Final answer:
In the reaction, bicarbonate (HCO₃⁻) acts as an acid and its conjugate base is carbonate (CO₃²⁻) after it donates a proton.
Step-by-step explanation:
In the acid-base reaction HCO₃⁻(aq) + H₂O(l) ↔ CO₃²⁻(aq) + H₃O⁺(aq), bicarbonate (HCO₃⁻) is the acid because it donates a proton to water, and carbonate (CO₃²⁻) is its conjugate base, as it is what remains after the bicarbonate ion loses a proton. According to the Brønsted-Lowry theory of acids and bases, an acid is a proton donor, while a base is a proton acceptor. Thus, when bicarbonate loses a proton, it forms its conjugate base, carbonate.