hello this is a keystage three chemistry video for pure and impure substances firstly what do we mean by pure substances well these are made of one substance only and again what do we mean by the word substance well we mean made of one either element or compound only so made of one element or compound we could take a look at an example of a Pure Element so an element would be something like oxygen so we've got a little diagram here of an oxygen tank if we release some of that oxygen we can see that if we were to magnify uh inside the gas there inside the oxygen gas we would see oxygen atoms only and if we take a look we'll see the oxygen atoms they do exist in pairs that's why we call oxygen gas O2 but you would only find oxygen atoms and that is pure oxygen what about a compound that is a pure substance we could take a look at a very common example here is some water in a container now you might think well water is H2O so it's got two different elements in it and it can't be pure but actually it is pure because if we were to zoom in on some of the water in the same way as we did with the oxygen you would see that we only have water molecules inside the water if it's pure water so these are two examples of pure substances one is a compound and one is an element what about mixtures or impure substances well these are made of more than one substance or two or more substances together and again by substances we mean elements or compounds and one common example of a mixture is air now you might say air is a pure substance or can be a pure substance however air is actually a mix mixture of many different gases so if we could magnify into some air we would see molecules of different gases oxygen water vapor carbon dioxide nitrogen and one or two others so that's one example of a mixture a very common example of a mixture there are others as well so for example we have air as we mentioned we also have milk is a mixture of proteins sugars lipids and a few other things we also have sea waterer which is a mixture of water and various different salts fruit juice is a mixture and also another example at the end here we have is paint which is different to the other substances but all of these are examples of mixtures for our pure substances now we've talked about this already so it's made of one either element or compound only one substance only and we have the example of water which is a compound but still pure oxygen gold as a random example of the periodic table and in fact any element of the periodic table would be a pure substance if it wasn't mixed with anything else and any single compound would also be a pure substance as well now there's one more thing I want to go through with this or in this video and that's the idea that pure substances have a single melting and boiling point what do we mean by this well let's have a look at a graph of something that's melting and boiling so we take the example of pure water that we have Frozen and we are warming up and we can look at a graph like this here's the boiling point of water here's the melting point of water and just so we have a little visual reference we've got some melting uh water some melting ice and some boiling water as well now as we would warm up some ice it would eventually get to 0° C and that's when it would start melting it stays at the same temperature while it's melting and only after it has all melted will the temperature rise further so there goes the uh increase in temperature and when it reaches the boiling point it stays at the same temperature while it all boils and then the temperature rises but if you look the melting point and the boiling point for pure water are at 0° for melting and 100° for boiling this is one very specific temperature and can help you identify that the liquid there is actually pure water what about if we have impure substances well these melt and boil over a range of temperatures so imagine we had some impure water for example had some salt in it we could do a graph similar to what we had before we've got the boiling point and melting point there and the graph would look slightly different in that the temperature would rise but at the melting point it's not flat and at the boiling point is not flat as it was in the last graph we'll be able to see that the melting point actually happens over a range of temperatures a little bit below zero and a little bit above zero and the boiling point also has a range of temperatures as well the boiling point actually gets a bit higher than 100° and goes on for a little bit more but the graph is not flat we have melting and boiling over a range of temperatures so let's summarize what we have learned in this video for pure substances these contain only one substance or material which could be an element or a compound for example oxygen water carbon dioxide I've added there or hydrogen so these are examples are different but still pure um remember they have specific and single melting and boiling points and this means that they only boil at at a very specific temperature and melt at a specific temperature and we had a graph that looked like this to help us understand that idea we can also actually make a note that when we have a specific boiling point or melting point we can identify the substance based on that boiling point and melting point so one example a very common example is that of water which boils at 100° C and freezes or melts at 0° C for our impure substances well these are when we have mixtures so we could describe an impure substance as a mixture but they contain two or more substances I.E elements and or compounds for example air sea waterer paint or juice these are all mixtures and they melt and boil over a range of temperatures and not at one specific temperature and we had a graph that showed that that looked a bit like this so there's our graph for melting and boiling of impure substances okay so this was a summary of what we mean by pure and impure substances and how we can use graphs to identify which one of those two we might have thank you for watching and I'll see you soon