[WilliamYWT]

In organic solvent, iodine appears purple, in water, iodine appears brown, if I add a spatula of iodine powder to 10mL 75% ethanol, what is its colour, and why? Many thanks!

[Archer]

Iodine is not very soluble in water, iodine solution has another component in it.

Do you know what this is and what it forms with I₂ to make it water soluble?

[WilliamYWT]

Um...This is exactly one of the question I want to ask, I know the component is ionic, but I searched through Google and still haven't found anything promising.

[Archer]

Google search for iodine solution

 http://en.m.wikipedia.org/wiki/Lugol's_iodine

This will get you started for aqueous solutions.

Do you understand why iodine in chloroform is purple, i.e. what interactions with visible light and I₂ are taking place?

[WilliamYWT]

Thanks for the aqueous one. For that in organic solvent, I know it's about electrons jumping between electron shells and releasing photons

[magician4]

in addition:

though KI₃ sporting the I₃^- anion is most often named when the solubility of iodine in KI-solutions is discussed, this by no means is the only species involved here.
In general, a bunch of anions [I_2n+1]^- is observed, and at times quite long chains are formed  (you might compare this to polysulfide-dianions)

the longer the linear part of the chain, the darker and the more intense the colour becomes, and with forced "long" linearity (like, for example, when incorporated to starch helices) even minor traces of polyiodine-anions (hence, indirectly, iodine) might become very intense, and hence detected.
(that's why we use starch in iodometry)

with no KI around, the colour results from (in neutral environment: weak) autodisproportionation of iodine in water: I₂ + H₂O  HIO + HI  HIO + I^- + H⁺
(with further processes like 5 HIO  2 I₂ + 2 H₂O + HIO₃ )
the iodine thus formed again will be involved with minor polyiodid-anion formation

that's why "solutions" of iodine in water appear to be slightly brownish-yellow

regards

Ingo

[WilliamYWT]

Thanks! I believe I receive much more education here than in school. You people are utterly amazing!

[magician4]

erraturm:

the iodine thus formed again will be involved with minor polyiodid-anion formation

I just realized that I misspelled badly here: of course, I meant iodide I^-

sorry for any confusion this might have caused

mea culpa

Ingo