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Most everything that a chemist does involves mixing things

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together in some way, so I thought now would be a good

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time to introduce some terminology and some ideas

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involved with mixtures.

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And in particular, I'll talk about homogenized or

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homogeneous mixtures.

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Homogenized implies that they were made homogeneous, but

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maybe they were homogeneous to begin with.

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So homogeneous mixtures, and you're probably asking what

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does homogeneous mean?

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It means uniform or consistent throughout, that there's not a

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lot of variation in the mixture itself.

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And the most common word or the example of this is

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homogenized milk.

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I don't know if you've had the privilege of directly milking

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a cow or a goat, but you'll find very quickly that if you

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do, that the fat, the milk fat and the non-milk fat,

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separates very quickly.

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So if this is regular, straight-from-the-udder milk,

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you'll have a layer of fat that shows up there, and all

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of this stuff over here is much more liquidy.

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What homogenized milk does is it makes sure that all of this

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fat is dispersed completely evenly through the milk.

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So that's why, when you go to your local grocery store and

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you buy homogenized milk, it's all nice and creamy

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throughout.

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And you don't get this-- I guess some people actually

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like it, but you don't get this nice sheen

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of fat at the top.

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And it all goes down a little bit smoother.

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So that's what homogenized means.

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So a homogeneous mixture is the same thing: even and

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consistent throughout.

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Now, that is further divided, depending on how large the

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particles that are diluted in the mixture are.

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So if we have a situation where the particles are larger

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than 500 nanometers-- and that might sound large, but it

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still isn't that big, because a nanometer is one-billionth

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of a meter.

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But if we have particles mixed in, say, water-- but it

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doesn't have to be mixed in a fluid, or especially it

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doesn't have to be water-- that are greater than 500

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nanometers, we're dealing with a suspension.

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And the one characteristic that people associate with a

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suspension is that whatever you suspend in it, whatever

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you mix in-- let's say I have a suspension here.

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Maybe it's water, just because it's easy for me to visualize.

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And I have some big particles here-- that they'll stay in

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the water for some amount of time, but eventually they'll

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deposit on the bottom of the container.

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Or sometimes, they'll actually float to the top.

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Depending on whether they're heavier or depending on their

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buoyancy, they'll either float to the top or the bottom.

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In order to get it back into the suspension state, you've

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got to shake the bottle.

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So two examples I can think of this.

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One is mixed paint, right?

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Before you paint your walls, you've got to make sure that

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the can is well shaken.

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Otherwise, you're going to get an inconsistent coat.

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The other, that's close to my heart, is chocolate milk.

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Because when you mix it up, it's nice and it seems

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homogeneous, right?

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It's nice.

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And I already have milk here.

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So right at first when you stir it nice, you have all the

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little chocolate clumps in there, at least the chocolate

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when I make it is like that.

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But then if you let it sit around for a long time,

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eventually all the chocolate is going to collect at the

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bottom of the glass.

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Actually, different parts of it.

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I've seen situations where the sugar all collects at the

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bottom and then you have these little clumps at the top.

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But you get the idea, that the mixture separates.

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And that's because the particles in either the paint

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or the chocolate milk are greater than 500 nanometers.

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Now, if we get to a range that's a little bit smaller

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than that, if we get to the situation where we're at 2 to

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500 nanometers, we're dealing with a colloid.

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That word, I remember in seventh grade, I think you

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learned it in science class: the colloid.

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And a friend and I, we thought it was a more appropriate word

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for some type of gastrointestinal problem.

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But it's not a gastrointestinal problem.

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It's a type of homogeneous mixture.

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And it's a homogeneous mixture where the particles are small

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enough that they stay suspended.

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So maybe they could call it a better suspension or a

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permanent suspension.

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So here the molecules are-- so let's say that's my mixture.

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So water, maybe it's water.

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It doesn't have to be water.

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It could be air or whatever.

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Now the molecules are small enough

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that they stay suspended.

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So the forces, either their buoyancy or the force--

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actually, more important, the forces between the particles

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and the intermolecular forces kind of outweigh these

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particles' tendencies to want to exit the

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solution in either direction.

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And so common examples of these-- well, the one I always

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think of, for me, the colloid is Jell-O.

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Jell-O is the brand name, but gelatin is a colloid.

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The gelatin molecules stay suspended in the-- the gelatin

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powder stays suspended in the water that you add to it, and

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you can leave it in the fridge forever and it just won't ever

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deposit out of it.

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Other examples, fog.

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Fog, you have water molecules inside of an air mixture.

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And then you have smoke.

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Fog and smoke, these are examples of aerosols.

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This is an aerosol where you have a liquid in the air.

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This is an aerosol where you have a solid in the air.

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Smoke just comes from little dark particles that are

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floating around in the air, and they'll never

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come out of the air.

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They're small enough that they'll always just float

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around with the air.

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Now, if you get below 2 nanometers-- maybe I should

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eliminate my homogenized milk.

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If you get below 2 nanometers-- I'm trying to

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draw in black.

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If you're less than 2 nanometers, you're now in the

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realm of the solution.

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And although this is very interesting in the everyday

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world, a lot of things that we-- and this is a fun thing

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to think about in your house, or when you encounter things,

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is this a suspension?

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Well, first, you should just think is it homogeneous?

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And then think is it a suspension?

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Is it eventually going to not be in the state it's in and

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then I'll have to shake it?

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Is it a colloid where it will stay in this kind of nice,

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thick state in the case of Jell-O or fog or smoke where

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it will really just stay in the state that

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it's already in?

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Or is it a solution?

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And solution is probably the most important in chemistry.

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Although people talk about colloids and suspensions, 99%

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of everything we'll talk about in

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chemistry involves solutions.

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And in general, it's an aqueous solution, when you

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stick something in water.

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So sometimes you'll see something like this.

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You'll see some compound x in a reaction and right next to

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it they'll write this aq.

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They mean that x is dissolved in water.

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It's a solute with water as the solvent.

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So actually, let me put that terminology here, just because

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I used it just now.

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So you have a solute.

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This is the thing that's usually whatever you have a

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smaller amount of, so thing dissolved.

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And then you have the solvent.

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This is often water or it's the thing

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that's in larger quantity.

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Or you can think of it as the thing that's all around or the

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thing that's doing the dissolving.

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For example, you could have sodium

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chloride in aqueous solution.

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That means it's in water.

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And what's happening is that the sodium and the chloride

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particles are dispersing.

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So sodium is positive.

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Chloride is negative, an ion, because it took away the atom

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from the sodium.

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But when you put it in the presence of water-- remember,

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water, you know, you have all the oxygen and the hydrogens.

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I've done this tons of times already.

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Oxygen and hydrogen.

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This is partially positive over here on this end.

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This is partially negative over here, so you'll have

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these larger-- the positive sodium cation will separate

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from the chloride and be attracted to the oxygen ends

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of the water.

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And then the chloride, the negative anion, will be

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attracted to the hydrogen ends of the water.

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That's what allows it to get dissolved.

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Because these ions have some charge, they like to mix in

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with the water, which has these hydrogens, or has this

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polarity to it.

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And see, the chlorine, I'll draw here.

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It will be over here with a minus charge.

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So this is probably the single most

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important thing to realize.

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And just so you get a sense of what 2 nanometers is, this is

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still pretty big.

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It allows for molecules that have anywhere from-- actually,

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a good number of atoms. If you think of even a fairly large

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atom, cesium, the cesium atom, which is one of the largest--

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at least one of the largest that you might encounter,

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there are larger-- is on the order of 2.6 angstroms. An

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angstrom is a tenth of a nanometer, so that's 0.26

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nanometers.

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So, for example, if you wanted a molecule that would get you

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out of the solution state and into the colloid, and we're

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talking in three dimensions here.

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So in three dimensions you could actually fit a lot of

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cesium atoms within a 2-nanometer diameter sphere.

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Cesium doesn't bond in that way, but I think you get the

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idea that this is a scale of, you know, on the order of 20

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to 30 atoms can be in this molecule.

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Actually, even more than that, especially if you have very

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small atoms like hydrogen.

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So the next question is how do you measure these things?

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And there's a lot of different ways to measure concentration.

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We already actually used one of them,

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which is mole fraction.

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And this is the number of moles of solute divided by the

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number of moles in the whole solution, or moles of solute

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plus moles of solvent.

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And we did this when we figured out the partial

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pressure problems. Because in order to figure out the

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partial pressure of something, you just figured out what the

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total pressure is, and then you said what is the mole

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fraction of, say, oxygen in the mixture?

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And then you multiply that times the partial pressure and

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you got the mole fraction.

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Now, the ones that show up a lot in chemistry-- and since

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their words are so similar can get a little confusing-- are

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molarity, not to be confused with morality.

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One day I'll make a video on that once I figure out enough

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about it-- and molality.

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And molarity, it sounds like the right one because it's

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almost like morality and it has the word molar in it,

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which is for me more intuitive than the word molal.

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But molarity in my mind is not a good measure because it's

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moles of solute, so what you're dissolving into it,

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divided by liters of solution.

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And the reason why I don't like molarity much-- and

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you'll see that molality is actually, at least in my

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opinion, more useful.

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But the reason why I don't like this is because liters of

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solution is not invariant.

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It changes, right?

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We've learned that a bunch.

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You know, pV equals nRT.

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The volume-- which liters is a measure of-- volume can vary

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with pressure and temperature.

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So the molarity is going to vary with pressure and

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temperature for the same solution.

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If you just take the same solution and take it to Denver

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or take it to Death Valley, the molarity of the solution

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is going to change.

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So, to me, that isn't that satisfying of a measure of

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concentration.

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Molality, on the other hand, is moles of solute.

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So the numerator in both cases is essentially the number of

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solute particles we have-- the number of particles we have

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divided by the mass of the solvent, or the kilograms of

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whatever we're being dissolved into.

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And the reason why this one is better is because no matter

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where you go, whether you're in Denver or Death Valley,

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moles aren't going to change.

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They didn't change here either.

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And the mass won't change.

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Now, the pressure and the volume and the temperature

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might change, but the mass won't change unless you're

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adding more or less solvent.

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So this, in my mind, is kind of the better one.

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And actually, I'll put a little contest on this video,

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if you all can think of good ways to remember the

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difference between molality and molarity.

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Because, frankly, I think this is one of the

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most-- it's not confusing.

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They're very simple definitions.

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But I think a lot of people get confused, especially a

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year or two out of taking chemistry class.

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If someone says, oh, what's the difference between

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molality and molarity?

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You're like, oh, there was a difference with volume and

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mass, but I forget which is which.

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And I'll leave it up to you guys to think of a good way to

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memorize the difference between the two.

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See you in the next video.