An aqueous solution of cobalt(II) chloride is pink due to the presence of octahedral [Co(H2O)6]2+[\text{Co}(\text{H}_2\text{O})_6]^{2+}[Co(H2O)6]2+ ions. When an excess of concentrated hydrochloric acid is added, the solution turns deep blue.
Which statement correctly describes the type of reaction and the change in coordination number of the cobalt ion during this process?
A ligand substitution reaction occurs, and the coordination number decreases from 6 to 4.
A ligand substitution reaction occurs, and the coordination number remains 6.
A redox reaction occurs, and the coordination number decreases from 6 to 4.
A deprotonation reaction occurs, and the coordination number remains 6.
63 exam-style questions on AQA A Level Chemistry Transition metals (A-level only), covering General properties of transition metals (A-level only), Substitution reactions (A-level only), Shapes of complex ions (A-level only), Formation of coloured ions (A-level only), Variable oxidation states (A-level only), and Catalysts (A-level only). Each one has a worked solution and a mark scheme showing where the marks go.