Vivienne Bradtke, Adelaide, Australia asked:
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?
Dr Peter Wothers, Department of Chemistry, University of Cambridge:
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.
The colours of materials come from the different optical frequency bands that they absorb or reflect. The same is true for ions in solution.
Hmm, but not all chemicals are as colourful as copper, and I think that's what the question is getting at.
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. Soul Surfer, Mon, 30th Jun 2008
Chromium forms compounds with so many different colours that the metal is named after this property.