Suspensions, colloids and solutions | Chemistry | Khan Academy

November 19, 2019 0 By Kody Olson


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 homogeneous mixtures. 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 homogenized milk. 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 throughout. 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 consistent throughout. 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 homogeneous, right? It’s nice. 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 permanent suspension. 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 nanometers. 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 concentration. 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.