Final answer:
In the chemical equation HCO₃⁻ + H₂O ⇌ OH⁻ + H₂CO₃, water (H₂O) acts as a Brønsted-Lowry acid because it donates a proton to the bicarbonate ion, forming OH⁻ and H₂CO₃.
Step-by-step explanation:
In the forward chemical equation represented by HCO₃⁻ + H₂O ⇌ OH⁻ + H₂CO₃, the role that water (H₂O) plays depends on whether we are looking at the forward or the reverse reaction. In the context of the provided equation, water is acting as the Brønsted-Lowry acid.
This is because in the forward reaction, water donates a proton (H+) to the bicarbonate ion (HCO₃⁻), forming hydroxide (OH⁻) and carbonic acid (H₂CO₃). As such, the correct answer to the question 'H₂O =' in the context of the given reaction is 'a. acid'.