The aqueous solution of transition metal salt changes colour from pink to blue, when concentrated hydrochloric acid is added to it. The changes in colour is due to:

Options

A. evolution of hydrogen that changes the oxidation state of the metal ion
B. change in the coordination number of the metal ion from 6 to 4 and formation of new species in solution
C. formation of a coordination complex of the metal ion with hydrochloric acid
D. protonation of the metal ion

Answer

To solve the question regarding the color change of a transition metal salt solution from pink to blue upon the addition of concentrated hydrochloric acid, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Transition Metal Salt**:
- The transition metal salt in question is likely cobalt(II) chloride hexahydrate, which is represented as CoCl₂·6H₂O. This compound is pink in color.

2. **Understand the Reaction with Hydrochloric Acid**:
- When concentrated hydrochloric acid (HCl) is added to the cobalt(II) chloride solution, it leads to a change in the coordination environment of the cobalt ion.

3. **Determine the Coordination Number**:
- In the initial state (before adding HCl), cobalt is in an octahedral complex with a coordination number of 6 (due to the six water molecules).
- Upon the addition of concentrated HCl, the coordination number changes from 6 to 4, forming a new complex, CoCl₄²⁻, which is tetrahedral.

4. **Analyze the Color Change**:
- The octahedral complex (Co(H₂O)₆²⁺) is pink, while the tetrahedral complex (CoCl₄²⁻) is deep blue. This significant change in color is attributed to the different geometries and the resulting electronic transitions.

5. **Conclusion**:
- The color change from pink to blue is due to the change in the coordination number of the cobalt ion from 6 to 4 and the formation of a new species (CoCl₄²⁻) in the solution.

### Final Answer:
The changes in color are due to the change in the coordination number of the metal ion from 6 to 4 and the formation of a new species in the solution.

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