Hey, guys. So in earlier videos, we've talked about phases and phase changes. In this video, I'm going to show you a more sort of visual representation of phases through what's called a phase diagram. So the kinds of problems you're going to see here are going to be mostly conceptual, not a lot of calculations. So I'm just going to go ahead and walk you through it and show you what you need to know. Let's check it out. So as I said before, it's a diagram and what this diagram does is it shows you the possible phases for a substance as a function of pressure, which is on the y-axis, and temperature, which is on the x-axis. So the idea here is that every substance has its own unique phase diagram. It's going to look different for copper, aluminum, and water, or whatever. So here's a pretty common one you're going to see for H2O, again, what we all commonly know as water. So the idea here is that if you pick a temperature and a pressure and you just figure out where these two lines intersect, then you're going to figure out what phase that material is in like a solid, or a liquid, or a gas depending on the different combinations of pressures and temperatures you have. In general, for water, there are 3 main regions. When you move towards higher pressures and lower temperatures, sort of the top left of the diagram, you have solid, which is where basically water turns into ice. That makes sense. Right? If you take water and you cool it down, you bring it down to a low temperature and it freezes. Now if you sort of heat it up and you have sort of like these sort of middle temperatures and middle pressures, you're going to have water. That's sort of what exists in like, you know, room temperature in like everyday conditions. And if you start to heat it up, if you start to go toward the right side of the diagram and then you're going to turn everything into gas. This is where we have steam. That should make that should make sense sort of from our everyday experience here. Now the boundaries between these three regions are actually where phase changes occur. So for example, the boundary between the solid and liquid is called the fusion curve. We've seen that word before when we talk about phase changes, we use the latent heat of fusion. So this is where solid becomes liquid. So the idea here is that anywhere, actually. So the idea here is that for any combination of pressure and temperature along this curve, you're going to have water, you're going to have ice that's melting into water and vice versa, water that's freezing into ice. So the other curve that we've seen or the other word that we've seen is vaporization. This is where you have liquid turns into gas. The idea is the same here. If you're anywhere along this curve, right, for any combination, there's an infinite possibility of pressures and temperatures. You're going to have water that boils into steam and steam that condenses into water, vice versa. The one that we haven't talked about yet before is called the sublimation. So the idea here is that there's actually some combinations of pressures and temperatures, usually very low temperatures or pressures, where you actually have a solid that doesn't first become a liquid and then a gas. It actually just turns straight from ice into steam. So this is called sublimation.
Alright. So these are the 3 curves you need to know. Besides that, there's really just 2 more important points on these diagrams that you need to know. The first one is called the triple points. Triple point happens where all the 3 curves meet together. So there's this one point right here where I'm gonna call the triple points, and this is actually let me mark that in red. So this triple point, it's basically where the where you have a special special temperature and pressure where all three phases can coexist. So oops, coexist. So if I had a container that was of H2O that was at 273 Kelvin and exactly this pressure, then I would actually have a container that has ice, water and steam all together in the same container and one phase wouldn't try to change into another one. So this is sort of where all the phases reach a sort of balance. The other point you need to know is called the critical point. The critical point always happens at the tip of the vaporization curve. So the idea here is that this is gonna be the critical point. The critical point is a special point. Again, it's a special temperature and pressure, like this is what it is for water, in which you have a substance that isn't just a liquid or a gas. It's actually both. It's kind of weird, but basically what happens is that if you have, you know, if you were on this part of the diagram, you would just strictly be a liquid. You would have a lot of the properties of a liquid. If you were over here on the diagram, you would have all the properties of gas. But for very high temperatures and very very high pressures, the distinction between liquid and gas kind of starts to go away. And in physics what we do, we don't have to, you know, get into the specifics, but we just call this generally a fluid. It doesn't it's not really liquid; it's not really a gas; it's kind of both. So this is the region where fluids sort of live.
Alright. So that's really all you need to know. Again, most of the problems are gonna be conceptual. Let's check out our example here. So we have a sample of H2O, which means we're going to use our diagram over here. We're at 250 Kelvin and we're at atmospheric pressure, which is 1,000 kilopascals. So the first problem here is to figure out what phase the sample is in. And as I said before, this is pretty straightforward. All you have to do is just mark the temperature on the diagram and the pressure, figure out where the two lines intersect. Alright? So the idea here is that if this is 273 then if I'm at 250, that's gonna be somewhere, I don't know, just over here. This is gonna be 250. Alright? And if this is at 610, but I'm trying to get to 1,000, that's gonna be somewhere over here. So all I have to do is just figure out where these two lines intersect. So I'm just gonna draw this line straight up and then this line straight across, and then this is going to be my phase. So for part a, that's part a, we're just going to have ice which is a solid. Whoops. This is gonna be ice. Yeah. That's a solid. Alright. Let's take a look at the second part now. For part b, what we want to do is we want to figure out what phase change is gonna occur if we do something to the sample. So we're gonna take this sample here and we're gonna increase the temperature and then we're going to keep the pressure constant. So the view here is we're going to have to move along this diagram in some direction and figure out which curve we're going to cross, what phase change happens first. So what happens is we're going to increase the temperature. Remember temperature increases along the right as you go to the right. But we're gonna keep the pressure constant, which means that we're actually just going to keep we're not going to move up or down from this particular point. Basically, we're just going to move in a horizontal line. So here's part b. The first curve that we cross is the fusion curve. So this is where ice becomes water. So the phase change that happens here is you're just gonna have fusion, which is, you know, another word for that is just gonna be melting. Now we're gonna do the same exact thing in part c, except now we're gonna change some of the some of the changes. Here we have that we're gonna decrease the pressure and we're gonna keep the temperature constant. So same idea here for part c, except now what we're going to do is we're going to start off from our ice sample and we're going to decrease the pressure. Remember, pressure goes up, as you move increases as you go up on the diagram, so it's gonna decrease as you move down. But now we're gonna keep the temperature constant, which means we're not gonna be moving left to right. We're only just gonna be going down. So as you move down, what happens is this part c, the first curve that you're gonna cross is gonna be the sublimation curve. So that that's what happens here. So you have the ice sample that just sublimes, so sublimation.
Alright. So that's it, guys. That's really all there is to it. You're just going to identify these things on the diagram and figure out what changes happen. Let me know if you have any questions and I'll see you in the next one.
