hi there I'm Jeremy Krug and we are moving on to unit 3 section 2 which is about how intermolecular forces affect the physical properties of liquids and and solids so we're going to start by talking about vapor pressure now if you can imagine a bottle of water or some other liquid and it's enclosed so we have a cap some sort of a lid on top of this water bottle or this bottle containing liquid well we know that that liquid is going to evaporate to some extent not all of it but some of it will and while that vapor is inside the closed bottle or closed container that vapor is going to push back down on the liquid and that pressure is called vapor pressure the pressure exerted by a vapor back down on the liquid from which it evaporated and different liquids have either higher or lower Vapor pressures generally speed taking as you raise the temperature of a liquid the vapor pressure of a liquid is going to increase I think intuitively that makes sense we know that when you raise the temperature you're going to evaporate more of the liquid and so if you have an enclosed container you'll have more Vapor trapped inside the bottle or the container and it'll create of course more pressure as it pushes back down on the liquid now another factor that we have to think about will be intermolecular forces liquids that have very weak intermolecular forces are going to evaporate much more easily and much more readily so since they evaporate more easily you have more vapor in the container and you'll have a higher vapor pressure so weaker intermolecular forces correlate to higher Vapor pressures and you can also go the other direction the stronger the intermolecular forces the better the molecules stick together the lower the vapor pressure is going to be now this also has to do with the boiling point when you have weaker intermolecular forces you have a lower boiling point that means it's going to be a whole lot easier to force those molecules in the liquid phase apart and they're going to fly off into the gaseous phase very easily we'll take a look at a graph of this here and here on this on this graph I have four different liquids and their Vapor pressures as a function of temperature so on the x-axis we have temperature in degrees Celsius and on the y-axis we have vapor pressure in kilopascals we have a dotted line that represents 101.3 kilopascals because that is normal atmospheric pressure at sea level so that's about one atmosphere so as we look at the graph we should be able to answer a few questions about it so for example can we estimate the vapor pressure of ethanoic acid at 120 degrees Celsius now this question is just a matter of reading the graph so we'd find 120 degrees Celsius which is right around here and we slide up to the curve for ethanoic acid and hopefully you can see that the answer should be something fairly close to about 110 kilopascals maybe a touch less than that but very close to 110 kilopascals how about this question at what pressure in kilopascals would water boil at 90 degrees Celsius well it's just a matter once again of reading the graph we find the 90 degree celsius line which is right around right around here it looks like and we slide up to the graph or to the curve for water which is right here and we go over to the y-axis and it looks like it's very close to 70 kilopascals now this shows us that you can change the the boiling point of water or pretty much any liquid just by changing the pressure so if you're at a higher elevation where you have less pressure less air pushing down on you water is going to boil at a lower temperature and likewise if you increase the pressure water is going to boil at a higher temperature which is what the graph shows as well that's how pressure a cooker Works actually now based upon this graph we should also be able to answer this question which of these liquids has the weakest intermolecular forces now think back to what we said a couple of slides ago we said that weaker intermolecular forces will correlate to higher vapor pressure so all we have to do is find pretty much any temperature which of these liquids has the highest vapor pressure at pretty much any temperature I mean just take a random temperature like maybe right here 35 degrees and you see that by far the answer is propanone so propanone is the answer it has the weakest intermolecular forces and as it turns out that also will have the lowest boiling point as well which which makes sense along with that question now let's shift phases and talk about solids specifically ionic solids now ionic solids are composed of ionic compounds which of course is no surprise as we talked about in unit 2 they are in a crystal lattice and so this is a picture that we've looked at before this might be a typical ionic compound let's say it's sodium chloride in which the sodium ions are these little represented by the purple circles or spheres and the chloride ions are the larger ones those little green or the somewhat larger green spheres that's how these ionic compounds are ordered this nice orderly arrangement this lattice now you probably know from your own experience that ionic compounds like salt are brittle and can be easily shattered and that has a lot to do with the way this Crystal is structured if you take a hammer and you hit it you know as long as this this Cube this this lattice is intact and it's not Disturbed it's fairly strong and it's strong because you have these electrostatic attractions you have the positives and the negatives that are that are attracting each other very strongly the positives and those negatives that's a very strong force to be honest what happens if you tap this with a hammer though well if you get this this lattice uh kind of off-kilter by just even one row and you get it to the point where maybe one of these green spheres is right next to a green and a purple is right next to a purple all of a sudden you have like charges next to each other and they repel and so literally it just forces itself apart and so just even a little tap with a hammer that gets this this lattice just off kilter by even one row it's just it just shatters very easily so that's how that works it has a lot to do with the charges in there now as you probably know as well in the case of salt and lots of others ionic solids will conduct electricity when they're dissolved in water or when they're melted as well so very easily conducting and that's because you have all these charges positives and negatives swimming around in the water and those charged particles allow the electrons to pass through very easily now another type of solid that you need to know about are molecular solids and these are solids composed of not ionic compounds but covalently bonded compounds and here we have the case of water we have these little water molecules as you can see and they're attaching to each other and so if this is a solid and it's water then what we're seeing here is ice now the forces that hold molecules together in a molecular solid aren't nearly as strong as they are in an ionic solid so what happens is these molecular solids are going to have fairly low boiling points and low melting points in the case of solids we should say so there are lots of examples of these molecular solids things like ice as we see here moth balls which we call naphthalene a lot of the polymers like this fairly easy to get these these uh these molecules to disassociate themselves from each other so we have ionic solids we have the molecular solids and there's another kind of solid as well covalent Network solids this is kind of a special type of bonding because there are some elements some covalent compounds that have a repeating network of covalent bonds and because of the of the repeating Network that's there you have a very strong structure very difficult to break in fact it is so strong it's actually as far as I can think of one of the very few types of bonds that's stronger than your run-of-the-mill ionic compound you might remember in the last section we said that ionic compounds have a really high melting point right well the only thing that I can think of that we're talking about in this class anyway that's higher than that would be covalent Network solids things like diamond and graphite silicon dioxide that's sand basically silicon carbide and so in the case of diamond we know that that's the hardest material known to humanity today that's naturally occurring anyway and if we have a submicroscopic model of what's going on there you'll notice that every carbon atom in diamond is attached to if I'm counting correctly four other molecules maybe more than that I don't know but they're they are connected in multiple dimensions multiple directions and so that makes this lattice or this this a network we should call it so strong it's almost impossible to break that Bond and so that's why diamond is such a a hard material silicon dioxide sand or quartz it's also very very hard and that's because of those covalent Network bonds it's you have a network that's connected in so many different dimensions and directions it's almost impossible to break try to take sand and burn sand can you burn it I mean no in fact sand is often used to put out fires if that's the case why is graphite used in pencils maybe how we're thinking you know graphite covalent Network solids why do we use graphite DNA just a regular pencil and we can write with graphite well if you look at the bonding for graphite what you have are these sheets here and so you have a sheet of carbon and in in between that sheet itself those bonds are extremely strong it's almost impossible to burn graphite you have to get the temperature up to thousands of degrees to burn graphite but the bonding in Between the Sheets from one sheet to another is actually extremely weak and so that's why you can take a pencil that has graphite in it and write on a piece of paper and those little individual sheets of graphite literally flake off onto the piece of paper but these are the substances that have covalent Network solids on the AP Chemistry exam you aren't going to be asked about too many of these generally Diamond graphite silicon dioxide and silicon carbide are usually the four that they will limit it to now let's apply this knowledge let's use what we know about chemical bonds to explain why carbon dioxide has such a low boiling point while silicon dioxide has such a high boiling point and if you look at the periodic table you might think you know Carbon and silicon are you know one is right above the other on the periodic table you would expect them to be pretty similar chemically but as it turns out they are very different what has to do with the fact that carbon dioxide is composed of individual molecules containing covalent bonds and so those forces that are keeping them together are relatively weak they're basically just uh London dispersion forces but silicon dioxide has covalent bonds but we're talking about a covalent Network solid and so the repeating lattice of bonds in those multiple directions will give silicon dioxide extra strength and that's why it has an extremely hot boiling point so this brings us to the end of section 3.2 hope you learned something about solids and liquids and how they are associated with each other if you learn something please give me a thumbs up and I hope to see you in my next video in which we're going to continue on to section 3.3 right there in unit 3. thanks for watching