Here's why:

* It reacts with water to form an acid: SO₂ dissolves in water to form sulfurous acid (H₂SO₃):

SO₂ (g) + H₂O (l) → H₂SO₃ (aq)

* It reacts with bases to form salts: SO₂ reacts with bases like sodium hydroxide (NaOH) to form sulfites:

SO₂ (g) + 2NaOH (aq) → Na₂SO₃ (aq) + H₂O (l)

These reactions are characteristic of acidic oxides.