Most everything that a chemist does involves mixing things
together in some way, so I thought now would be a good
time to introduce some terminology and some ideas
involved with mixtures.
And in particular, I'll talk about homogenized or
Homogenized implies that they were made homogeneous, but
maybe they were homogeneous to begin with.
So homogeneous mixtures, and you're probably asking what
does homogeneous mean?
It means uniform or consistent throughout, that there's not a
lot of variation in the mixture itself.
And the most common word or the example of this is
I don't know if you've had the privilege of directly milking
a cow or a goat, but you'll find very quickly that if you
do, that the fat, the milk fat and the non-milk fat,
separates very quickly.
So if this is regular, straight-from-the-udder milk,
you'll have a layer of fat that shows up there, and all
of this stuff over here is much more liquidy.
What homogenized milk does is it makes sure that all of this
fat is dispersed completely evenly through the milk.
So that's why, when you go to your local grocery store and
you buy homogenized milk, it's all nice and creamy
And you don't get this-- I guess some people actually
like it, but you don't get this nice sheen
of fat at the top.
And it all goes down a little bit smoother.
So that's what homogenized means.
So a homogeneous mixture is the same thing: even and
Now, that is further divided, depending on how large the
particles that are diluted in the mixture are.
So if we have a situation where the particles are larger
than 500 nanometers-- and that might sound large, but it
still isn't that big, because a nanometer is one-billionth
of a meter.
But if we have particles mixed in, say, water-- but it
doesn't have to be mixed in a fluid, or especially it
doesn't have to be water-- that are greater than 500
nanometers, we're dealing with a suspension.
And the one characteristic that people associate with a
suspension is that whatever you suspend in it, whatever
you mix in-- let's say I have a suspension here.
Maybe it's water, just because it's easy for me to visualize.
And I have some big particles here-- that they'll stay in
the water for some amount of time, but eventually they'll
deposit on the bottom of the container.
Or sometimes, they'll actually float to the top.
Depending on whether they're heavier or depending on their
buoyancy, they'll either float to the top or the bottom.
In order to get it back into the suspension state, you've
got to shake the bottle.
So two examples I can think of this.
One is mixed paint, right?
Before you paint your walls, you've got to make sure that
the can is well shaken.
Otherwise, you're going to get an inconsistent coat.
The other, that's close to my heart, is chocolate milk.
Because when you mix it up, it's nice and it seems
And I already have milk here.
So right at first when you stir it nice, you have all the
little chocolate clumps in there, at least the chocolate
when I make it is like that.
But then if you let it sit around for a long time,
eventually all the chocolate is going to collect at the
bottom of the glass.
Actually, different parts of it.
I've seen situations where the sugar all collects at the
bottom and then you have these little clumps at the top.
But you get the idea, that the mixture separates.
And that's because the particles in either the paint
or the chocolate milk are greater than 500 nanometers.
Now, if we get to a range that's a little bit smaller
than that, if we get to the situation where we're at 2 to
500 nanometers, we're dealing with a colloid.
That word, I remember in seventh grade, I think you
learned it in science class: the colloid.
And a friend and I, we thought it was a more appropriate word
for some type of gastrointestinal problem.
But it's not a gastrointestinal problem.
It's a type of homogeneous mixture.
And it's a homogeneous mixture where the particles are small
enough that they stay suspended.
So maybe they could call it a better suspension or a
So here the molecules are-- so let's say that's my mixture.
So water, maybe it's water.
It doesn't have to be water.
It could be air or whatever.
Now the molecules are small enough
that they stay suspended.
So the forces, either their buoyancy or the force--
actually, more important, the forces between the particles
and the intermolecular forces kind of outweigh these
particles' tendencies to want to exit the
solution in either direction.
And so common examples of these-- well, the one I always
think of, for me, the colloid is Jell-O.
Jell-O is the brand name, but gelatin is a colloid.
The gelatin molecules stay suspended in the-- the gelatin
powder stays suspended in the water that you add to it, and
you can leave it in the fridge forever and it just won't ever
deposit out of it.
Other examples, fog.
Fog, you have water molecules inside of an air mixture.
And then you have smoke.
Fog and smoke, these are examples of aerosols.
This is an aerosol where you have a liquid in the air.
This is an aerosol where you have a solid in the air.
Smoke just comes from little dark particles that are
floating around in the air, and they'll never
come out of the air.
They're small enough that they'll always just float
around with the air.
Now, if you get below 2 nanometers-- maybe I should
eliminate my homogenized milk.
If you get below 2 nanometers-- I'm trying to
draw in black.
If you're less than 2 nanometers, you're now in the
realm of the solution.
And although this is very interesting in the everyday
world, a lot of things that we-- and this is a fun thing
to think about in your house, or when you encounter things,
is this a suspension?
Well, first, you should just think is it homogeneous?
And then think is it a suspension?
Is it eventually going to not be in the state it's in and
then I'll have to shake it?
Is it a colloid where it will stay in this kind of nice,
thick state in the case of Jell-O or fog or smoke where
it will really just stay in the state that
it's already in?
Or is it a solution?
And solution is probably the most important in chemistry.
Although people talk about colloids and suspensions, 99%
of everything we'll talk about in
chemistry involves solutions.
And in general, it's an aqueous solution, when you
stick something in water.
So sometimes you'll see something like this.
You'll see some compound x in a reaction and right next to
it they'll write this aq.
They mean that x is dissolved in water.
It's a solute with water as the solvent.
So actually, let me put that terminology here, just because
I used it just now.
So you have a solute.
This is the thing that's usually whatever you have a
smaller amount of, so thing dissolved.
And then you have the solvent.
This is often water or it's the thing
that's in larger quantity.
Or you can think of it as the thing that's all around or the
thing that's doing the dissolving.
For example, you could have sodium
chloride in aqueous solution.
That means it's in water.
And what's happening is that the sodium and the chloride
particles are dispersing.
So sodium is positive.
Chloride is negative, an ion, because it took away the atom
from the sodium.
But when you put it in the presence of water-- remember,
water, you know, you have all the oxygen and the hydrogens.
I've done this tons of times already.
Oxygen and hydrogen.
This is partially positive over here on this end.
This is partially negative over here, so you'll have
these larger-- the positive sodium cation will separate
from the chloride and be attracted to the oxygen ends
of the water.
And then the chloride, the negative anion, will be
attracted to the hydrogen ends of the water.
That's what allows it to get dissolved.
Because these ions have some charge, they like to mix in
with the water, which has these hydrogens, or has this
polarity to it.
And see, the chlorine, I'll draw here.
It will be over here with a minus charge.
So this is probably the single most
important thing to realize.
And just so you get a sense of what 2 nanometers is, this is
still pretty big.
It allows for molecules that have anywhere from-- actually,
a good number of atoms. If you think of even a fairly large
atom, cesium, the cesium atom, which is one of the largest--
at least one of the largest that you might encounter,
there are larger-- is on the order of 2.6 angstroms. An
angstrom is a tenth of a nanometer, so that's 0.26
So, for example, if you wanted a molecule that would get you
out of the solution state and into the colloid, and we're
talking in three dimensions here.
So in three dimensions you could actually fit a lot of
cesium atoms within a 2-nanometer diameter sphere.
Cesium doesn't bond in that way, but I think you get the
idea that this is a scale of, you know, on the order of 20
to 30 atoms can be in this molecule.
Actually, even more than that, especially if you have very
small atoms like hydrogen.
So the next question is how do you measure these things?
And there's a lot of different ways to measure concentration.
We already actually used one of them,
which is mole fraction.
And this is the number of moles of solute divided by the
number of moles in the whole solution, or moles of solute
plus moles of solvent.
And we did this when we figured out the partial
pressure problems. Because in order to figure out the
partial pressure of something, you just figured out what the
total pressure is, and then you said what is the mole
fraction of, say, oxygen in the mixture?
And then you multiply that times the partial pressure and
you got the mole fraction.
Now, the ones that show up a lot in chemistry-- and since
their words are so similar can get a little confusing-- are
molarity, not to be confused with morality.
One day I'll make a video on that once I figure out enough
about it-- and molality.
And molarity, it sounds like the right one because it's
almost like morality and it has the word molar in it,
which is for me more intuitive than the word molal.
But molarity in my mind is not a good measure because it's
moles of solute, so what you're dissolving into it,
divided by liters of solution.
And the reason why I don't like molarity much-- and
you'll see that molality is actually, at least in my
opinion, more useful.
But the reason why I don't like this is because liters of
solution is not invariant.
It changes, right?
We've learned that a bunch.
You know, pV equals nRT.
The volume-- which liters is a measure of-- volume can vary
with pressure and temperature.
So the molarity is going to vary with pressure and
temperature for the same solution.
If you just take the same solution and take it to Denver
or take it to Death Valley, the molarity of the solution
is going to change.
So, to me, that isn't that satisfying of a measure of
Molality, on the other hand, is moles of solute.
So the numerator in both cases is essentially the number of
solute particles we have-- the number of particles we have
divided by the mass of the solvent, or the kilograms of
whatever we're being dissolved into.
And the reason why this one is better is because no matter
where you go, whether you're in Denver or Death Valley,
moles aren't going to change.
They didn't change here either.
And the mass won't change.
Now, the pressure and the volume and the temperature
might change, but the mass won't change unless you're
adding more or less solvent.
So this, in my mind, is kind of the better one.
And actually, I'll put a little contest on this video,
if you all can think of good ways to remember the
difference between molality and molarity.
Because, frankly, I think this is one of the
most-- it's not confusing.
They're very simple definitions.
But I think a lot of people get confused, especially a
year or two out of taking chemistry class.
If someone says, oh, what's the difference between
molality and molarity?
You're like, oh, there was a difference with volume and
mass, but I forget which is which.
And I'll leave it up to you guys to think of a good way to
memorize the difference between the two.
See you in the next video.
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