Here's why:

* Basic oxides react with water to form bases (alkalies). For example, sodium oxide (Na₂O) reacts with water to form sodium hydroxide (NaOH), a strong base.

* Acidic oxides react with water to form acids. Carbon dioxide reacts with water to form carbonic acid (H₂CO₃), a weak acid.

The reaction of carbon dioxide with water is as follows:

```

CO₂(g) + H₂O(l) ⇌ H₂CO₃(aq)

```

This reaction is reversible, but the equilibrium favors the formation of carbonic acid.

Therefore, due to its ability to form an acid when reacting with water, carbon dioxide is classified as an acidic oxide.