Group 7: Halogens
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What colours are solutions of halogens in water and solutions of halogens in hydrocarbon solvents, and why are they that colour?
Ulex says
Chlorine in water is a very pale green – this is the colour due to chlorine molecules, Cl2, and is similar to the colour of the gaseous element itself. In hydrocarbon solvents the colour is also green.
Bromine in water varies in colour from yellow (dilute) to orange (concentrated). The colour is similar in hydrocarbon solvents. The colour is that of the molecules, Br2.
Iodine in hydrocarbon solvents is purple, as in iodine vapour, the colour of the molecules, I2. Iodine is practically insoluble in water. To get an aqueous solution, it is dissolved in potassium iodide solution. It reacts reversibly with this to give the complex ion I3-. This gives the solution a yellow colour (dilute) or red (concentrated).
There are two complications with all the foregoing remarks. The first is that the halogens react with water reversibly to give halide and halate(I) ions, X- (aq) and XO-(aq). The second is that the halogens react with hydrocarbons in slow substitution reactions so that the colours fade gradually.
Fluorine reacts violently with almost anything you try to dissolve it in!
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updated: 30 March 2006
