Colour of cobalt chloride solution is:

Options

A. pink
B. black
C. colourless
D. green

Explanation

### Step-by-Step Solution:

1. **Identify the Compound**: The compound in question is cobalt chloride, which is represented as CoCl₂.

2. **Determine the Oxidation State of Cobalt**: In cobalt chloride, cobalt is in the +2 oxidation state (Co²⁺).

3. **Electron Configuration**: The electron configuration of cobalt in the +2 oxidation state is [Ar] 3d⁷. This means that cobalt has 7 electrons in its d-orbitals.

4. **Understanding d-Orbital Splitting**: In a complex, the d-orbitals split into two energy levels: the lower energy d₂g and the higher energy eₕ. For Co²⁺ with 3d⁷ configuration, the filling of d-orbitals will be as follows:
- d₂g: 3 electrons (each orbital singly occupied first)
- eₕ: 4 electrons (the remaining 4 electrons will occupy the higher energy orbitals)

5. **Color Due to d-d Transitions**: The presence of unpaired electrons in the d-orbitals allows for d-d transitions. When light hits the cobalt chloride solution, some wavelengths are absorbed, and the color observed is the complementary color of the absorbed light.

6. **Hydration State**: Cobalt chloride can exist in two forms:
- Anhydrous CoCl₂: This form is blue in color.
- Hydrated CoCl₂·6H₂O: This form is pink in color.

7. **Conclusion**: Since the question refers to cobalt chloride solution, which is typically hydrated in solution, the color of cobalt chloride solution is pink.

### Final Answer:
The color of cobalt chloride solution is pink.

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