Here's why:

* Definition of an acid: An acid is a substance that donates hydrogen ions (H+) when dissolved in water.

* Hydrogen sulfate's behavior in water: When hydrogen sulfate dissolves in water, it releases hydrogen ions (H+) and sulfate ions (SO₄²⁻), making the solution acidic.

The chemical equation for this reaction is:

H₂SO₄ (aq) + H₂O (l) → H₃O⁺ (aq) + HSO₄⁻ (aq)

Note: Hydrogen sulfate is also commonly known as sulfuric acid, one of the strongest mineral acids.