HSO₄⁻ can act as both a base and an acid because it is an amphiprotic species. This means it can donate a proton (act as an acid) or accept a proton (act as a base).

Here's a breakdown:

As an acid:

* HSO₄⁻ can donate a proton (H⁺) to form SO₄²⁻ (sulfate ion):

HSO₄⁻ ⇌ H⁺ + SO₄²⁻

As a base:

* HSO₄⁻ can accept a proton (H⁺) to form H₂SO₄ (sulfuric acid):

HSO₄⁻ + H⁺ ⇌ H₂SO₄

This dual behavior is due to the presence of the hydrogen atom attached to the sulfur atom in HSO₄⁻. This hydrogen can either be released as a proton (acidic behavior) or can be added to form another bond (basic behavior).

In summary:

HSO₄⁻ is an amphiprotic species, meaning it can act as both an acid and a base depending on the reaction environment.