Phase changes | Middle school chemistry | Khan Academy

- [Instructor] Ice melts
at 0 degrees Celsius.

But if we take something like gold,

then it'll melt only about
1000 degrees Celsius.

Similarly, water boils at
around 100 degrees Celsius.

But if we take something like nitrogen,

well, it'll boil at a very low temperature

of -200 degrees Celsius,
slightly about that.

But the big question is
why do different materials

have different melting and boiling points?

Let's find out.

Now, to answer this question,

we need to ask a much
more fundamental question.

What keeps stuff together?

Well, if you were to look into, you know,

if you could zoom in

and look at the molecule
at the atomic level,

we'll find that all
these atoms and molecules

are actually being
attracted to each other.

There is a force of attraction
that's keeping them together.

In fact, you've probably
witnessed this force of attraction

when you've seen two drops merging to form

a bigger drop, okay?

So this force of attraction

keeps all the particles together.

And turns out that this
force of attraction

purely depends on the types of particles.

So for example, the strength
of this force of attraction

between water molecules would be different

than that between gold atoms, right?

But another thing you can see

is that particles also
have kinetic energy.

What does that depend on?

Well, kinetic energy depends
purely on temperature.

In fact, temperature is a measure

of the average kinetic
energy of the particles.

At high temperature,

the average kinetic energy is very high.

And at low temperature,

the average kinetic energy is very low.

Kinetic energy only
depends on temperature.

It has nothing to do

with which particles we're dealing with.

So you can see there are
two things over here.

First, we have the force of attraction

that's trying to keep them together.

This purely depends on
the type of particle,

but it has nothing to do with temperature.

On the other hand, we have kinetic energy

that has nothing to do
with the particle type,

but it purely depends on temperature.

And what's interesting

is that these two are kind of opposite.

The attraction force is
trying to keep them together,

whereas the kinetic energy is
trying to make the molecules

go farther away from each other.

And it's the balance between
these two that decide

what the melting and the
boiling points would be.

So let's take a concrete
example to understand that.

Let's take ice at a very low temperature,

say -10, -15 degrees Celsius.

At this temperature, again,

if we were to zoom in,

we will see the atoms and molecules

all stuck together due
to the attractive force.

And they also have kinetic energy

that's trying to make them go apart.

However, at this temperature,

it turns out the kinetic
energy is very low,

so low that the force of attraction

actually locks them into
places giving us a solid.

And the way I like to visualize this

is by using some bar graph.

So here's the force of attraction

of the water molecules over here,

and here is the kinetic energy.

Look, the level of
kinetic energy is very low

relative to the force of attraction.

And as a result, you get a solid.

But now comes the big question,

what happens if we start heating it?

Why don't you pause the video

and think about what will happen
to the force of attraction

and the kinetic energy
as we start heating it?

Will it increase, decrease,
what happens to them?

Pause and think about it.

All right, what happens with
the force of attraction?

Nothing, because that only depends

on the types of atoms and molecules.

It has nothing to do with temperature.

Whereas what happens with
the kinetic energy, ooh, ooh!

That increases with temperature,

which means as we hit this up,

the temperature rises

and the kinetic energy
will start increasing.

At one particular point,

the kinetic energy of these
particles will be high enough

that it can partially overcome
the forces of attraction.

And when that happens,

the atoms and molecules will
no longer be locked in place.

They will start moving around.

This is when solid turns into liquid.

In our case, ice starts
turning into liquid water.

And this temperature at which it happens,

for water, it happens to
be about 0 degrees Celsius.

And that temperature where liquid turns,

sorry, solid turns into liquid,

is what we call the melting point.

So the melting point of
water is 0 degrees Celsius.

Now let's keep heating it up further.

What happens as we heat it up?

Again, nothing happens with
the force of attraction,

but the kinetic energy will keep rising.

And at one particular point,

it will be high enough

that it can fully overcome
the force of attraction.

And then that happens,

these molecules will now be free,

almost completely free from each other,

freely moving about.

In other words, our liquid
starts turning into gas,

water starts turning into steam.

So the temperature at which this happens

is what we call the boiling point.

And for water, that boiling point

happens to be at 100 degrees Celsius.

And if you further heat it,

well, the steam just gets hotter,

nothing else will happen.

So when the kinetic energy
is too low to overcome

any amount of attraction, we have solid.

When the kinetic energy is high enough

to partially overcome
the force of attraction,

we have liquid.

And when the kinetic energy is high enough

to fully overcome the force of attraction,

we get a gas.

All right, now let's
reverse the whole thing.

Let's cool down our gas
and see what happens.

Again, nothing will happen
to the force of attraction

because it does not depend on temperature,

but the kinetic energy will reduce

and eventually when it goes
below the boiling point, look!

It will no longer be
able to fully overcome

the force of attraction,

which means the gas will turn into liquid.

We call this condensation,

and this point is called
the condensation point.

And you can clearly see
the condensation point

is the same thing as the boiling point.

And we've seen this before.

For example, when you,
you know, hold a plate

over, say boiling water,

we see liquid drops, that's condensation.

The steam over here has temperature

lower than the condensation point,

lower than 100 degrees Celsius

so it condenses into liquid water.

And that's why you see
the drops over there.

Okay, and what happens

if we were to reduce the
temperature even more?

Well, again, the kinetic
energy will keep reducing.

And when it's below the
melting point, look!

It will no longer be able to overcome

any force of attraction,

which means the liquid
will turn back into solid.

We call this the freezing point.

And you can see the freezing point

is the same as the melting point.

And therefore, when
liquid water, you know,

is below 0 degrees Celsius,

it freezes into ice.

Okay, so the key thing
that we see over here

is that the boiling point
and the melting point

depends a lot on the force
of attraction, right?

Now, here's a question:

What if we consider a material like gold?

Well, it turns out for gold,

the force of attraction is much higher

than that of water.

Now, actually, the attraction

is much higher in gold compared to water.

So the graph should be
much higher over here.

But don't worry about that.

But this means now the kinetic
energy needed to partially

and fully overcome the force of attraction

would be much higher than before.

And as a result, the freezing
point or the melting point

and the boiling point would
be much higher than before.

For gold, it turns out to be, you know,

about 1000 degrees Celsius

and about like close to
3000 degrees Celsius.

That's why for gold,

you need a much, much higher temperature

for it to melt.

Okay, what about nitrogen?

Well, it terms for nitrogen,

the force of attraction
is much, much lower.

And therefore, the melting
point and the boiling points

would be much lower.

And that's why it boils

at a much lower temperature

of -196 degrees Celsius actually.

That's why at room temperature,

nitrogen is a gas.

So long story short,

the temperature at
which the kinetic energy

can partially overcome
the force of attraction

is what we call the melting
or the freezing point.

That's when you have a phase
change from solid to liquid

or liquid to solid if
you're cooling it down.

And similarly, the temperature
at which the kinetic energy

is high enough to fully overcome
the force of attraction,

that's what we call the boiling point.

That's when you get a phase
change from liquid to gas,

or again, if you're cooling
it down, from gas to liquid.

And look, since these temperatures

purely depend upon how strong or weak

the attractive force is and that, in turn,

depends upon which types of
particles we are dealing with,

types of atoms and molecules
we're dealing with.

That's the reason why the
boiling points and melting points

of different particles,
different substances

would be different.