And what I want you to think about,

if you had a pure sample of each,

which of those pure samples would have

the highest boiling point, second highest,

third highest, and fourth highest?

Pause this video, and try to figure that out.

All right, now to figure that out,

it really just boils down to

which of these has the highest intermolecular forces

when they're in a liquid state?

Because if you have high intermolecular forces,

it would take a lot of energy or a higher boiling point

to really overcome those intermolecular forces

and get to a gas state.

So let's think about the intermolecular forces

that we have studied.

So I will start with hydrogen bonds,

hydrogen bonds.

'Cause you could really view those,

those are the strongest of the dipole-dipole interactions,

and they're going to be stronger

than your London dispersion forces.

We can see that diethyl ether won't form hydrogen bonds.

We don't see any bonds between hydrogen

and an oxygen, a nitrogen, or a fluorine.

Ethanol has one oxygen-hydrogen bond.

Methanol also has one oxygen-hydrogen bond.

Water has two oxygen-hydrogen bonds.

So if I had to rank the hydrogen bond contribution

to the intermolecular forces,

I would put water as number one

'cause it can form the most hydrogen bonds.

I would put methanol and ethanol as a tie for second.

And then I would put diethyl ether last

'cause it can't form hydrogen bonds.

So just looking at this,

I know that water's going to have the highest boiling point.

Diethyl ether is going to have the lowest boiling point.

But what about the difference between methanol and ethanol?

And we could think about other types of dipole forces,

but not a lot that you could intuit just by eyeballing them.

They might actually have similar dipole moments

on a molecular basis.

But we can think about London dispersion forces.

I'll do this in a different color.

So London dispersion forces.

And if we're just trying to,

actually I'll rank all of them.

So London dispersion forces are proportional to

how polarizable a molecule is,

which is proportional to how large its electron cloud is,

which is proportional to its molar mass.

And it's clear that diethyl ether

has the highest molar mass,

followed by ethanol,

followed by methanol,

followed by water.

How did I know that?

Well, you literally can take atoms away

from the diethyl ether to get to an ethanol.

And you can literally take atoms away

from that to get to a methanol.

And you can literally take atoms away

from that to get to a water.

So we know that this is the order of molar mass.

And so London dispersion forces,

I wouldn't make that change the ranking

between water or diethyl ether because these are going

to be a lot weaker than those hydrogen bonds.

But they can be useful for the tiebreaker

between ethanol and methanol.

And so my overall ranking on boiling points,

the highest boiling point I would put would be water,

followed by, since ethanol won the tiebreaker,

followed by ethanol,

followed by methanol,

and then the lowest boiling point would be diethyl ether.

And if we look at the actual data,

it's consistent with what we just talked about.

We can see very clearly

that water has the highest boiling point,

ethanol is second,

methanol is third,

and diethyl ether was fourth,

completely consistent with our intuition.

Now, what's also interesting here,

you might have noticed, is this thing called vapor pressure.

And you might have also noticed that vapor pressure seems

to trend the opposite way as boiling point.

The things that have the high boiling point

have the low vapor pressure,

and the things that have the low boiling point

have a high vapor pressure.

So what are we talking about, why,

about vapor pressure, and why do we see this relationship?

And I'm not going to go deep into vapor pressure.

There'll be other videos on that on Khan Academy.

But just to get you a sense,

imagine a closed container here.

And I put one of these,

a sample of one of these molecules in a liquid state,

and I'm gonna just draw the molecules,

clearly not drawn to scale, as these little circles.

And the temperature matters,

so let's say that this is at 20 degrees Celsius.

Now, you might notice, at 20 degrees Celsius,

it's lower than the boiling point

of all of these characters.

So for the most part, they're going to be in a liquid state,

but we know that not every one of these molecules is moving

with the exact same kinetic energy.

The temperature, you could view as a measure

of the average kinetic energy of the molecules,

but they're all bumping around into each other,

in different positions, with different amounts of velocities

and therefore different kinetic energies.

And so every now and then, you're going to have a molecule

that has the right position and the right kinetic energy

to escape and get into the vapor state,

into a gaseous state.

And so that's going to keep happening.

But then the things that are in the gaseous state,

every now and then they're bumping into each other,

and they're bumping into the sides of the container.

And every now and then,

they might approach the surface

with the right kinetic energy, with the right position,

so that they get recaptured by the intermolecular forces

and enter a liquid state.

And so you can imagine, this will keep happening

where things go from liquid, and then they go to vapor.

But then when that vapor gets high enough

or when you could say the vapor pressure gets high enough,

remember, that pressure's just from

the vapor molecules bouncing around,

then you will get to some form of an equilibrium.

And you could imagine,

the things that have a lower boiling point,

that means they have lower intermolecular forces,

more of the vapor is going to form,

and so you're going to have a higher vapor pressure

before you get to equilibrium.

On the other hand, things with high intermolecular forces,

fewer of those molecules are going to break away,

and so you're going to have a lower vapor pressure

when you get to that equilibrium.

And you can see that very clearly here.

So I will leave you there.

We got a little bit of practice,

seeing everything we've seen so far,

and we learned a little bit about vapor pressure

and how that relates to intermolecular forces

and boiling point.