States of matter
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States of Matter
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States of Matter Follow-Up
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Specific Heat, Heat of Fusion and Vaporization
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Chilling Water Problem
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Phase Diagrams
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Van Der Waals Forces
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Covalent Networks, Metallic, and Ionic Crystals
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Vapor Pressure
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Suspensions, Colloids and Solutions
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Solubility
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Boiling Point Elevation and Freezing Point Suppression
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Change of State Example
Phase Diagrams Understanding and interpreting phase diagrams
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- All of the phase changes we've been doing so far have been
- under constant pressure conditions, and, in
- particular, with the problems that I've been doing with
- water phase changes in the last couple of videos, it was
- at atmospheric pressure, at least at sea level atmospheric
- pressure, or at 1 atmosphere.
- So it was done-- well, I'll explain this
- diagram in a second.
- But we all know that in the universe, pressure isn't
- always constant and it definitely isn't always
- constant at 1 atmosphere.
- 1 atmosphere was defined as the pressure at
- sea level on Earth.
- Obviously, pressure will vary wildly if you go to smaller
- planets or larger planets, or have thicker atmospheres, or
- if we're just doing different types of applications dealing
- with gases and liquids and solids.
- So what I've drawn here is a phase diagram.
- Let me write that down.
- And there are many forms of phase diagrams. This is the
- most common form that you might see in your chemistry
- class or on some standardized test, but what it captures is
- the different states of matter and when they transition
- according to temperature and pressure.
- This is the phase diagram for water.
- So just to understand what's going on here, is that on this
- axis, I have pressure.
- On the x-axis, I have temperature, and at any given
- point, this diagram will tell you whether you're dealing
- with a solid, so solid will be here, a liquid will
- be here, or a gas.
- For example, if I told you that I was at 0 degrees, let's
- say 0 degrees is right there, if I'm at 0 degrees Celsius
- and 1 atmosphere, where am I?
- So 0 degrees, 1 atmosphere, I'm right at
- that point right there.
- So I'm at a boundary point between solids and liquids at
- 1 atmosphere of pressure, right?
- This is when we're at 1 atmosphere of pressure.
- So this coincides with our traditional notion of when ice
- freezes or when it melts at 0 degrees.
- If we made the pressure higher, what happens?
- Well, then ice starts melting at a lower temperature, right?
- So this is pressure going up, so pressure going up, let's
- say-- I don't know what this is.
- This is maybe 10 atmospheres, ten times Earth's atmospheric
- pressure at sea level, then all of a sudden, the
- temperature at which solid turns into liquid-- this
- transition is solid to liquid --the temperature at which
- that happens will go down.
- Likewise, if we lower the pressure, if we go to Denver
- and it's a mile high, pressure is lower because we have less
- of the atmosphere above us, then all of a sudden, the
- freezing point increases, so the freezing point will be
- something above 1 degree.
- This isn't drawn completely to scale, but the idea is your
- ice would actually freeze a little bit faster and would
- freeze at a higher temperature in Denver than it would at the
- bottom of the Dead Sea or in Death Valley at some below sea
- level point on the planet.
- Now, this transition is the transition between
- anything and gas.
- And we're very familiar, this is 1 atmosphere.
- And remember, this is water we're dealing with.
- This is the diagram for water, so at 1 atmosphere, this is
- kind of the stuff that we're used to seeing.
- Let me draw a line here.
- So at 1 atmosphere, 0 degrees is where solid, or ice, turns
- into liquid water.
- And then we go up here, so we keep going at a higher,
- higher, higher temperature, and then here, this would be,
- since we're at 1 atmosphere, this is 100 degrees Celsius
- right there.
- And that's the point at 1 atmosphere of pressure where
- liquid turns into gas, or water vaporizes,
- or the liquid boils.
- All of those are acceptable ways to think about that.
- But what happens when we go to low pressure?
- Once again, let's take our little trip to Denver.
- So that's Denver right there.
- It's not that drastic.
- I'm just doing that for education purposes.
- Or even better, let's say Mount Everest. Mount Everest,
- very low pressure there.
- Then our freezing point, we already said that goes up when
- you lower the pressure, and your boiling point goes down,
- so it's much easier to boil something on the top of Mount
- Everest than it is to boil it at the bottom or at the lowest
- point in Death Valley or the Dead Sea.
- The intuition behind that is if I have a liquid, a bunch of
- molecules in liquid form, and they're touching each other,
- but they have enough kinetic energy to move past each
- other, so they're flowing past each other, they're kind of
- rubbing up against each other, one of the reasons why they
- don't just evaporate, why this guy doesn't just jump up
- there, is that there's air above him.
- There's air pressure.
- And air pressure, we've learned about this
- when we did PV nRT.
- That's a bunch of gas molecules, and the pressure
- they're creating is essentially caused by their
- temperature and their kinetic energy.
- And they sit there, and they bounce, and they essentially
- keep these heavier molecules from going up.
- They keep them from essentially separating from
- each other and turning into a gas.
- So the more pressure you have, the harder it is for these
- guys to escape.
- On the other hand, if we're in a vacuum, if we're doing this
- on the surface of the moon and there's none of these guys
- there, then just a little slight bump.
- Even though this guy's still a little bit attracted to over
- here, they're still attracted to each other.
- But just a little bit of bump, since there's no pressure up
- here on the surface of the moon, might allow this guy to
- escape and go straight to a gas.
- So when you lower the pressure, it's just that much
- easier to go from liquid to gas or even from solid to gas.
- And you might say, Sal, that's a bizarre
- concept, solid to gas.
- It turns out, if you get to low enough pressures here, I
- mean, let's say this is-- Actually, there's probably not
- stuff here.
- This is probably close to a vacuum right here.
- You could go from ice-- So if you took ice and you were on
- the moon and you were at the right temperature-- this is
- maybe some negative degrees Celsius temperature; I don't
- know what the exact temperature is --your ice on
- the moon would go directly from ice to a gas.
- Because there's this huge vacuum here, so these
- molecules would say, hey, there's all this space to fill
- and if they just get bumped a little bit, they're just going
- to escape and turn into a gas.
- You might say, oh, Sal, that's a strange phenomenon.
- It only exists on the moon.
- And to rebut that comment, I've drawn the phase diagram
- for carbon dioxide.
- It's all around you.
- You're exhaling it as we speak.
- Your plants in the room are hopefully inhaling it, but
- carbon dioxide at 1 atmosphere has a very different behavior
- than water.
- This is carbon dioxide at 1 atmosphere.
- Just so you know, this scale is definitely
- not drawn to scale.
- The difference between 1 atmosphere and 5 atmospheres
- is not the same as between 5 atmospheres and 73.
- Likewise, this is not drawn to scale here.
- This is a much larger distance than this.
- If I had to really draw it to scale, I'd have to stretch
- this chart out or do a
- logarithmic chart or something.
- But anyway, I was talking about carbon dioxide.
- So this is carbon dioxide solid, and this is gas, and
- this is liquid carbon dioxide.
- So at 1 atmosphere, let's say you live at sea level, like
- you're in New Orleans, I guess that's a little bit below sea
- level-- that's where I grew up --if you were able to get your
- fridge down to minus 80 degrees Celsius, the carbon
- dioxide would actually freeze.
- And you're actually not too unfamiliar with that, or at
- least you haven't been if you've gone to some-- I don't
- know if they still use it for smoke machines or for visual
- effects on stage, but this is dry ice.
- It's frozen carbon dioxide.
- If you're at sea level atmospheric pressure, as soon
- as you get above this minus 78 and 1/2 degrees Celsius, it
- sublimates to gas.
- So that process, where you go straight from a solid to a
- gas, is sublimation.
- And that's why dry ice, when you see it, you don't see
- liquid dry ice or you don't see it at standard pressures.
- I've never seen liquid carbon dioxide.
- In fact, to get liquid carbon dioxide, you have to get above
- 5 atmospheres so you have to get above five times the sea
- level pressure on Earth, and you're really not going to see
- that in natural conditions on Earth.
- You might see that on Jupiter or Saturn where you have
- tremendous pressures because of the gravity and all of the
- atmosphere above you.
- Liquid carbon dioxide, you might see-- I don't know if
- Jupiter actually has carbon, but you'll probably see it on
- other huge massive planets that are gas giants.
- But on Earth, this process is just called sublimation.
- It's just a neat word.
- Or it's sublimating.
- It's going straight from solid to gas and it's something
- you've seen with dry ice.
- Now, there's a couple other interesting points here and
- you're probably already noticing them.
- This right here is called the triple point, because right
- here at this-- Well, in the case of carbon dioxide, at 5
- atmospheres and minus 56 degrees Celsius, the carbon
- dioxide is in a state of equilibrium between the ice,
- the liquid and the gas.
- It's a little bit of all of the three.
- And if you just nudge it in one direction or another by
- nudging the pressure or the temperature,
- it'll go in that direction.
- Similarly, water's triple point is right here.
- It's at a much lower pressure than we're
- used to dealing with.
- This is 0.611 kilopascals, or just 611 pascals, which is
- 5/1000 of an atmosphere.
- So if you go down to 5/1000 of an atmosphere and you go a
- little bit above 0 degrees Celsius, you're at the triple
- point of water.
- where water can take on any of these states if you just nudge
- it in one direction or another.
- Now, the other interesting point on these
- charts is up here.
- This is the critical point.
- Sounds very important.
- Critical point.
- And that's the point at which if you increase the
- temperature beyond that or the pressure beyond that, you're
- dealing with a supercritical fluid.
- It sounds very exciting.
- So above here, you have a supercritical fluid.
- So very high temperature, very high pressure.
- It's so high temperature that it wants to be a gas, but
- you're putting so much pressure on it that it wants
- to be a fluid, so it's a little bit of both.
- And actually, in the case of water, supercritical water is
- actually used as a solvent.
- Because you can imagine, it's kind of like liquid water in
- that things can dissolve in it, but it's so high
- temperature and it can diffuse into solids that it's really
- good at just getting whatever you want out of whatever
- you're trying to clean or somehow get into or get salt
- put into the water.
- So this is supercritical fluid and it's a fun
- thing to think about.
- But anyway, I just wanted to expose you to these phase
- diagrams. Everything I've done so far was at a constant
- pressure and I changed the temperature, but you can also
- read them the other way.
- If I'm at 100 degrees, and I go from-- Well, let's say I'm
- at 110 degrees, where at sea level is comfortably in the
- gaseous phase for-- So this is 110 degrees for water.
- It's water vapor.
- But if I were to increase the pressure and I keep increasing
- the pressure and maybe I dig a hole or something or I go into
- the ocean, then it's going to condense into water or it's
- going to condense into a liquid.
- If I did that experiment here, when I increase the pressure,
- I'm going to reverse sublimate.
- And I think I wrote down a word for what that is.
- Let me see if I wrote it down someplace.
- Oh, no, I didn't.
- I didn't write it down.
- But essentially it's something like condense, but the word is
- escaping me at the second.
- It's something on the word of
- condensing or falling together.
- Anyway, I forget the word, but it'll go straight from a gas
- to a solid.
- So these are pretty neat diagrams. They actually tell a
- lot about different substances and then tell you what happens
- when the pressure or the temperature changes.
Be specific, and indicate a time in the video:
At 5:31, how is the moon large enough to block the sun? Isn't the sun way larger?
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