Ammonium hydroxide is a weak base because

Options

A. It has low vapour pressure
B. It is only slightly ionized
C. It is not a hydroxide of any metal
D. It has low density

Explanation

### Step-by-Step Text Solution

1. **Definition of Ammonium Hydroxide**: Ammonium hydroxide (NH₄OH) is a solution of ammonia (NH₃) in water. It is commonly referred to as ammonia water.

2. **Understanding Weak Bases**: A weak base is defined as a base that does not completely ionize in solution. This means that when a weak base is dissolved in water, only a fraction of the base molecules dissociate into ions.

3. **Ionization of Ammonium Hydroxide**: When ammonium hydroxide is dissolved in water, it partially ionizes to form ammonium ions (NH₄⁺) and hydroxide ions (OH⁻):
\[ NH₄OH \rightleftharpoons NH₄^+ + OH^- \]
This reaction indicates that not all NH₄OH molecules dissociate into ions.

4. **Degree of Ionization (α)**: The degree of ionization (α) is a measure of the extent to which a substance ionizes in solution. For ammonium hydroxide, α is between 0 and 1, indicating that it only partially ionizes.

5. **Conclusion**: Since ammonium hydroxide does not fully ionize in solution, it is classified as a weak base. If a base were to completely ionize (α = 1), it would be considered a strong base.

### Final Answer
Ammonium hydroxide is a weak base because it only partially ionizes in solution, meaning that it does not completely dissociate into ions when dissolved in water.

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