Phase Diagrams

by Jared Rovny

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    00:01 Our last topic is the topic of a Phase Diagram.

    00:05 And a phase diagram looks something like this.

    00:07 We would plot the pressure of a material versus the temperature of that material.

    00:12 And then we talk about the phases of that material at different temperatures.

    00:16 So for example here, this would be a very simple one dimensional phase diagram in which we are talking about a particular pressure.

    00:24 So at atmospheric pressure, we already know how a liquid water behaves.

    00:28 We know that below zero degree Celsius it's a solid; between zero and 100 degrees Celsius it's a liquid; and from 100 and on we would think of it as a gas.

    00:37 But now were going to do something different. We are going to allow ourselves to change the pressure and see what happens to the freezing point and the boiling point of water, and watch if they move to a different temperature if we are at different pressures.

    00:50 And in fact that's exactly what happens. And so now what we've done is we've allowed ourselves to change the pressure to a different pressure, maybe a higher pressure or a lower pressure if you were for example living at a higher altitude.

    01:01 At these different pressures. The boiling point and the freezing point of your water will move to a different temperature.

    01:07 And the temperature at which water boils or freezes is in fact going to depend on whatever pressure you are at.

    01:14 So in this particular diagram we could see we have a region where liquid is solid, where liquid water is solid.

    01:20 That is at low temperatures and at higher pressures.

    01:23 We can see the liquids region which has a particular temperature and pressure region that it resides in as well, and then we have a separate gas region in which water is a simply vapor.

    01:33 It turns out that water has a special property for this phase diagram showing the temperature and pressure, which is that the line between the solid and liquid for water is in fact slightly reversed because it turns out that solid water or ice is less dense than a liquid water.

    01:50 So when water freezes, the density of that water goes down because the water expands slightly to become ice.

    01:57 So for this reason, we have slightly a different phase diagram for water and it has some key interesting properties to it.

    02:04 So seeing that this line is bent backwards, imagine that you had some solid water or ice at a particular point.

    02:11 If you look at that point and simply raise the pressure of that ice, you could turn the ice into water without needing to change the temperature at all.

    02:20 This is in fact what makes ice so slippery if you're walking on it.

    02:24 Because if you step on ice, you've increased the pressure of the ice which can make a very thin layer right on the surface of the ice turn into water, and that makes it very slippery for your feet as you walk across ice.

    02:35 On these phase diagrams we can also identify something called the triple point which is exactly the point where this three phases -- the solid, liquid and gas -- are at the same point.

    02:45 They're in equilibrium with each other right at the triple point, right at the particular temperature and pressure for that substance.

    02:52 And each substance would have its own triple point where these three phases are in equilibrium.

    02:57 Finally, something we just should be aware of but won't be needing to go into the details of, if we increase the temperature and the pressure to very high levels, we can in fact come to new phases of matter.

    03:09 So we could call this a super critical fluid.

    03:11 It's not so important that we know any particular properties of these super critical fluids.

    03:17 There won't be super detailed questions about these fluids, but it is important to understand that the solid, liquid, and gas phases are not the only phases of matter.

    03:26 There can be other ones at more exotic or uncommon temperatures and pressures.

    03:31 So this brings us to the end of our discussion on the phase diagrams And how the different phases interact with each other, as well as to the end of our discussion on thermodynamics and this whole lecture series.

    03:42 I highly recommend you do a lot of practice problems and ask a lot of questions as you go and prepare for your exams.

    03:49 And for the last time, thanks for listening.

    About the Lecture

    The lecture Phase Diagrams by Jared Rovny is from the course Thermodynamics and Thermochemistry.

    Included Quiz Questions

    1. At pressures lower than the atmospheric pressure the boiling point of water is above 100° C
    2. Under pressure ice can melt into water
    3. The triple point is a point on the pressure-temperature diagram which the three phases are in equilibrium
    4. At very high temperatures and pressures phases of matter different from solid, liquid or vapor can exist
    5. The triple point can vary for different substances

    Author of lecture Phase Diagrams

     Jared Rovny

    Jared Rovny

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