Here's why:

* Weak acid: When CO₂ dissolves in water, it reacts to form carbonic acid (H₂CO₃):

CO₂ (g) + H₂O (l) ⇌ H₂CO₃ (aq)

* Carbonic acid is a weak acid: It only partially ionizes in water, meaning it doesn't donate all its hydrogen ions (H⁺) to the solution. This results in a relatively low concentration of H⁺ ions, making the solution weakly acidic.

* Equilibrium: The reaction between CO₂ and water is an equilibrium reaction. This means that the formation of carbonic acid is reversible, and some of the carbonic acid will decompose back into CO₂ and water.

Important Note: While CO₂ acts as a weak acid in water, it's not a base. It doesn't have the ability to accept protons (H⁺) like a base would.