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Phase Diagrams - Concept

Teacher/Instructor Kendal Orenstein
Kendal Orenstein

Rutger's University
M.Ed., Columbia Teachers College

Kendal founded an academic coaching company in Washington D.C. and teaches in local area schools. In her spare time she loves to explore new places.

Phase diagrams graphically depict the state of matter in varying temperatures and pressures. The x-axis of a phase diagram is always temperature while the y-axis is always pressure. There is a point on a phase diagram called the triple point at which all three phases of matter exist simultaneously.

So we're going to talk about phase diagrams, phase diagrams are graphical interpretation of pressure versus temperature.

okay so basically you can fig- you can figure out if you don't know the temperature or pressure of any substance you can figure out which phase it is using a phase diagram so and we also can see what pressure and temperature is needed for different phase changes going from solid to liquid to vapour so let's like let's look at this graph in itself and try understand what this is saying, so we have 3 major lines here we have one going up this way and we have one coming down here so this, this separation here between the solid and vapour, this point where if any time they crosses this line, we're going to have sublimation occuring. Any time it crosses this line here it's going from a solid to a liquid or liquid to solid either way, we're going to have either freezing or melting going on. Notice that one atmosphere which is that we know one atmosphere in 0 degree Celsius is our normal freezing point and melting point.

Any time that you have temperature pressure within this region your substance will be so- will be a liquid anything down here is starting to be a vapour or gas. We have the reason why we call this normal is because one atmosphere we call this normal atmosphere pressure and we also know this is our boiling point at 100 degrees Celsius we know the water at a 100 degrees.

Okay there are a couple other things in here that you might not know, one is when they're all in a centric here that was what we called the triple point actually there is a there is a temperature and pressure at which all three phases will coexist together and we're going to call that the triple point.

There's also something up here called the critical point, the critical point is a place where anything any pressure or temperature that's higher than the critical point there's no way liqu- the liquid phase will exist it will only the substance will only exist in the gaseous phase so this actually kind of tells us a lot of things in terms of subst- which where the substance is in terms of the state of matter that is in.

Alright, so let's look at something else, this was water, so let's look at something else, it's another phase diagram we can talk about each substance has its own phase diagram and this is carbon dioxide and there is a couple differences in carbon dioxide than there was in water one main difference if you look at both of them, one main difference is this line here okay notice this line versus this line okay water is very unique in the sense that it will have its solid form is more dense sorry lesser dense than it's liquid form so as we continue to add pressure to the solid formula it will actually change to a liquid state and if we continue to add pressure you notice it'll go up its going to eventually change into liquid state because liquid state actually more dense than the solid state whereas in most substances if you increase pressure it's going to stay in it's solid state because the solid state is actually more dense than liquid state so you're going to see a line looks like this.

Alright notice carbon dioxide we're going to call that dry ice carbon di- the reason we call it dry ice is because here we have some dry ice actually with us, let me get some out for you alright the reason why we call it dry ice is because we have it a solid form but yeah it's sublimate meaning it goes straight from solid to gas okay this is used sometimes in like movie studios or to make that gas the gas appearance the gaseous appearance, so if you notice this goes straight from the solid state skip skipping the liquid state going straight to the solid state I mean straight to the gaseous state and we call this sublimation. Alright so we're going to actually demonstrate with triple point will look like by putting dry ice in this container and seeing if we can make it liquid somehow so let me try and put my safety goggles on because safety is always first and we are going to put some little pieces of, we're going to, this is really cold so I really shouldn't be doing that but it's not going to hurt me unless hold it for too long, little pieces [IB] alright so what I'm going to do with this oops I'm going to, alright so what I'm going to do with this is I'm going to close this up and so now I have my dry ice in here and my dry ice is sublimating as you saw before and so it's going straight to a gaseous state meaning and also making a pressure inside this container much higher okay so if we look back and if you remember back at our graph you notice this is continue still sublimating, okay and what we want to do is we want to continue the to make the pressure high enough where it will sublimating and actually go to a triple point where we can see all these in the same time and how we're going to do that is going to lower the temperature and notice from the side we notice that inside this container is really cold as you can see from the impressions of the bottle and also as you can imagine its getting really pressurized in here because the gas is continuing to sublimate and this is going to increase the pressure inside the bottle so hopefully we'll get to the triple point, we can shake it up a little bit as you can see like on the side over here maybe you can see like the liquid inside here so now all three phases are existing making this the triple point. We can see in the bottom like maybe some liquid here, liquid CO2.

Alright so now we're going to open the bottle and see what happens whoo and this is all the gas that's escaping there's a tons of pressure in here they're 5 atmospheric pressure which is 5 times the regular atmosphere that we actually living in at sea level so actually let's go back here and just make sure we understand what will happen is here is our normal temperature around here we know that [IB] sorry carbon dioxide is in gaseous state what we did we lowered the, the temperature by putting it inside the container and make increase of pressure as it sublimates inside the pressure and we're able to create the triple point where all threes substances coexist, so this graph actually tells us a lot of things about the different types of substances in different types of forms that they can actually have in pressure and temperature and how they're related.