[sharbeldam]

So there is this Le Chatelier Principle lab and the students need to test the color of ions in a solution, (K+ is what color, Cl- is what color) using solutions of KCl, FeCl3, FeSCN+2

so I understand that you can tell the color of the solution itself, but how can you tell the color of a specific ion, like let's say we had a solution of XY (ionic compound) X+ has a specific color and Y- has a specific color, how can I tell the color of X+ alone?

Thanks for the help in advance.

[Corribus]

Usually only one of the ions is colored (absorbs visible light). There are a few exceptions, but it's almost always the metal cation, particularly if the metal is a transition metal.

[sharbeldam]

Oh that explains alot!
but what about for instance FeSCN+2
the Fe is yellow and non metal SCN is colorless, why is the complex red and not yellow?

say i didnt know Fe+3 was yellow, how would i know its yellow from the dark red Fe(SCN)+2 complex?

[AWK]

The iron(III)-thiocyanate ion complex has a very characteristic intense blood-red color.

[sharbeldam]

Yea but i meant how would I know Fe+3 was yellow :/ ?if the solution is dark red.

[AWK]

I wrote the "complex ion" correctly - Iron(III) forms covalent bonds with thiocyanate ions from [FeSCN]²⁺ up to [Fe(SCN)₅·H2O]^2- depending on the concentration of thiocyanate ion.
The yellow-brown color of the iron(III) salt is due to the hydrolysis of the salt. In the qualitative analysis laboratory, the solution of iron(III) nitrate is slightly acidified with nitric acid and then the salt solution is colorless.

[sharbeldam]

So realistically speaking, there is no way in jupiter the student can know that Fe+3 ion is yellow, if they have Fe(SCN)+2 complex solution since it's somehow an exception to what the first chemist said (in this thread)

BUT if i had FeCl3 and it was yellow, then i can know that the yellow color comes from Fe+3 ions

[AWK]

[Fe(SCN)]²⁺ is not Fe³⁺ because of the covalent bond.

[sharbeldam]

Ohh. Got it now! Thank you so much both.

[Corribus]

For the most part, this is qualitative analysis and you just have to know what ions are which colors. It's just something you memorize or look up. Of course there are reasons for the colors, but you don't really need to know them to do the analysis.

You would run into problems when cations and anions are different colors in the same solution, or if one cation is changing color during a reaction. In those cases you'd use more analytical means to measure what the colors are. But if this is a basic introductory lab, it's usually set up to avoid those kind of complexities.