 When you look at the periodic table in group 17, you see fluorine, chlorine, bromine, and iodine. And what we want to look at is, what is the periodic trend of activity? In other words, which is more reactive? What we're going to do is we're going to compare the chemistry of what will replace what by putting these compounds in a sequence of solutions. In each of the test tubes, we have cyclohexane. And in the first one, I'm going to add potassium fluoride. And you can see that we have two levels in our test tube. The solution of potassium fluoride is in the bottom and the cyclohexane is at the top. In the second one, we're going to put the chloride ion. You see the same thing happen. And bromide ion. And in this one, we're going to put an iodine ion. Now we have fluorine, chlorine, bromine, and iodine. And what I'm going to do is to add a solution of bleach. This is household bleach. And there's chlorine gas in here, and it will come out of solution. So what we want to do is to see if chlorine will replace the fluorine, bromine, or iodine. From this we can see that chlorine does not replace the fluoride, but chlorine does replace the bromide and the iodide. This time we're going to add just a little bit of bromine in a water solution. So we have bromine as a compound. What we see here is that the bromine did not replace the fluoride. The chloride or the bromide, but it did push the iodide out of solution. And so you can see iodine down in the bottom.