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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
    Commented Jan 6, 2020 at 9:56

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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
    Commented Jan 6, 2020 at 3:55
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    $\begingroup$ I edited the answer. See the last paragraph. $\endgroup$
    – ACR
    Commented Jan 6, 2020 at 4:14
  • $\begingroup$ Cu (II) is only slightly blue in solution, except if you have ammonia in there at pH>8. $\endgroup$
    – Karl
    Commented Jan 6, 2020 at 5:49
  • $\begingroup$ commons.wikimedia.org/wiki/File:Kupfer(II)-Ionen1.jpg $\endgroup$
    – Karl
    Commented Jan 6, 2020 at 5:57
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    $\begingroup$ I disagree that this is mostly a memorization question. These types of questions are based on the (admittedly simplistic) assumption that color comes from $d \rightarrow d$ transitions in transition metal complexes. That means $d^0$ and $d^{10}$ central metal atoms have colorless complexes, while others are colored. Permanganate and dichromate are the big exceptions ($d^0$ but colored) that you have to memorize, but otherwise, the intent is that students will count $d$ electrons to figure out the answer here. $\endgroup$
    – Andrew
    Commented Jan 6, 2020 at 23:16

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