While going through my AP chem prep book again, I came across questions asking about the color of certain aqueous solutions. For example:

Which of the following substances will produce a colorless aqueous solution?

A) $\ce{Zn2(NO3)2}$
B) $\ce{Cu(SO4)}$
C) $\ce{K2Cr2O7}$
D) $\ce{Co(NO3)2}$

I know that zinc compounds are generally colorless, so I picked that (and it was correct), but I have no idea how to predict the colors of the rest of the compounds.

So, to answer this question (and questions similar to this), would I need to use pure memorization of the common elements that produce color?

If so, where can I find information about these compounds and the color they produce?

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    $\begingroup$ The question is probably trying to tell whether you know the difference between a transition metal and other metal ions. Knowing whether there is a colour is the point not knowing what the colour is. $\endgroup$ – matt_black Jan 6 at 9:56

There is no simple way to predict beforehand as far as I know. This is a general knowledge type of chemistry question, you will have to know or memorize the colors of common anions and cations. It will be much easier, if you do so group wise and know the colors of the first row of transition element ions.

Any object which absorbs a certain portion of visible light's spectrum, will appear colored to us. For colored solutions, we will always consider absorption of light in the visible region. So if you see a green solution, it is not reflecting green light, it is absorbing a certain portion of the visible spectrum and the transmitted light [from the solution] appears green to us. Keep this concept in mind because a lot of students have this misconception.

You should know that sulfate ion is colorless but copper (II) ion is blue so most likely copper sulfate solution is blue. In part c, you should know the color of potassium ion and dichromate ion and then guess the color. Do the remaining exercise for others.

It will be very difficult to predict if both ions are colored because it is not like that we are mixing two paints, a yellow paint with blue paint will show a green color. You will have to study the absorption spectrum of that salt. The theory of color of ions and that of solids is quite complex. It was developed by physicists. You can start from crystal field theory if you wish to see how the color of transition metal ions is explained but I will not recommend that you explore it if you are in school.

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  • $\begingroup$ What about when sometimes its noted that compounds "vary in color". How does this happen if each ion has a set color? Also, it is always an equal distribution between the ions? For example, if you had a blue ion and a red ion in a compound - would it always produce an even purple? $\endgroup$ – TheGodlyBeast Jan 6 at 3:55
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    $\begingroup$ I edited the answer. See the last paragraph. $\endgroup$ – M. Farooq Jan 6 at 4:14
  • $\begingroup$ Cu (II) is only slightly blue in solution, except if you have ammonia in there at pH>8. $\endgroup$ – Karl Jan 6 at 5:49
  • $\begingroup$ commons.wikimedia.org/wiki/File:Kupfer(II)-Ionen1.jpg $\endgroup$ – Karl Jan 6 at 5:57
  • $\begingroup$ @Karl, It is funny that you would raise this point to split hairs because we can never define what $slightly$ means spectroscopically. Do you recall dissolving copper nitrate in water in high concentration? It is such a beautiful deep blue solution but I would never call it slightly blue. $\endgroup$ – M. Farooq Jan 6 at 5:58

You have to memorize the colours for insoluble compound. For soluble compounds, after dissolution, ions form aquated complex with water. In complexes, if CFSE value of a complex is zero, then it will be colourless(except a few compounds,where colour arises due to ligand to metal charge transfer LMCT). (For transition elements) If the ion forms d0 or d10 outermost electronic configuration, then it is always colourless. 3d5 configuration for Weak Field Ligand(O or Halogen donor site) in octahedral complexes are colourless. The 3d ions colours are given in the table (aquated complex) *Mn+2 also shows light pink colour,almost colourless and Cr+3 mostly shows green colour except the hydrated complex.

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  • $\begingroup$ Sorry, "You have to memorize the colors for an insoluble compound. For soluble compounds, after dissolution, ions form aquated complex with water." this is all wrong and not true in general. $\endgroup$ – M. Farooq Jan 6 at 14:35

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