Why is copper(II) sulphate solution blue?
If white light (ordinary sunlight, for example) passes through copper(II) sulphate solution, some wavelengths in the light are absorbed by the solution. Copper(II) ions in solution absorb light in the red region of the spectrum. The light which passes through the solution and out the other side will have all the colours in it except for the red. We see this mixture of wavelengths as pale blue (cyan). The diagram gives an impression of what happens if you pass white light through copper(II) sulphate solution. Working out what colour you will see isn’t easy if you try to do it by imagining “mixing up” the remaining colours. You wouldn’t have thought that all the other colours apart from some red would look cyan, for example. Sometimes what you actually see is quite unexpected. Mixing different wavelengths of light doesn’t give you the same result as mixing paints or other pigments. You can, however, sometimes get some estimate of the colour you would see using the idea of complementary c
