![]() ![]() Blue and yellow are complementary colours red and cyan are complementary and so are green and magenta. An internet search will throw up many different versions!Ĭolours directly opposite each other on the colour wheel are said to be complementary colours. The diagram shows one possible version of this. If you arrange some colours in a circle, you get a "colour wheel". You can, however, sometimes get some estimate of the colour you would see using the idea of complementary colours. Mixing different wavelengths of light doesn't give you the same result as mixing paints or other pigments. Sometimes what you actually see is quite unexpected. You wouldn't have thought that all the other colours apart from some red would look cyan, for example. Working out what colour you will see isn't easy if you try to do it by imagining "mixing up" the remaining colours. ![]() The diagram gives an impression of what happens if you pass white light through copper(II) sulphate solution. We see this mixture of wavelengths as pale blue (cyan). The light which passes through the solution and out the other side will have all the colours in it except for the red. Copper(II) ions in solution absorb light in the red region of the spectrum. If white light (ordinary sunlight, for example) passes through copper(II) sulphate solution, some wavelengths in the light are absorbed by the solution. Why is copper(II) sulphate solution blue? Anyone choosing to use this spectrum as anything more than an illustration should be aware that it lacks any pretence of accuracy! The colours are only an approximation, and so are the wavelengths assigned to them. Important: This isn't a real spectrum - it's a made-up drawing. The diagram shows an approximation to the spectrum of visible light. Visible light has wavelengths from about 400 to 750 nm. Visible light is simply a small part of an electromagnetic spectrum most of which we can't see - gamma rays, X-rays, infra-red, radio waves and so on.Įach of these has a particular wavelength, ranging from 10 -16 metres for gamma rays to several hundred metres for radio waves. You will know, of course, that if you pass white light through a prism it splits into all the colours of the rainbow. Why do we see some compounds as being coloured? Be aware that this is only an introduction to what can grow into an extremely complicated topic. The color of these ions is pH dependent, as indicated by the color changes when the above reactions take place.This page is going to take a simple look at the origin of colour in complex ions - in particular, why so many transition metal ions are coloured. If a shift in pH causes the solution to become more acidic (i.e. The equilibrium equation can be represented byīy Le Chatelier’s Principle, if certain conditions (concentration, temperature, pressure, volume, etc.) are changed, the amount of each ion present in solution is affected. I will try to take a different approach to your question (in terms of acidity and basicity).Ĭhromate (yellow) and dichromate (orange) ion are at equilibrium in solution. The complementary color of blue is red slash orange, and that is in fact the color we see in the dichromate ion!Īt the heart of all this is the principle that the colors we see are those wavelengths of light which on average are not absorbed by a large number (on the order of Avogadro's number) of molecules.Īn approach like this will only be reliable for very similar molecules like the two we have here. That means, if one of the bonds in the chromate ion, and thus two of the bonds in the dichromate ion, were absorbing a longer wavelength like we said earlier, on average we would expect something just longer than purple-ish, like blue, to be absorbed. Thus, in the case of the chromate ion, we see yellow, and across from yellow is the purple-ish region. Now to the color wheel! It is a general chemistry (often unexplained) fact that the color we see is the complementary color of the wavelength of a bond's vibration. This means that bond will vibrate at a lower frequency, and because frequency and wavelength are inversely related, that bond will absorb a longer wavelength of light. ![]() If you look at the structure of the chromate and dichromate ions next to each other (see here for structures: ), the only major difference between the two is that the Cr-O bond joining the two chromate ions (missing an oxygen) is now a single bond. I'm really excited for this because I get to reference the almighty color wheel!! Fair warning, this answer is much more qualitative than quantitative, but that's more interesting sometimes anyways. ![]()
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