 Hi students, welcome to Year 12 Chemistry and Module 5, Equilibrium and Acid Reactions. This is video number 19 and we're going to be talking a little bit about determining some of the solubility rules. In actual fact, most of what we're going to be doing here is trying to determine how we use these rules to tell us a little bit about what happens when substances are dissolved in water. So we know that not all ionic substances are equally soluble in water. Some are very soluble, some only slightly soluble and some dissolve to such small concentrations as to be virtually insoluble. So how do we know? Well there's two main ways that we can determine whether or not a particular ionic substance is going to dissolve in water and also to what extent. The first one you will do in class, obviously we can't test every single combination of ions to determine whether they're all soluble and to what extent, but we will be able to run a range of different types of tests to give you an idea of which substances do actually form precipitates and you've already started that process of looking at different types of precipitation reactions. The other way is to have a look at some of the rules for precipitates. If we can have a general way of trying to figure out when we have two substances being mixed together whether we can eliminate some of the ions and see if what we have left over actually forms a precipitate then that's probably the easiest way to do it. Up until now most of these ions have been given to you in tables but we need to start to work on ways of seeing if we can memorise some of the general rules to just help us a little bit with some more common examples. One of the most important things when you are carrying out this sort of activity is to identify the spectators. So there are certain ions which remain in the solution. They will not form a precipitate, usually they don't form a precipitate at all and therefore we can eliminate them from our conversation. So the first thing that we need to do is if we have a combination of solutions such as potassium chloride and silver nitrate then what we need to do firstly is say okay the potassium chloride is going to dissociate into potassium ions and chloride ions and the silver nitrate is going to dissociate into silver ions and nitrate ions. So that's all good. But then we need to have some rules and those rules are the ones that are really important because they help us to determine what happens next. Well one thing you may remember is the group one metals form soluble salts. So that means that potassium being a group one metal is always going to form something that is soluble so therefore it's not going to be part of a precipitate. One other general rule that we may remember is that all nitrates are soluble therefore nothing nitrate will form a precipitate either so we have a nitrate over here so it disappears. So our green circled substances over here are our spectators. They're going to remain in the solution and not form a precipitate so we can eliminate them. So what that leaves us with therefore we have the silver ion and the chloride ion and then we need to determine from the level of solubility whether or not there will be a precipitate. If the concentration of ions is greater than the solubility then we will have a precipitate.