we are ready to start talking about section 4.4 and we are going to shift gears a little bit now we've been talking about adding or subtracting energy in the form of heat and we've been talking about raising or lowering the temperature of an object but now we're going to start thinking about phase change and connecting that back to the ideas that we've been developing so far with energy and molecular theory so let's think about phase change and you see that we're using again water as our example because it's pretty easy and it's very intuitive because we're all familiar we're really familiar with water in the liquid state and ice water in the solid state and steam water vapor water in the state of a gas so the first thing is adding or subtracting energy and raising or lowering the temperature you can take your water in the liquid form and you can heat it up make it warmer water increase the temperature it's still liquid water or you can cool it down subtract energy lower the temperature still liquid water but now we need to think about what's happening when we change the phase what happens when excuse me you put an ice cube tray in the freezer you've got liquid water you put it in the freezer and you come back later and now you've got solid ice cubes what happened well the water froze right what happened or you put that panel water on the stove and you add energy you heat it up and then starts to boil and it makes steam and the water turns from liquid to vapor what happened so a couple of things about phase change that we want to understand is the mechanism is not the same when we're talking about changing the temperature you're adding energy where does that energy go what does the substance do with that energy and if you're raising or lowering the temperature you're just making the molecules either move faster if you're raising the temperature and adding energy or move more slowly if you are subtracting energy and lowering the temperature the molecular motion decreases now let's go to the phase change and what i want you to see on the graph here is this is just general it's not specific to because notice there's no numbers on either of our axes this is not specific to any one particular material it's the general shape of the curve you're going to see for any material so start at the beginning go all the way over to the origin start at the left side and let's say we've got an ice cube and when you have an ice cube in the freezer it's colder than freezing so zero degrees celsius or 32 degrees fahrenheit that's the freezing point that's the temperature at which liquid water will freeze into solid ice but you can be colder than that your freezer has to be colder than that so you've got a solid material your ice and it's at negative five celsius just whatever and what happens when you add energy to that solid cube of ice it doesn't instantly turn into a puddle of water if that ice is at -5 and i'm going to use celsius temperatures here then adding energy is going to first of all increase the temperature the ice cube is going to stay an ice cube but what are you doing you're adding energy the molecules are going to move more but the molecules because it's in the solid state the molecules are still fixed in position that's what makes it a solid with respect to each other those molecules are locked in place they can move rotate they can vibrate back and forth but they're still locked into position with respect to each other so the temperature increases until you get to that melting point so now we're at 0 degrees celsius and at 0 degrees celsius this is our temperature for phase change the melting point now what's going to happen well you're going to keep adding energy and when you add energy you're not going to end up with now a solid block of ice at one degree celsius a solid block of ice at two degrees celsius what happens is a transition in the way the energy is used and so at that melting point what i want you to see on the graph notice it gets flat and notice the y-axis is temperature so at that melting point and if you keep adding energy that energy isn't going to be used to keep raising the temperature what's going to happen instead is the phase change okay so the phase change what happens during that phase change well we just said that if it's solid those molecules are locked in position so the bond between the molecules is keeping those molecules rigidly fixed in position with respect to each other you're going to use that energy to break those bonds now if you're going from the solid to the liquid phase you're not going to break the bonds completely remember our cohesive bonding idea you're going to break some of the bonds some of the other bonds will get weaker and what melting means going from the solid to the liquid remember our liquid phase there's not zero cohesive bonds but there's weaker cohesive bonds between the molecules and the water molecules aren't rigidly fixed in position anymore with respect to each other the fluid flows so that's what's happening during that melting the energy is not going to be used to make the individual molecules move more rotate or vibrate it's going to be used to weaken the bonds and break some of those bonds and that's going to make those molecules no longer rigidly positioned with respect to each other so you keep adding energy that curve stays flat that curve stays flat because you're not using the energy to increase the temperature in the form of that molecular motion you're using the energy to weaken and break the bonds eventually you've changed the phase you've added enough total energy to change the phase and now it's all liquid so you've taken that ice cube and now it is a puddle of water it's a puddle of water at zero degrees it's not frozen in a block of ice anymore now it's in the liquid phase but it's still at the same temperature it's still at that zero degrees celsius what happens next well we keep adding energy and what happens next is now that the water is in the liquid phase we're going to use the energy again to increase the molecular motion and that's going to raise the temperature so you keep adding energy the temperature keeps increasing the water gets warmer the temperature keeps increasing the water gets warmer until you get to that boiling point and now we're going to do the same thing all over again with our phase change from the liquid to the gas phase and that phase change the same process happens you get to that temperature 100 degrees celsius the water is at 100 degrees celsius and if you add more energy you're not going to have liquid water at 101 degrees celsius you're not going to have liquid water at 102 105 degrees celsius because you're gonna again change the way the energy gets used so the energy now again will be used to break the bonds and going from the liquid to the gas phase now you really are breaking more bonds than you are weakening bonds because remember what makes the difference between the liquid and the gas the gas has the weakest cohesive bonding very low and a lot of times zero cohesive bonding between molecules so the gas phase remember you can pour a glass of water but you can't pour yourself a glass of water vapor remember that idea that the gases have no fixed volume and no fixed shape so you're using that energy to vaporize or boil the water and again 100 degrees constant temperature because you're not using the water to increase your not using the energy to increase the temperature of your water once you've got all of that water vaporized let's say you've kept your steam that water vapor let's say you've kept it in a container now if you keep adding energy now again the same thing will happen now that energy will be used to increase the molecular motion and that's going to raise your temperature so you've got those steps on the graph and the places where that graph is flat that's where the phase change happens where that phase change happens the energy is focused on either making or breaking bonds because now we can do the whole thing in reverse we can start with the gas and we can start taking energy away and when we slow down we reduce the temperature and slow down the molecular motion eventually you get to that boiling point but now we're coming down instead of going up in temperature and you'll get condensation to go from the gas to the liquid and what happens there well the molecules are slowing down enough that they can now form cohesive bonds so you're going to form bonds instead of break bonds and now once all of your gas molecules have slowed down enough and started forming cohesive bonds now it's condensed into that liquid form condensed into liquid water and if you keep taking energy away then you're going to reduce the temperature we're going right back down the curve and you take that liquid water at 100 degrees and now you take the energy away it's at 99. keep taking energy away it's at 95. keep taking energy away until your water is still liquid but now you're at zero degrees now you're back down to that melting point but now we're going the other direction the freezing point instead of going from solid to liquid we're going to go from liquid to solid same thing is going to happen we're not going to keep decreasing the temperature you're not going to get liquid water at zero degrees and then liquid water at minus one liquid water at minus five because what will happen is the formation of those cohesive bonds the water molecules will start to form those bonds and now those two molecules are fixed in position with respect to each other and then another one forms and another one forms and you've probably seen ice crystals the formation of the bonds molecule to molecules spreads out and water does in the solid form have a crystalline structure you've seen snowflakes that is solid water in its crystalline form once the whole thing is solidified you can then keep making it colder you can make it colder than zero degrees the water is still going to be solid at this point but then the temperature decreases okay so i know we've only covered one slide here we've got a ways to go yet in section 4.4 but i'm going to stop the video here keep this one a little bit short and we'll come back and we'll take a closer look at the equations and how the math works out