Solution A contains the compound [Cu(H₂O)₄]Cl₂ - AQA - A-Level Chemistry - Question 7 - 2017 - Paper 1

The chloride ions in this compound are not considered to be ligands because Cl⁻ does not donate a lone pair of electrons to the copper ion (Cu²⁺) and is instead present in the compound as a counterion.

The ionic equation for the reaction is:

$[Cu(H₂O)₄]^{2+} + 4 NH₃ \rightarrow [Cu(NH₃)₄]^{2+} + 4 H₂O$

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

The blue-green solid C is identified as CuCO₃, or copper carbonate.

Reagent D is hydrochloric acid (HCl). The ionic equation for the reaction is:

$Cu²⁺ + 2 Cl⁻ \rightarrow [CuCl₂]$

Colorimetry cannot be used to determine the concentration of solutions containing [CuCl₂] because copper ions in this form do not absorb frequencies of visible light. This is due to the electron configuration of the metal ion, which does not allow for transitions that would lead to visible absorption.