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 Initial State**:
- The transition metal salt in aqueous solution is initially pink. This color is typically associated with a specific coordination complex of the metal ion, which we can denote as \( M(H_2O)_6^{2+} \).

2. **Understand the Effect of Concentrated HCl**:
- When concentrated hydrochloric acid (HCl) is added to the solution, it introduces chloride ions (Cl⁻) into the system. This can lead to a change in the coordination environment of the metal ion.

3. **Reaction with Chloride Ions**:
- The chloride ions can replace water molecules in the coordination sphere of the metal ion. The reaction can be represented as:
\[ M(H_2O)_6^{2+} + 4Cl^- \rightarrow MCl_4^{2-} + 6H_2O \]
- In this reaction, the coordination number of the metal ion changes from 6 (with water) to 4 (with chloride ions).

4. **Color Change**:
- The change in coordination number and the nature of the ligands (from water to chloride) results in a different electronic environment around the metal ion, leading to a change in the color of the solution. The new complex \( MCl_4^{2-} \) is blue.

5. **Conclusion**:
- The observed color change from pink to blue is due to the change in the coordination number of the metal ions from 6 to 4 and the formation of a new species in the solution.

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