 In the last video, we looked at how models can change and the importance of empirical evidence and observation in helping us to clarify our understanding of chemistry and also of the nature of matter. We have had a number of different models for acids and bases over hundreds of years and our models have continued to be refined on the basis of observations that we see. So one of the things that we want to look at is the fact that up until our senior years, the Arrhenius model is a pretty good model. You remember the Arrhenius model is about hydrogen ions in solution. So as long as there are hydrogen ions in solution, this model works fine. But the problem is that occasionally when we look at bases, hydroxide ions are not necessarily a part of what we can see being created by the substance, which is basic. So here's an example of a reaction. So zinc carbonate plus an acid producing zinc ions, carbon dioxide and water. So we know this is an example of a neutralization reaction. We've looked at a few of these now and we say we know this is neutralization. So if it's a neutralization reaction, it's produced water and it's produced carbon dioxide, then it gets a tick. So therefore, we must have an acidic substance and a basic substance. So the H plus ions fits our Arrhenius model nicely. That's OK. But the zinc carbonate does not. There's a bit of a problem here. Something is not quite fitting our Arrhenius model. So therefore, this is an observation that we can make that is contradictory our model, the model of bases that was suggested by Arrhenius. The other one is the example that I briefly mentioned on the previous video, which is ammonia. And if you soak, say, some cotton wool in each of these two solutions, ammonia and hydrochloric acid, then the vapors that are given off can combine and produce a chemical reaction. So if it's the gas, we would call it hydrogen chloride. It's dissolved in water. It becomes hydrochloric acid and it forms this white smoke here. And this white smoke is a solid, and it's called ammonium chloride. Now, this is a reaction that's occurring with no water. So if there is no water, then there are no H plus nor OH minus ions. So here's a couple of examples of chemical reactions that we see where we have one which produces water as a neutralization reaction but doesn't have a hydroxide ions obviously being produced, another one which doesn't even produce water at all. So what's going on? How do we classify these and how does our model need to change in order to incorporate these types of examples?