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Evaporating cold aqueous chromium(III) chloride produces violet crystals. However when hot aqueous chromium(III) chloride crystallizes, green crystals result.

Could someone please explain the chemistry behind this... I mean both the crystals contain $Cr^{3+}$, so the difference in colour cannot be due to two different oxidation states. Also, they contain the same ligand $Cl^-$. Can someone please tell me where I am going wrong?

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Well, actually when we evaporate aqueous chromium(III) chloride, the hydrate of the crystal will be gone, so we get anhydrous chromium(III) chloride ($\ce {CrCl3}$). This type of anhydrous produce violet color.

But if we crystallizes aqueous of the chromium(III) chloride, the hydrate won't go, so we get hydrous chromium(III) chloride (usually hexahydrate, like $\ce {CrCl3.6H2O}$). This crystal produce dark green color.

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  • $\begingroup$ lambda23: could it also be due to the $Cl-$ ion acting as both a ligand and an anion? $\endgroup$
    – Eliza
    Apr 2, 2014 at 9:19

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