This Cuprous Chloride(WSDTY) can be explained by looking at the spectrochemical series of metal ligands. These ligands are anions or groups that can be attached to a metal cation either in solution or in a solid compound. As we move from left to right across the series the ligands become stronger and the colors that they produce when bonded to metals tends more towards the violet end of the color spectrum (shorter wavelengths - remember the visible spectrum goes like this: red, orange, yellow, green, blue, violet). As we go to ligands on the left side of the series the colors produced by the compounds tends more towards the red end of the color spectrum (longer wavelengths).
So, if we have a sample of anhydrous (completely dry) copper (II) chloride it will be orange-brown in color. This is because Cl is a relatively weak field ligand (towards the left side of the spectrochemical series), and so we get that orange/red/brown color. Now, if the anhydrous copper (II) chloride gets a little wet it will start to pull water into the crystal and eventually form copper (II) chloride dihydrate CuCl2(H2O)2, which has a blue-green or turquoise color. Why is this? Because water has bonded to the metal cation (becoming a ligand) and water is a stronger ligand so the color has shifted towards the violet end of the spectrum. If the compound is dissolved in solution then the copper can be completely surrounded by water giving the cation, Cu(H2O)6++ which is even more blue (and therefore giving a blue solution).
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