[Window_Knight]

So in our science class we did a lab where we took a 1.0M solution of copper (II) nitrate and mixed it with a 1.5M solution of sodium hydroxide. The balanced equation is Cu(NO₃)₂ + 2Noah = 2NaNO₃ + Cu(OH)₂. Then we took that and heated it in a hot water bath until no more change occurred and the copper (II) hydroxide decomposes into Cu(OH)₂ = CuO + H₂O. We then removed it from the hot water bath and added a 1.5M solution of hydrochloric acid and it reacts with the copper (II) oxide to form 2HCl + CuO = H₂O + CuCl₂. This was officially the end of the experiment but we accidentally heated the solution and it changed from a blue to a green. We asked the teacher and replicated results on multiple solutions and the teacher didn't know why this happened. I was reading in other places that the aqueous solution of CuCl₂ turned green and yellow if there were additional Cl^- atoms. The ratio was supposed to be 1:1:1 on all 3 solutions so i was wondering if this was a measuring error or if I was off on my equations. Not really as important as intriguing, I was just wondering if anyone had any insight as to why this was happening.
Thanks ahead of time,
                     an interested student.
p.s. If there is something I can do to improve my question, please let me know so that I can do that in the future.

[Hunter2]

You are right. The blue color of the copper solution is coming from [Cu(H₂O)₄]²⁺. It turns to green if chloride is present and some of the water is exchanged. [CuCl₄]^2-

[Borek]

In other words: copper in water is already complexed, with water molecules acting as ligands (ligand is an ion, or molecule, that attaches to the central ion in compounds known as coordination complexes). These ligands can be replaced by some other ligands, and the new compound will usually have a different color. In your case water molecules are replaced by Cl^- anions.

[Window_Knight]

So to put this in extremly simple english that my class would understand (they are not the sharpest bunch) there are extra Cl atoms in th solution that enter the CuCl₂ when it is heated to make CuCl₄ and turns the solution green?

[Borek]

Close. Solution contained excess HCl, so there were plenty of free chloride anions Cl^- around. Note - these were not atoms of Cl. Same element, but different form.

[Corribus]

By the way, the reason the color changes is because the ligands surrounding the metal affect the energy spacing of the metal's d-orbitals.  The color of copper (and many transition metal) salts arises from absorption of light.  Electrons in lower energy d-orbitals absorb light and are excited into higher energy (empty) d-orbitals, and the frequency (color) of light absorbed is dependent on how far apart these orbitals are in energy.  When water ligands are replaced by chloride ligands, the energy levels of the d orbitals change because of the electric fields created by their electron rich character.  This impacts the color of light absorbed and hence results in a change of their apparent color that you observe in solution.