Why when you add chlorine to a solution containing bromide ions does it change colour?
That depends if the solvent is a protic solvent like water so it may ionize or a aprotic solvent where it may not ionize like carbon tetrachloride. If you look at a periodic chart of the elements and look horizontally at the VII Family you will see F then Cl then Br then I then At in descending order. Any element above another element in the same Family will displace the element beneath it because it is more electro negative or electro positive as in the case of the I Family of alkaline metals. So if the solution is a protic solvent and the Bromide ions have ionized then after the Chlorine gas dissolves in that liquid and ionizes with the water it will begin to displace the Bromide ion in solution and the Bromine will precipitate out of solution as a free element.
