In the given reaction:
\[ \text{HCl} + \text{H}_2\text{O} \rightarrow \text{Cl}^- + \text{H}_3\text{O}^+ \]
we can analyze the roles of the substances involved:
- HCl is a strong acid that donates a proton (H^+) during the reaction.
- H2O acts as a base because it accepts a proton from HCl, resulting in the formation of H3O^+ (hydronium ion).
- The Cl^- ion is the conjugate base of HCl, formed when HCl loses a proton.
With this understanding, we can assess the statements provided:
- H2O is an acid, and Cl is its conjugate base. (Incorrect: H2O is acting as a base, not an acid, and Cl^- is the conjugate base of HCl.)
- HCl is an acid, and Cl is its conjugate base. (Correct: HCl donates a proton to form Cl^-, making Cl^- the conjugate base of HCl.)
- H2O is an acid, and H3O^+ is its conjugate base. (Incorrect: H2O is acting as a base, not an acid, and H3O^+ is the conjugate acid, not the base.)
- HCl is an acid, and H3O^+ is its conjugate base. (Incorrect: HCl is the acid, but H3O^+ is its conjugate acid, not a base.)
Thus, the correct statement is:
HCl is an acid, and Cl is its conjugate base.