WEBVTT

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- [Instructor] Ice melts
at 0 degrees Celsius.

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But if we take something like gold,

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then it'll melt only about
1000 degrees Celsius.

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Similarly, water boils at
around 100 degrees Celsius.

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But if we take something like nitrogen,

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well, it'll boil at a very low temperature

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of -200 degrees Celsius,
slightly about that.

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But the big question is
why do different materials

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have different melting and boiling points?

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Let's find out.

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Now, to answer this question,

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we need to ask a much
more fundamental question.

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What keeps stuff together?

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

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if you could zoom in

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and look at the molecule
at the atomic level,

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we'll find that all
these atoms and molecules

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are actually being
attracted to each other.

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

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

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when you've seen two drops merging to form

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a bigger drop, okay?

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So this force of attraction

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keeps all the particles together.

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And turns out that this
force of attraction

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purely depends on the types of particles.

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So for example, the strength
of this force of attraction

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between water molecules would be different

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than that between gold atoms, right?

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But another thing you can see

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is that particles also
have kinetic energy.

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What does that depend on?

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Well, kinetic energy depends
purely on temperature.

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In fact, temperature is a measure

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of the average kinetic
energy of the particles.

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At high temperature,

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the average kinetic energy is very high.

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And at low temperature,

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the average kinetic energy is very low.

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Kinetic energy only
depends on temperature.

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It has nothing to do

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with which particles we're dealing with.

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So you can see there are
two things over here.

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First, we have the force of attraction

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that's trying to keep them together.

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This purely depends on
the type of particle,

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but it has nothing to do with temperature.

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On the other hand, we have kinetic energy

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that has nothing to do
with the particle type,

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but it purely depends on temperature.

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And what's interesting

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is that these two are kind of opposite.

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The attraction force is
trying to keep them together,

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whereas the kinetic energy is
trying to make the molecules

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go farther away from each other.

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And it's the balance between
these two that decide

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what the melting and the
boiling points would be.

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

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Let's take ice at a very low temperature,

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say -10, -15 degrees Celsius.

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At this temperature, again,

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if we were to zoom in,

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we will see the atoms and molecules

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all stuck together due
to the attractive force.

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And they also have kinetic energy

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that's trying to make them go apart.

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However, at this temperature,

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it turns out the kinetic
energy is very low,

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so low that the force of attraction

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actually locks them into
places giving us a solid.

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And the way I like to visualize this

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is by using some bar graph.

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So here's the force of attraction

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of the water molecules over here,

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and here is the kinetic energy.

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Look, the level of
kinetic energy is very low

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relative to the force of attraction.

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And as a result, you get a solid.

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But now comes the big question,

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what happens if we start heating it?

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Why don't you pause the video

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and think about what will happen
to the force of attraction

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and the kinetic energy
as we start heating it?

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Will it increase, decrease,
what happens to them?

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Pause and think about it.

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All right, what happens with
the force of attraction?

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Nothing, because that only depends

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on the types of atoms and molecules.

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It has nothing to do with temperature.

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

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That increases with temperature,

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which means as we hit this up,

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the temperature rises

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and the kinetic energy
will start increasing.

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At one particular point,

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the kinetic energy of these
particles will be high enough

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that it can partially overcome
the forces of attraction.

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And when that happens,

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

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They will start moving around.

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This is when solid turns into liquid.

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In our case, ice starts
turning into liquid water.

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And this temperature at which it happens,

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

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And that temperature where liquid turns,

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sorry, solid turns into liquid,

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is what we call the melting point.

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So the melting point of
water is 0 degrees Celsius.

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Now let's keep heating it up further.

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What happens as we heat it up?

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Again, nothing happens with
the force of attraction,

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but the kinetic energy will keep rising.

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And at one particular point,

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it will be high enough

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that it can fully overcome
the force of attraction.

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And then that happens,

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these molecules will now be free,

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almost completely free from each other,

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freely moving about.

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In other words, our liquid
starts turning into gas,

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water starts turning into steam.

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So the temperature at which this happens

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is what we call the boiling point.

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And for water, that boiling point

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happens to be at 100 degrees Celsius.

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And if you further heat it,

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well, the steam just gets hotter,

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nothing else will happen.

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So when the kinetic energy
is too low to overcome

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any amount of attraction, we have solid.

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When the kinetic energy is high enough

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to partially overcome
the force of attraction,

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we have liquid.

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And when the kinetic energy is high enough

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to fully overcome the force of attraction,

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we get a gas.

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

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

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Again, nothing will happen
to the force of attraction

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because it does not depend on temperature,

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but the kinetic energy will reduce

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

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It will no longer be
able to fully overcome

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the force of attraction,

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which means the gas will turn into liquid.

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We call this condensation,

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and this point is called
the condensation point.

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And you can clearly see
the condensation point

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is the same thing as the boiling point.

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And we've seen this before.

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

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over, say boiling water,

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we see liquid drops, that's condensation.

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The steam over here has temperature

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lower than the condensation point,

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lower than 100 degrees Celsius

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so it condenses into liquid water.

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

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Okay, and what happens

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if we were to reduce the
temperature even more?

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Well, again, the kinetic
energy will keep reducing.

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

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It will no longer be able to overcome

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any force of attraction,

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which means the liquid
will turn back into solid.

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We call this the freezing point.

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And you can see the freezing point

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is the same as the melting point.

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And therefore, when
liquid water, you know,

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is below 0 degrees Celsius,

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it freezes into ice.

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Okay, so the key thing
that we see over here

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is that the boiling point
and the melting point

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depends a lot on the force
of attraction, right?

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Now, here's a question:

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What if we consider a material like gold?

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Well, it turns out for gold,

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the force of attraction is much higher

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than that of water.

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Now, actually, the attraction

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is much higher in gold compared to water.

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So the graph should be
much higher over here.

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But don't worry about that.

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But this means now the kinetic
energy needed to partially

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and fully overcome the force of attraction

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would be much higher than before.

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

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and the boiling point would
be much higher than before.

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

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about 1000 degrees Celsius

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and about like close to
3000 degrees Celsius.

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That's why for gold,

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you need a much, much higher temperature

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for it to melt.

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Okay, what about nitrogen?

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Well, it terms for nitrogen,

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the force of attraction
is much, much lower.

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And therefore, the melting
point and the boiling points

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would be much lower.

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And that's why it boils

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at a much lower temperature

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of -196 degrees Celsius actually.

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That's why at room temperature,

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nitrogen is a gas.

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So long story short,

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the temperature at
which the kinetic energy

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can partially overcome
the force of attraction

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is what we call the melting
or the freezing point.

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

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

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And similarly, the temperature
at which the kinetic energy

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is high enough to fully overcome
the force of attraction,

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that's what we call the boiling point.

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

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

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And look, since these temperatures

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purely depend upon how strong or weak

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the attractive force is and that, in turn,

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depends upon which types of
particles we are dealing with,

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types of atoms and molecules
we're dealing with.

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

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of different particles,
different substances

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would be different.