Skip to main content
Pearson+ LogoPearson+ Logo
Start typing, then use the up and down arrows to select an option from the list.


Learn the toughest concepts covered in Physics with step-by-step video tutorials and practice problems by world-class tutors

20. Heat and Temperature

Phase Diagrams, Triple Points and Critical Points


Phase Diagrams

Play a video:
Was this helpful?
Hey guys. So in earlier videos, we've talked about phases and phase changes in this video, I'm gonna show you a more sort of visual representation of phases through what's called a phase diagram. So the kind of problems you're gonna see here are gonna be mostly conceptual, not a lot of calculations. So I'm just gonna go ahead and walk you through 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 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 gonna look different for copper and aluminum and water or whatever. So here's a pretty common one you're gonna see for H 20 again, but we all commonly know as water. So the idea here is that if you pick a temperature any pressure and you just figure out where these two lines intersect, then you're gonna 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 that you have in general. For water. There's three 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 gonna have water, that's sort of what exists in like room temperature, like everyday conditions. And if you start to heat it up, if you start to go towards the right side of the diagram and then you're gonna turn everything into gas, this is where we have steam that it sort of makes that should make sense, sort of my 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 talked about phase changes, we use the latent heat of fusion. So this is where solid becomes liquid. So the idea here is that anywhere extraction. So the idea here is that for any combination of pressure and temperature along this curve, you're gonna have water or you're gonna 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 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 gonna 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 this is the three curves you need to know besides that, there's really just two 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 three 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 let's co exist. So if I had a container that was of H20. 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 that's sort of where all the phases reached some sort of balance. The other point you need to know is called the critical points. The critical point always happens at the tip of the vaporization curve. So the idea here is that this is going to 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 is 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 and we don't have to 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. Um, 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 gonna be conceptual, let's check out our example here. So we have a sample of H 20, which means we're gonna use our diagram over here, we're at 250 kelvin and we're at atmospheric pressure, which is the 1000 kill pascal's. So the first problem we have 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 2 that's going to be somewhere, I don't know just over here, this is gonna be to 50. Alright. And if this is at 6 10, but I'm trying to get to 1000 that's going to be somewhere over here. So all we 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 gonna be my face. So for part A that's part A. We're just gonna have ice, which is a solid. Oops, 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 wanna do is we want to figure out what phase change is going to occur if we do something to the sample. So we're gonna take this sample here and we're going to increase the temperature and then we're going to keep the pressure constant. So the gears are gonna have to move along this diagram in some direction and figure out which curve we're gonna cross, what phase change happens first. So what happens is we're going to increase the temperature, Remember temperature increases along the rights as you go to the right, but we're gonna keep the pressure constant which means that we're actually just gonna keep and we're not gonna move up or down from this particular point, basically we're just gonna 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, it's just gonna be melting. Now we're gonna do the same exact thing in part C. Except now we're going to change some of some of the some of the changes here, we have that we're going to decrease the pressure and we're gonna keep the temperature constant. So the same idea here for part C. Except now we're gonna do is we're gonna 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 going to decrease as you move down but now we're going to keep the temperature constant which means we're not going to be moving left to right, we're only just gonna be going down. So as you move down, what happens if this part, see the first curve that you're gonna cross is going to be the sublimation curve so that that's what happens here. So you have the ice sample that just sublime so sublimation. Alright, so that's it guys, that's really all there is to it. You're just gonna 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 the next one.