 The situation is different when we look at a couple of different types of curves and we'll just briefly touch on these because they're all things you're going to look at in a bit more detail in class. The problem that we have with a weak acid and a strong base is the weak acid is going to form a strong conjugate base, so therefore we're going to have a basic salt and we're also going to have water. So this time our salt is not neutral. It's actually a salt that is going to have a pH greater than 7. So two things to notice about this. First of all you've got this rapid initial rise which we didn't see previously and that's because the weak acid is not fully ionized and therefore the addition of the base is going to shift the equilibrium. It's going to use up all of the H plus ions and therefore shift the equilibrium in order to produce more H plus ions. As that process continues we'll eventually have as many H plus ions as there are from that weak acid and we're going to go through our equivalence point. So again we've got this vertical section which we saw in the previous graph but the vertical section is shorter and it's also not at the same equivalence point. You can see this time the equivalence point is actually in the basic range. So we've produced a basic salt as well as water. Hopefully you're seeing that the same sort of thing logic we can apply to the third situation and the third situation is one where we have a strong acid plus a weak base. The problem this time is the weak base is going to form a strong conjugate acid and therefore we're going to have an acidic salt plus water. As a result of this we're going to have our base pushing its way through very slowly reacting with the acid but then once we get to that equivalence point again you can see we get the vertical region of the graph but this time the equivalence point is actually at a pH which is below 7 because the salt that we're producing is an acidic salt. There's a lot more reasons why this stuff is happening and we need to go into it in a little bit more detail and you will and of course the other problem is if this is happening when we've previously chosen phenolphthalein as a good choice of base of indicator now we can't do that because phenolphthalein is going to change beyond where our graph is going to emerge from the equivalence point. So this time we don't use phenolphthalein usually we'll choose something different like methyl orange which changes quite low in the pH range so it'll allow us to see that change occurring. I haven't added the titration of the weak acid and the weak base because if you've got a weak acid and a weak base they're both shifting equilibrium and it takes weeks so we don't do it. Simply put we don't look at reactions between weak acids and weak bases so we'll put them to one side just focus on these other three look at some examples have a look at the application of the indicators and continue to refine your techniques. Good luck and thanks for watching.