Solution A contains the compound [Cu(H2O)6]Cl2 - AQA - A-Level Chemistry - Question 7 - 2017 - Paper 1

The chloride ions (Cl-) do not donate a lone pair of electrons to the central copper ion, which is required for a species to be classified as a ligand.

Equation:

$[Cu(H2O)6]^{2+} + 4NH_3 \rightarrow [Cu(NH_3)_{4}]^{2+} + 6H_2O$

Colour: The colour of solution B is deep blue or royal blue.

The blue-green solid C is copper carbonate (CuCO3).

Identify of reagent D: Hydrochloric acid (HCl).

Equation:

$[Cu(H2O)_{6}]^{2+} + 4Cl^{-} \rightarrow [CuCl_4]^{2-} + 6H_2O$

Colorimetry cannot be used because the color of the solution does not correlate linearly with concentration due to the presence of chloride ions. The electron configuration of the metal ion leads to light absorption characteristics that prevent accurate colorimetric analysis.