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I had a question asking about the colour of the solution of chromium(III) chloride in my exam, all my colleagues have different answers, I couldn't find any exact answer after searching!

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  • $\begingroup$ Related: How hydrate isomerism results difference in colours? $\endgroup$ – Loong May 10 '16 at 18:07
  • $\begingroup$ It depends on the amount of time since the solution has been created, the temperature, and possibly present excess chloride ions. $\endgroup$ – aventurin May 10 '16 at 18:21
  • $\begingroup$ wikipedia says it it dark green when exist as hydrate. $\endgroup$ – Nilay Ghosh May 11 '16 at 4:41
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The color of the solution may vary as long it is not in equilibrium.

According Bjerrum and Bjerrum1 a $\ce{1\mathrm{M}}$ chromium (III) chloride solution in equilibrium at 25° C contains about 13% monochloro and 85% hexaqua ions.

The absorption spectrum published shows minima at $\ce{480 \mathrm{nm}}$ (blue) for the hexaqua and $\ce{510 \mathrm{nm}}$ (green) for the monochloro complex.

          absorption minimum colors

When we integrate over the spectrum we get the following colors.

          colors integrated over the whole spectrum

However, it should be noted that the colors vary with spectral optical depth (concentration, thickness of the medium).

1 Bjerrum, M.J. and Bjerrum, J., 1990. Estimation of Small Stability Constants in Aqueous Solutions. The Chromium(III) Chloride System. Acta Chem. Scand. 44: 358-363.

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