The chloride of one of the following elements is a stable covalent compound in solid state but ionic in their solutions. It is

Options

A. B
B. Al
C. In
D. Tl

Answer

To solve the question, we need to identify which chloride of the given elements (boron, aluminium, indium, or thallium) is a stable covalent compound in the solid state but behaves as an ionic compound in solution.

### Step-by-Step Solution:

1. **Identify the Chlorides of Each Element**:
- Boron: BCl3 (Boron trichloride)
- Aluminium: AlCl3 (Aluminium chloride)
- Indium: InCl3 (Indium trichloride)
- Thallium: TlCl3 (Thallium trichloride)

2. **Analyze the Nature of Each Chloride**:
- **BCl3**: This is a covalent compound due to the small size and high electronegativity of boron.
- **AlCl3**: In the solid state, AlCl3 is primarily covalent due to the presence of covalent bonds between Al and Cl. However, when dissolved in water, it dissociates into Al³⁺ and Cl⁻ ions, exhibiting ionic behavior.
- **InCl3**: Similar to AlCl3, it is covalent in solid form but can also dissociate in solution.
- **TlCl3**: This compound also shows covalent characteristics in solid form but behaves more ionic in solution.

3. **Determine the Correct Answer**:
- Among the options, **AlCl3** is the most notable for being covalent in solid form and ionic in solution. The high charge density of Al³⁺ leads to polarization of the chloride ions, which contributes to its ionic character in solution.

4. **Conclusion**:
- Therefore, the chloride that is a stable covalent compound in solid state but ionic in solution is **Aluminium chloride (AlCl3)**.

### Final Answer:
The chloride of aluminium (AlCl3) is a stable covalent compound in solid state but ionic in their solutions.

---