Why do copper compounds come in different colours?

29 June 2008

Question

Can you tell me why, copper the metal is “copper in colour”, yet it is blue when in solution with sulphate, copper carbonate is colourless in solution, and when you flame test the element it is a green flame?

Answer

Dr Peter Wothers, Department of Chemistry, University of Cambridge:Metals in general reflect all of the light energy that comes on to them but copper doesn't reflect all of them. It absorbs part of the spectrum. It absorbs the bluey part of the light and maybe some of the green light and reflects all the coppery coloured light which comes back in to our eyes. That's what happens with the metal.

In compounds copper sulphate, the blue colour is due to the light energy being used to promote or excite electrons that are in the atom of the copper when it's combined with other things such as the sulphate or carbonate ions and so on. In solution what you actually have - in the same way when you dissolve salt in water you end up with sodium ions and chloride ions not bound together any longer as they are in the crystals but surrounded by water - the water interacts with the copper ions. The colour that you see isn't really copper sulphate, it's copper ions surrounded by lots of water.

Copper carbonate the solid doesn't have the same water there and this is usually a greenish colour. Incidentally the copper sulphate, the crystals itself are blue but that's because they also have water trapped in their crystals. If you heat them up and drive out the water they actually go white and colourless. It's the waters there that are interacting with the copper ions.

Finally the flame test, why does the element test produce a green flame? This again is energy being used to excite the electrons in the atoms or ions. When this energy is returned, is given out again as the electrons fall back down to their low energy levels it gives out only part of the spectrum. It gives out green light.

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