Step 1: Understanding the Concept:
According to the Brønsted-Lowry theory:
1. An Acid is a proton (\( H^+ \)) donor.
2. A Base is a proton (\( H^+ \)) acceptor.
In any Brønsted-Lowry acid-base reaction, there are two conjugate acid-base pairs.
The general form is: \( \text{Acid}_1 + \text{Base}_2 \rightleftharpoons \text{Acid}_2 + \text{Base}_1 \).
Step 2: Detailed Explanation:
Let's analyze the given chemical reaction:
\[ HCl(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + Cl^-(aq) \]
1. \( HCl \) loses a proton (\( H^+ \)) to become \( Cl^- \). Therefore, \( HCl \) is acting as the proton donor (\( \text{Acid}_1 \)).
2. \( H_2O \) accepts the proton from \( HCl \) to become \( H_3O^+ \). Therefore, \( H_2O \) is acting as the proton acceptor (\( \text{Base}_2 \)).
3. In the reverse reaction, \( H_3O^+ \) would donate a proton to become \( H_2O \). Thus, \( H_3O^+ \) is the conjugate acid of \( H_2O \) (\( \text{Acid}_2 \)).
4. Similarly, \( Cl^- \) would accept a proton in the reverse reaction to become \( HCl \). Thus, \( Cl^- \) is the conjugate base of \( HCl \) (\( \text{Base}_1 \)).
Step 3: Evaluating Options:
(1) \( H_3O^+ \): This is the conjugate acid (\( \text{Acid}_2 \)).
(2) \( H_2O \): This is the base that accepts the proton (\( \text{Base}_2 \)).
(3) \( Cl^- \): This is the conjugate base (\( \text{Base}_1 \)).
(4) \( HCl \): This is the primary acid (\( \text{Acid}_1 \)).
Step 4: Final Answer:
By identifying the proton acceptor in the forward reaction, we conclude that \( H_2O \) is \( \text{Base}_2 \).