 We've already had a look at some of the different ways in which different substances behave, how we classify them on the basis of their ability to donate or accept a proton, or indeed their ability to do both, but there's another way that we describe acids and bases, and that's on the basis of their strength or concentration. It's very important that you don't mix these two terms up, and we have some common phrases that we use that confuse terms like strong and concentrated. So let's just make sure that we're really clear about the difference between each of these. What we need to do is look at the fact that strength is a measure of the ionization of a particular substance, or if it's a base, often its ability to dissociate into ions. On the other hand, concentration is simply a ratio of the solute to solvent. So how much acid, for example, there might be in relation to how much water. The diagram from one of the texts shows you how we can have all four of these different descriptors when we describe different types of acid solutions. So you can see the first picture is about having something which is both strong and concentrated. The second picture is a dilute, strong but dilute solution. The third one is a weak but concentrated, and the final one is a weak and dilute. So these are the ways that we would describe each of these, but how do we work that out? How do we know what's what? Well, let's look at each of these concepts in just a little bit more detail on its own. So the first thing we need to do is look at acid strength. So strength is equivalent to the degree of ionization. If you like to look at it a different way, it's the ratio of molecules to ions. Therefore, if we think about it as the ratio of molecules to ions, then if the ratio of molecules to ions is low, so few to many, then we have a strong solution. If it is many to few, then we have a weak solution. So let's look at the two diagrams from the previous slide, blowing up a little to help us. So in this first one, you can see this is a strong solution, and that's because we have no molecules. There's no molecular forms here. They are all ions, okay, positive and negatively charged particles in the solution. So they're all ions. So this is completely ionized. All of the molecules have been ionized often through the interaction with water in order to form a strong solution. On the other hand, we have a weak solution over here. You can see there are lots and lots of molecules. These molecules have not ionized, and we have only few ions. So where we have a lot of molecules and not many ions, the ratio of molecules to ions is high, lots of molecule forms and very few ion forms, and as a result we have a weak solution. So strength is related to degree of ionization. Do we have lots of molecules or do we have lots of ions? Lots of ions equals strong.