we're back for our second lecture on section 4.1 and we're going to talk about phases of matter we're going to talk about phases of matter because that is what gets us to this kinetic molecular theory that we haven't yet defined that is the title of the section but we haven't yet defined it so phases of matter why are we looking at this particular photograph because what you are seeing in this particular photograph is an example of all three phases of matter you have water in its liquid state you have ice which is water in the solid state but it's still water right and then you have clouds in the sky and clouds are composed primarily of water vapor which is water in the gas phase of matter but it's still water that's the key so let's talk about these different phases what makes those phases different and then loop it back to our concept of kinetic theory so let's start with the ice cubes let's start with water in the solid phase and the ice cube the frozen water stays a cube it maintains its shape why because the cohesive bonds the cohesive force the cohesive bonds between the water molecules is strong and that keeps the water molecules locked into place with respect to the other water molecules and so as a result of those very strong cohesive bonds the ice cube stays a cube so it has a shape and it stays that shape and however much volume it occupies that stays the same as well so solids not just ice but all solid matter has a fixed volume and a fixed shape and you put the ice cube in a glass and it stays a cube solids don't flow and the obvious next step once we've put the ice cubes in a glass is to pour some water into the glass there's the example the counter example fluids flow so a liquid pour the water into the glass you still have water molecules it's still all water molecules to make a water molecule you take two hydrogen atoms hydrogen is an element and you stick it to one oxygen atom oxygen is an element and you have h2o and i'm i know you know the formula h2o two hydrogens stuck to an oxygen so the ice cube the water is still water h2o the liquid water in the glass is still water h2o and so now what have we done to the cohesion there's still cohesive bond how do you know that there is still cohesion between the water molecules because you can pour the water into the glass and it stays in the glass and you know that it's weaker cohesion because now the water molecules are not rigidly held in place with respect to all the other water molecules you can pour the water so the water molecules think about holding hands you can hold hands and be fixed firmly in place or you can still hold hands but be free to move and stop holding hands with this person but then hold hands with that person but the water stays in the glass which means that the fluids take the shape of their containers however much you pour in the glass that volume of water is going to stay the same but if you pour that volume of water into a glass that has a different shape it's going to take that shape you put an ice cube into a glass it doesn't matter what shape the glass is the ice cube is still going to stay a cube so we have cohesive bonds the water molecules are still interacting with each other but they're not holding on as tight and water molecules can move around and hold hands with one molecule or stop holding hands and hold hands with a different molecule weaker cohesive bonds but there's still cohesive bonding and finally we get to gases so we're looking at the fog over the golden gate bridge and fog is a cloud on the ground which means that you're looking at water vapor and in the case of a gas so a cloud of water vapor your cohesive bonds are going to be absolutely minimal very very weak sometimes literally zero but typically with the case of water molecules you'll find very weak cohesive bonds but not completely zero and so what does that mean the weaker you make those bonds the harder it is to contain the matter so you can put ice cubes in a glass you can pour water in the glass but you cannot get yourself a glass of water vapor you can't pour water vapor into a glass gases can't be held in an open container now you could put water vapor in a glass and put a lid on it but if the glass is open you can't keep the water in the glass so what happens in terms of the volume in the sha and the shape just like a liquid a gas is going to take the shape of its container let's assume we've got a closed container we've got a closed container the gas is going to take the shape of the container but now what is different than pouring yourself a glass of water is if you pour yourself a glass of water vapor and put a lid on it that water vapor is going to fill the volume of the glass the water vapor or any gas will expand to fill its container the molecules are weakly bound and they're moving around and there is no reason not to move around all the way to the top and all the way to the bottom and all the way to the sides of whatever the container is the other thing that you can do with a gas that you can't do with solids and liquids is compress them so let's say you take that glass of water and instead of just putting a lid on it let's say you've got a piston and you can push it down into the glass if you've got water vapor in there you can push it down and you can take those water vapor you can take that water vapor those water molecules and make the container smaller they're still going to bounce around inside that container if you try to do that with liquid water okay so you've filled up your container and you push your piston down once you get to the water line keep trying to push that piston down you're not going to be able to you're not going to be able to compress that water into a smaller volume of water same thing with the ice cube obviously if you're pushing down on that piston maybe you can crush the ice cube but but the cube itself you can't take that cube and squeeze it into a smaller cube so fluids flow the idea of fluids both liquids and gases both are considered to be fluid states of matter because fluids flow you can pour them they don't have a fixed shape so the solid has a fixed shape but the liquid and the gas don't and notice the difference in this this picture between the liquid and the gas the liquid sits at the bottom of the container and the cohesive bonds between the liquid molecules keep them all bundled together into a group but with the gas you have extremely low cohesive bonds so the molecules of whatever you've got in there are not bundling together on the bottom they're just everywhere within the container and the idea of motion now when we observe the difference between the gas and the liquid that gives us this idea of moving molecules because when you look at that solid cube you are yeah that's not moving there's nothing there that's moving but that is a little bit deceptive let's think about motion and let's make sure that we understand that there's a lot of different types of motion so if we have a solid cube of anything of ice a solid cube of metal a solid cube of anything you want to pick you look at that and you think well it stays in its shape so the molecules can't be moving around can they and because of those strong cohesive bonds this molecule and this molecule are next door neighbors and they stay next door neighbors that's what makes it a solid this molecule can't just go off over here and stop holding hands with this one and go over and hold hands with a different one you can do that with a fluid but even though you have this fixed bond and these two molecules are not going anywhere with respect to each other there can still be motion these molecules can still be moving but they're still in position right next to each other and there's a lot of different motions that these molecules can still be doing even if they're not sliding around past each other and that is what's happening in every solid material every solid material the molecules that make up the material are moving they are moving in ways that don't break the bond between that cohesive bond between adjacent molecules but you can still move and what this one is doing and what this one is doing don't have to be the same thing so those molecules are in motion and that is maybe a little hard to wrap your head around with a solid material with liquid and gas materials the motion of the molecules is much easier to conceptualize and it's much easier to illustrate if we have liquid matter so notice the experiment we're doing here we're putting a drop of blue dye into a flask of water and if we leave that sit we don't stir it notice that we're just going to leave it sit we're not going to stir it up so we're not going to mechanically agitate like jewels paddle wheels but what's going to happen anyway even if we don't stir those dye molecules are going to distribute through that flask of water and it's going to take a little time you come back five minutes later you come back 10 minutes later and you will notice the distribution of the molecules has changed how do you know well because you can see the color that's how you know that's what makes it obvious okay so maybe that's not an experiment that you've ever done but have you ever poured yourself a coke over some ice cubes sure you have maybe it's not a coke whatever but poured a drink that isn't water over ice cubes that are water and what happens over time now we're not gonna we're not quite ready that comes later in the chapter to talk about the ice melting so we're gonna ignore the ice melting but what happens if you leave the glass on the counter you forget about it you go about your day you come back and it's hours later the ice has melted what's happened to the liquid is there a layer of water from that melted ice on the bottom of the glass and then all the coke is floating on the top it's all mixed together and then you pour it down the sink because nobody wants a flat watered down coke right but there's an example that of that molecular motion i'm sure you've experienced and when we talk about the molecular motion for gases it is even more obvious because everybody everybody has had the experience look at our first two bullet points it's called diffusion that molecular motion that happens is called diffusion and you have experienced this you've been in the other room when someone in the kitchen started cooking something you're in the other room how do you know they're cooking something because it smells delicious how do you how did you smell that how did you smell that because the molecules of whatever it is they're cooking traveled through the air to wherever you happen to be in the house and literally went up your nose so you don't have to be in the kitchen to be able to smell what's cooking you don't have to be in the frying pan to smell the bacon you can be two rooms away that is molecular motion bacon molecules up your nose from two rooms away and you've experienced that maybe not with bacon but you've experienced that and so right there that is also giving us these energy clues for kinetic theory that's giving us energy clues that the stronger the cohesive bond solids have the strongest even though the molecules are moving those cohesive bonds are strong those cohesive bonds are not being broken because the cube of ice stays the cube of ice that's got to be the lowest energy the molecules in the solid the cohesive bond but they're still moving and not doing necessarily the same thing but the energy is low and how do we know it's low energy because they're not breaking the bond and so as we move from solid to liquid to gas how do you change an ice cube into a puddle of water you add energy it melts why does it melt because you left the glass on the counter and the counter isn't freezing cold the counter is room temperature what happens you add energy and what happens to turn that solid into the liquid it absorbs energy and what does it do with that energy well the molecules start to move more and the solid that bond is strong but now we're going to add energy and those molecules start to move more and they start to break their bonds not every bond gets broken but you break enough bonds and now you have a fluid where the water can slide around with respect to itself how do you turn the liquid water into water vapor well you know how to do that too you add energy you put a pan of water on the stove you light the stove that water boils and makes steam steam is water vapor and so there is the answer this is what kinetic theory is it turns out to be sounds pretty high level but it turns out to be very intuitive that our phases of matter are energy related and the lowest energy phase of matter would be the solid phase and the highest energy phase of matter is the gas phase and that is where we end section 4.1 and we'll come back and do section 4.2 very shortly