Which of the following will be the most hydrated salt ?
(1) NaCl (2) `MgCl_2` (3) `AlCl_3` (4) `SiCl_4`

Explanation

To determine which of the given salts is the most hydrated, we need to analyze the salts based on their charge density, which is defined as the charge of the ion divided by its radius. The higher the charge density, the more strongly the ions attract water molecules, leading to greater hydration.

### Step-by-step Solution:

1. **Understanding Hydration**:
- Hydration refers to the process where water molecules surround and interact with ions in a solution. The degree of hydration depends on the charge density of the cation.

2. **Identifying the Salts**:
- The salts given are:
1. NaCl (Sodium chloride)
2. MgCl₂ (Magnesium chloride)
3. AlCl₃ (Aluminum chloride)
4. SiCl₄ (Silicon tetrachloride)

3. **Determining the Charge of Each Cation**:
- Na⁺ has a charge of +1.
- Mg²⁺ has a charge of +2.
- Al³⁺ has a charge of +3.
- Si⁴⁺ has a charge of +4.

4. **Considering Atomic Radii**:
- As we move from Na⁺ to Si⁴⁺, the atomic size decreases due to increasing nuclear charge, which pulls the electrons closer to the nucleus.

5. **Calculating Charge Density**:
- Charge density is calculated as:
\[ \text{Charge Density} = \frac{\text{Charge}}{\text{Atomic Radius}} \]
- Higher charge and smaller radius result in higher charge density.

6. **Comparing Charge Densities**:
- NaCl (Na⁺): Low charge density due to +1 charge and larger radius.
- MgCl₂ (Mg²⁺): Moderate charge density due to +2 charge.
- AlCl₃ (Al³⁺): Higher charge density due to +3 charge.
- SiCl₄ (Si⁴⁺): Highest charge density due to +4 charge.

7. **Conclusion on Hydration**:
- Since lower charge density leads to greater hydration, NaCl will have the highest degree of hydration because it has the lowest charge density among the given salts.

### Final Answer:
The most hydrated salt among the given options is **NaCl**.