CAIE A Level Chemistry 9701 · Chemistry of transition elements
An aqueous solution of cobalt(II) chloride is pink. When concentrated hydrochloric acid is added, the solution turns blue. The blue species is the tetrahedral complex ion .
Explain, in terms of the splitting of d orbitals and the absorption of light, why the colour changes from pink to blue when the ligand water is replaced by chloride ions.
7 marks, so the mark scheme is looking for about 7 separate creditworthy points. Making the same point twice in different words earns it once.
Explain means: Give the reasons or the mechanism: why or how something happens. Link each point to the next with because, so or which means. A list of facts with no links usually loses the reasoning marks.
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