Anhydrous cobalt(II) chloride is blue in blue in colour but on dissolving in water it changes to pink in colour because

Options

A. its oxidation state changes
B. Its magnetic character changes
C. its coordination number changes
D. in water it shows fluorescence

Explanation

To understand why anhydrous cobalt(II) chloride changes color from blue to pink upon dissolving in water, we can break down the explanation step by step.

### Step-by-Step Solution:

1. **Identify the Anhydrous Form**:
- Anhydrous cobalt(II) chloride is represented as CoCl₂. In its anhydrous form, it has a tetrahedral geometry and appears blue in color.

2. **Dissolution in Water**:
- When anhydrous cobalt(II) chloride is dissolved in water, it undergoes a hydration process. The cobalt ions interact with water molecules.

3. **Formation of Hydrated Cobalt(II) Chloride**:
- The reaction can be represented as:
\[ \text{CoCl}_2 + 6\text{H}_2\text{O} \rightarrow \text{[Co(H}_2\text{O)}_6]^{2+} + 2\text{Cl}^- \]
- In this process, cobalt(II) chloride forms a complex ion, [Co(H₂O)₆]²⁺, which has an octahedral geometry.

4. **Change in Coordination Number**:
- The coordination number of cobalt changes from 4 in the tetrahedral anhydrous form to 6 in the octahedral hydrated form. This change in coordination number affects the electronic environment around the cobalt ion.

5. **Color Change**:
- The change in geometry and coordination number alters the d-orbital splitting of the cobalt ion. As a result, the color observed changes from blue (in the tetrahedral form) to pink (in the octahedral form).

6. **Conclusion**:
- Thus, the color change from blue to pink upon dissolving anhydrous cobalt(II) chloride in water is primarily due to the change in coordination number and the resulting change in the geometry of the cobalt complex.

### Final Answer:
The color change from blue to pink is due to the change in coordination number from 4 to 6 when cobalt(II) chloride is hydrated.

---