Question

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

• 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 1

Answer: (B)

change in the coordination number of the metal ion from 6 to 4 and formation of new species in solution

$\left[M\right(H_{2}O{)}_6{]}^{2+}$ is pink in colour.
When concentrated $HCl$ is added, a new species $[MC{l}_4{]}^{-}$ is formed.
The coordination number of the central metal ion changes from 6 to 4.
Hence, the colour changes to blue.