Alkali metals dissolve in liquid `NH_(3)` then which of the following observations is not true:

Options

A. It becomes paramagnetic
B. Solution turns into blue due to solvated electrons
C. It becomes diamagnetic
D. Solution becomes conducting

Answer

To solve the problem regarding the behavior of alkali metals when dissolved in liquid ammonia (NH₃), we will analyze each observation provided in the options and identify which one is not true.

### Step-by-Step Solution:

1. **Understanding Alkali Metals in Liquid NH₃**:
- Alkali metals (Li, Na, K, Rb, Cs) are known to dissolve in liquid ammonia. When they dissolve, they form solvated metal ions and solvated electrons.

2. **Formation of Solvated Electrons**:
- When an alkali metal (M) dissolves in NH₃, it reacts to form a solvated metal ion (M⁺) and releases hydrogen gas (H₂). The reaction can be summarized as:
\[ M + NH₃ \rightarrow M^+ + NH₂^- + \frac{1}{2} H₂ \]
- The NH₂⁻ ion contains a lone pair of electrons from nitrogen, which contributes to the formation of solvated electrons.

3. **Color of the Solution**:
- The presence of solvated electrons gives the solution a characteristic blue color. This is a well-known observation when alkali metals are dissolved in liquid ammonia.

4. **Paramagnetism of the Solution**:
- The solvated electrons are free and unpaired, which makes the solution paramagnetic. Paramagnetic substances are attracted to magnetic fields due to the presence of unpaired electrons.

5. **Diamagnetism**:
- A substance is considered diamagnetic if all its electrons are paired. Since the solution contains unpaired solvated electrons, it cannot be diamagnetic.

6. **Conductivity of the Solution**:
- The presence of solvated ions (M⁺) and solvated electrons allows the solution to conduct electricity. Therefore, the solution is conducting.

7. **Identifying the Incorrect Observation**:
- Now, we analyze the observations:
- (a) It becomes paramagnetic - **True**
- (b) Solutions turn blue due to solvated electrons - **True**
- (c) It becomes diamagnetic - **Not True** (this is the incorrect observation)
- (d) Solution becomes conducting - **True**

### Conclusion:
The observation that is **not true** is that "it becomes diamagnetic." Therefore, the correct answer is option (c).