 It's important that you have just a bit of an overview of how ideas change in science. This is a kind of nice little Venn diagram because most of what has happened in the changing of our models as our understanding of acids and bases has grown is that we've realized that there are certain substances that behave in a way that we've described as acidic or basic, but our definitions haven't covered all of those examples. So the earliest ideas about acids is that of Antoine Lavoisier and his idea was that acids are substances that contain oxygen. So if we have a look at a substance like sulphuric acid, this is an acid and it does contain oxygen. So therefore it's a little tick in the Lavoisier definition, but another very common acid that we know about is hydrochloric acid and hydrochloric acid certainly does not have any oxygen in it. So Lavoisier's definition, while it's correct for some acids, is certainly not correct for all. So we needed a better definition. So Humphrey Davy came up with a better definition. He came up with a definition of displacable hydrogen and he was looking at certain types of reactions, particularly reactions involving metals. And we now know that certain metals, certainly active metals can displace less active metals out of a solution, push them out of the solution. Metals can also push hydrogen ions out of the solution and we get the production of hydrogen gas. So this worked out quite nicely and you can kind of, if you match Davy with displacement, the 2Ds, give you a little bit of an idea about how you might remember that Davy's definition was based on displacable hydrogen. So this was really an observation about reactions. So one very common one we might have seen is if we react magnesium and hydrochloric acid, then we know that we're going to have the magnesium pushing or displacing the hydrogen from the solution. It'll come out as hydrogen gas, nice, easy one for a pop test. And we'll leave the salt magnesium chloride behind. So this was a better definition and incorporated the acids that were part of the Lavoisier definition, but extended on those to include certainly things like hydrochloric acid, which now does fit in terms of its reactions with metals for the Davy definition. But it still didn't go quite far enough. The definition that we probably gave you in the junior school is the one about hydrogen ions and hydroxide ions. So acids are substances that ionize in water to produce hydrogen ions and bases produce hydroxide ions. This is a better and more widespread definition. It enables us to look at things like concentration of acids and bases as it allows us to identify some specific relationship between the hydrogen ion concentration and the strength of the solution. This was a good definition and for all intents and purposes in our junior school, the Arrhenius definition gets a big tick because all of the acids that we encounter and all of the acid reactions that we use in our junior science years pretty much all involve acids that liberate hydrogen ions in solution. So it's a very good definition. It's a very broad definition and you can see as we go through this scale, these circles are getting bigger and bigger and it incorporates all of the known information that we had up to that point, but still there's a few exceptions that it doesn't quite work for. The examples of where it doesn't work, we will explore in future videos, but for now the definition that we're going to be concentrating on pretty much for the rest of this module is the Bronsted-Lowry definition. Now Bronsted-Lowry is two people, not one, two different scientists who both were working on acids and bases and who recognized that the H plus is actually the element hydrogen that has lost an electron and therefore has now become dominantly just a single proton. So the most common isotope of hydrogen and hydrogen one that has one proton and no neutron so if it loses its outer shell electron which is in that first shell then it's just left as a proton and this proton does all sorts of very interesting things in reactions between different substances. One quick example which we will have a look at a little bit later on and you can find these very easily on the internet is if you are able to mix the vapors of ammonia and hydrogen chloride and I'm going to call it hydrogen chloride not hydrochloric acid because both of these are gases then you find a little kind of white smoke being produced. That white smoke is ammonium chloride and it's a reaction between the NH3 and the HCl. It's not about carrying out a neutralization reaction in solution so there are no hydrogen ions present but nevertheless it fulfills our definition of an acid and a base but it only does so if we consider the fact that Bronsted, Lauri acids are proton donors. Now if we were to draw this in a little bit more detail which we will in class we find that nitrogen has three bonds and a lone pair or an unbonded pair of electrons which are also present and it's that unbonded pair of electrons that actually attracts the proton from the acid from the hydrogen chloride. That moving into there that attraction between the proton and the unbonded electrons is just that slight extension on the Bronsted, Lauri definition which is our Lewis acid. So while we describe acids in terms of the Bronsted, Lauri definition as proton donors we describe them for Lewis acids as electron acceptors, they're ones that accept electrons from other species.