 12.35. Recognized strong acids typically have negative pKa values (hydrochloric acid, for example, is about −7). Phosphoric acid's first-step pKa is positive and far from zero, indicating limited ionization; the second and third steps are even weaker, and the third (pKa3 ≈ 12.35) is negligible.
Comparison with recognized strong acids shows a clear gap.
Hydrochloric, nitric, sulfuric (first step) and perchloric acid dissociate almost completely in water and are far more acidic than phosphoric acid. At any concentration, phosphoric acid cannot reach their degree of ionization, so it cannot exhibit "very strong" acidity.
It fails the definition-based test.
The criterion for a strong acid is complete dissociation. Phosphoric acid does not meet this criterion and, by chemical definition, cannot be classified as a strong acid; it belongs to the medium-strength category.
3. Why phosphoric acid is not a weak acid
Its first-step dissociation is clearly higher than that of typical weak acids.
Phosphoric acid has pKa1 ≈ 2.15, well below that of acetic acid (pKa ≈ 4.76) and carbonic acid (pKa1 ≈ 6.35). The smaller the pKa, the stronger the acid; the first-step ionization of phosphoric acid is therefore far greater than that of these typical weak acids.
At the same concentration it is clearly more acidic.
In a 0.1 mol/L aqueous solution, phosphoric acid gives a pH of about 1.6, whereas acetic acid at the same concentration gives about 2.9-the hydrogen-ion concentration of phosphoric acid is roughly 20 times higher.
Its reactivity and industrial uses reflect medium-strength acidity.
Phosphoric acid reacts with metal oxides, hydroxides and carbonates to form phosphates, and is widely used in phosphating, etching, pickling and pH adjustment. In these applications its acidity is exploited to a degree that typical weak acids such as acetic and carbonic acid cannot provide, which shows its strength is not "weak".
It fails the definition-based test.
The acidity of phosphoric acid lies between strong and weak acids: it does not reach the "complete dissociation" standard of strong acids, yet it is clearly stronger than typical weak acids. Calling it a weak acid is therefore also inaccurate; classifying it as a medium-strength acid reflects the facts.
