 I just want to give you a little bit of background here, mainly because this is something that you'll be doing during class time, we'll be making and testing natural indicators and there's a number of different types. In fact it's likely that you've already in your previous science experience made a natural indicator, particularly something like red cabbage, it's such a good substance for using as an indicator. But what are indicators? Well, indicators are just plant dyes which change colour in the presence of an acid acid or base via an equilibrium reaction. So we can bring some of what we understood from the equilibrium topic into this topic to give us a little bit of an idea of what's happening with indicators. What we want to do now of course though in terms of indicators is see if we can explain exactly what's going on. So here's an example that we might use to help us to determine what's happening in terms of acid base indicators. So as with any equilibrium there has to be both a forward and a reverse reaction occurring and both occurring at the same rate with no macroscopic changes for equilibrium to be established. The other important thing about indicators is there should be some significant colour difference between the two different sides of the equilibrium and you will have noticed that in the last topic when we carried out a few different equilibrium shifts in order to see the equilibrium position changing from the left to the right and the colours changing as a consequence of that. So indicators do the same thing. What's important about an indicator though is that it's affected by acidical basic solutions. So we talked in the last video about the fact that acids are going to have these hydrogen ions in solution. Therefore they're going to increase the hydrogen ions. So if the concentration of the hydrogen ions is to increase, Alicia Tellier's principle tells us that the equilibrium will shift in such a way as to reverse or to counteract that change which means to try and bring the concentration of the hydrogen back down and therefore it does that by shifting in this case to the left which means we favour the solution that has a red appearance. Likewise if we add a base the reverse happens, let me put that in green, so if we add a base what often happens is that the hydrogen ions are accepted so if you think about a base like sodium hydroxide this is going to be neutralising the hydrogen ions and therefore their concentration will drop and so Alicia Tellier's principle says well what we want to do then is we want to increase that and then we're going to have to shift it to the right which means we're going to favour the blue solution. So this is how indicators work, there's a equilibrium that's established and then there's a shift that occurs as we add acid and base and of course we hopefully will be aware of the fact that if we continue to do this with the same solution add some acid, you'll turn it red, add some base, you'll turn it blue, add a little bit more acid, you can neutralise that base and eventually the acid concentration will be sufficient to push it back into the red region. So we definitely have an equilibrium happening here and we have the colours that are telling us where that equilibrium is and whether the solution is more acidic or more basic.