Displacement reactions of the halogens

In this experiment we see some examples of displacement reactions. The experiment uses water as a control. The purpose of the water is to show the effect of dilution. If the brown iodine solution is added to a colourless solution, with which it does not react, it will become paler in colour. This is because the same amount of coloured iodine is now spread out over a larger volume. Students often mistake this for a chemical reaction, saying that as the solution is paler there must be less iodine there. Look at the following video and try to decide whether a chemical reaction has taken place or whether any change can be explained solely by dilution. We use hydrocarbon solvent to distinguish between any iodine or bromine that might be produced.

Video - displacement reactions of the halogens

Try to write the equations for all of the reactions you have seen and then check them here. In the last experiment where iodine is added to each test tube, you should see that the colour of the iodine after hydrocarbon solvent is added is a much darker brown in the test tube on the right. Is this just poor experimentation or is there a chemical explanation? Check your answer here.