Let's analyze the given reaction:
\[ \text{HCl} + \text{H}_2\text{O} \rightarrow \text{Cl}^- + \text{H}_3\text{O}^+ \]
In this reaction:
- HCl donates a proton (\( \text{H}^+ \)) to water (\( \text{H}_2\text{O} \)), making HCl an acid.
- When \( \text{HCl} \) loses a proton, it forms \( \text{Cl}^- \), which is the conjugate base of the acid \( \text{HCl} \).
- Water (\( \text{H}_2\text{O} \)) accepts a proton to form \( \text{H}_3\text{O}^+ \), making \( \text{H}_2\text{O} \) act as a base in this reaction.
- The \( \text{H}_3\text{O}^+ \) ion, formed from water after it accepts a proton, is the conjugate acid of the base \( \text{H}_2\text{O} \).
Now, let's evaluate the response options:
- H₂O is an acid, and Cl⁻ is its conjugate base. (Incorrect, because H₂O acts as a base here.)
- HCl is an acid, and H₃O⁺ is its conjugate base. (Incorrect, because H₃O⁺ is the conjugate acid of the base H₂O, not the conjugate base of HCl.)
- H₂O is an acid, and H₃O⁺ is its conjugate base. (Incorrect, because H₂O acts as a base here.)
- HCl is an acid, and Cl⁻ is its conjugate base. (Correct statement.)
Thus, the correct statement about the reaction is:
HCl is an acid, and Cl⁻ is its conjugate base.