Naked Science Forum
General Science => Question of the Week => Topic started by: thedoc on 24/06/2008 18:40:16
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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?
Asked by Vivienne Bradtke, Adelaide, Australia
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Dr Peter Wothers, Department of Chemistry, University of Cambridge:
[img float=right]http://www.thenakedscientists.com/HTML/uploads/RTEmagicC_585px-Copper_sulfate.jpg.jpg[/img]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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The colours of materials come from the different optical frequency bands that they absorb or reflect. The same is true for ions in solution.
The neutral element and various compounds (some if which may involve ionic bonds) and different ions in solutions all have different absorption characteristics so there is no reason why they should not have different colours.
There are sometimes however complementary relationships between colours of material for example gold is yellow because it reflects more yellow light but gold can be hammered so thin that it becomes translucent and looking through very thin gold the colour is blue because the yellow light is being reflected.
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Hmm, but not all chemicals are as colourful as copper, and I think that's what the question is getting at.
Chris
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Not all chemicals have interactions that fall in the optical frequency bands many of them have interations that fall inot the infra red and ultra vioplet frequencies which offer far more bandwidth than the optical bands. What are called "transition elements" on which the only changes between their neihbours are more likely to have changes at optical frequencies so many compounds and salts involving transition elements are strongly coloured. Transition elements also tend to have several diffferent ionisation states available to them and so exhibit quite a complex chemistry with the same element being responsible for several different colours of ions in solution.
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Chromium forms compounds with so many different colours that the metal is named after this property.
CrO3 is red, Na2Cr2O7 is orange, NaCrO4 is yellow, Cr2O3 is green, CrCl2 (at least in soliution in water) is blue, (indigo isn't a real colour IMO so I'm missing it out of this rainbow), and mixed crystals of alum and chrome alum are violet.
It forms a black oxide (CrO2) and some brown mixed- oxidation state complexes.
About the only colour I can think of that you don't get is white.