When `NH_(4)Cl` is added to a solution of `NH_(4)OH :`

Options

A. the dissociation of `NH_(4)OH` increases
B. the concentration of `OH^(-)` increases.
C. the concentration of both `OH^(-)` and `NH_(4)^(+)` increase
D. the concentration of `OH^(-)` ion decreases.

Explanation

To solve the question "When NH₄Cl is added to a solution of NH₄OH," we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Components:**
- NH₄OH (ammonium hydroxide) dissociates into NH₄⁺ (ammonium ion) and OH⁻ (hydroxide ion) in solution.
- NH₄Cl (ammonium chloride) dissociates into NH₄⁺ and Cl⁻ (chloride ion) when added to the solution.

2. **Write the Dissociation Equations:**
- For NH₄OH:
\[ NH₄OH \rightleftharpoons NH₄^+ + OH^- \]
- For NH₄Cl:
\[ NH₄Cl \rightleftharpoons NH₄^+ + Cl^- \]

3. **Analyze the Effect of Adding NH₄Cl:**
- When NH₄Cl is added to the NH₄OH solution, the concentration of NH₄⁺ ions in the solution increases due to the additional NH₄⁺ ions from NH₄Cl.

4. **Apply the Common Ion Effect:**
- The increase in NH₄⁺ concentration will affect the equilibrium of the dissociation of NH₄OH. According to Le Chatelier's principle, if a system at equilibrium is disturbed by changing the concentration of one of the components, the system will shift in a direction that counteracts the change.
- In this case, the equilibrium will shift to the left (backward direction) to reduce the concentration of NH₄⁺ ions.

5. **Determine the Effect on OH⁻ Concentration:**
- As the equilibrium shifts to the left, the concentration of OH⁻ ions will decrease because the formation of NH₄OH will consume OH⁻ ions.

6. **Conclusion:**
- Therefore, when NH₄Cl is added to a solution of NH₄OH, the concentration of OH⁻ ions decreases.

### Final Answer:
The concentration of OH⁻ ions decreases.