States of matter | States of matter and intermolecular forces | Chemistry | Khan Academy
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States of matter | States of matter and intermolecular forces | Chemistry | Khan Academy

I think we’re all reasonably
familiar with the three states of matter in our
everyday world. At very high temperatures
you get a fourth. But the three ones that we
normally deal with are, things could be a solid, a liquid,
or it could be a gas. And we have this general notion,
and I think water is the example that always comes
to at least my mind. Is that solid happens
when things are colder, relatively colder. And then as you warm up, you
go into a liquid state. And as your warm up even more
you go into a gaseous state. So you go from colder
to hotter. And in the case of water, when
you’re a solid, you’re ice. When you’re a liquid, some
people would call ice water, but let’s call it
liquid water. I think we know what that is. And then when it’s in the gas
state, you’re essentially vapor or steam. So let’s think a little bit
about what, at least in the case of water, and the analogy
will extend to other types of molecules. But what is it about water that
makes it solid, and when it’s colder, what allows
it to be liquid. And I’ll be frank, liquids are
kind of fascinating because you can never nail them
down, I guess is the best way to view them. Or a gas. So let’s just draw
a water molecule. So you have oxygen there. You have some bonds
to hydrogen. And then you have two extra
pairs of valence electrons in the oxygen. And a couple of videos ago, we
said oxygen is a lot more electronegative than
the hydrogen. It likes to hog the electrons. So even though this shows
that they’re sharing electrons here and here. At both sides of those lines,
you can kind of view that hydrogen is contributing an
electron and oxygen is contributing an electron on
both sides of that line. But we know because of the
electronegativity, or the relative electronegativity of
oxygen, that it’s hogging these electrons. And so the electrons spend a
lot more time around the oxygen than they do around
the hydrogen. And what that results is that
on the oxygen side of the molecule, you end up with a
partial negative charge. And we talked about
that a little bit. And on the hydrogen side of the
molecules, you end up with a slightly positive charge. Now, if these molecules have
very little kinetic energy, they’re not moving around a
whole lot, then the positive sides of the hydrogens are very
attracted to the negative sides of oxygen in
other molecules. Let me draw some
more molecules. When we talk about the whole
state of the whole matter, we actually think about how the
molecules are interacting with each other. Not just how the atoms are
interacting with each other within a molecule. I just drew one oxygen, let
me copy and paste that. But I could do multiple
oxygens. And let’s say that that hydrogen
is going to want to be near this oxygen. Because this has partial
negative charge, this has a partial positive charge. And then I could do another
one right there. And then maybe we’ll have, and
just to make the point clear, you have two hydrogens
here, maybe an oxygen wants to hang out there. So maybe you have an oxygen that
wants to be here because it’s got its partial
negative here. And it’s connected to two
hydrogens right there that have their partial positives. But you can kind of see
a lattice structure. Let me draw these bonds, these
polar bonds that start forming between the particles. These bonds, they’re called
polar bonds because the molecules themselves
are polar. And you can see it forms
this lattice structure. And if each of these molecules
don’t have a lot of kinetic energy. Or we could say the average
kinetic energy of this matter is fairly low. And what do we know is average
kinetic energy? Well, that’s temperature. Then this lattice structure
will be solid. These molecules will not move
relative to each other. I could draw a gazillion more,
but I think you get the point that we’re forming this kind
of fixed structure. And while we’re in the solid
state, as we add kinetic energy, as we add heat, what
it does to molecules is, it just makes them vibrate
around a little bit. If I was a cartoonist, they way
you’d draw a vibration is to put quotation marks there. That’s not very scientific. But they would vibrate around,
they would buzz around a little bit. I’m drawing arrows to show
that they are vibrating. It doesn’t have to be just left-right it could be up-down. But as you add more and more
heat in a solid, these molecules are going to
keep their structure. So they’re not going to move
around relative to each other. But they will convert that heat,
and heat is just a form of energy, into kinetic energy
which is expressed as the vibration of these molecules. Now, if you make these molecules
start to vibrate enough, and if you put enough
kinetic energy into these molecules, what do you think
is going to happen? Well this guy is vibrating
pretty hard, and he’s vibrating harder and harder as
you add more and more heat. This guy is doing
the same thing. At some point, these polar bonds
that they have to each other are going to start not
being strong enough to contain the vibrations. And once that happens, the
molecules– let me draw a couple more. Once that happens, the molecules
are going to start moving past each other. So now all of a sudden, the
molecule will start shifting. But they’re still attracted. Maybe this side is moving here,
that’s moving there. You have other molecules
moving around that way. But they’re still attracted
to each other. Even though we’ve gotten the
kinetic energy to the point that the vibrations can kind of
break the bonds between the polar sides of the molecules. Our vibration, or our kinetic
energy for each molecule, still isn’t strong enough to
completely separate them. They’re starting to slide
past each other. And this is essentially what
happens when you’re in a liquid state. You have a lot of atoms that
want be touching each other but they’re sliding. They have enough kinetic energy
to slide past each other and break that solid
lattice structure here. And then if you add even more
kinetic energy, even more heat, at this point it’s
a solution now. They’re not even going to be
able to stay together. They’re not going to be able
to stay near each other. If you add enough kinetic energy
they’re going to start looking like this. They’re going to completely
separate and then kind of bounce around independently. Especially independently if
they’re an ideal gas. But in general, in gases,
they’re no longer touching each other. They might bump into
each other. But they have so much kinetic
energy on their own that they’re all doing
their own thing and they’re not touching. I think that makes intuitive
sense if you just think about what a gas is. For example, it’s hard
to see a gas. Why is it hard to see a gas? Because the molecules are
much further apart. So they’re not acting on the
light in the way that a liquid or a solid would. And if we keep making that
extended further, a solid– well, I probably shouldn’t
use the example with ice. Because ice or water is one of
the few situations where the solid is less dense
than the liquid. That’s why ice floats. And that’s why icebergs don’t
just all fall to the bottom of the ocean. And ponds don’t completely
freeze solid. But you can imagine that,
because a liquid is in most cases other than water,
less dense. That’s another reason why you
can see through it a little bit better. Or it’s not diffracting– well I
won’t go into that too much, than maybe even a solid. But the gas is the
most obvious. And it is true with water. The liquid form is definitely
more dense than the gas form. In the gas form, the molecules
are going to jump around, not touch each other. And because of that,
more light can get through the substance. Now the question is, how do we
measure the amount of heat that it takes to do
this to water? And to explain that, I’ll
actually draw a phase change diagram. Which is a fancy way of
describing something fairly straightforward. Let me say that this is the
amount of heat I’m adding. And this is the temperature. We’ll talk about the states
of matter in a second. So heat is often denoted by q. Sometimes people will talk
about change in heat. They’ll use H, lowercase
and uppercase H. They’ll put a delta
in front of the H. Delta just means change in. And sometimes you’ll hear
the word enthalpy. Let me write that. Because I used to say
what is enthalpy? It sounds like empathy,
but it’s quite a different concept. At least, as far as my neural
connections could make it. But enthalpy is closely
related to heat. It’s heat content. For our purposes, when you hear
someone say change in enthalpy, you should really
just be thinking of change in heat. I think this word was really
just introduced to confuse chemistry students and introduce
a non-intuitive word into their vocabulary. The best way to think about
it is heat content. Change in enthalpy is really
just change in heat. And just remember, all of these
things, whether we’re talking about heat,
kinetic energy, potential energy, enthalpy. You’ll hear them in different
contexts, and you’re like, I thought I should be using
heat and they’re talking about enthalpy. These are all forms of energy. And these are all measured
in joules. And they might be measured
in other ways, but the traditional way is in joules. And energy is the ability
to do work. And what’s the unit for work? Well, it’s joules. Force times distance. But anyway, that’s
a side-note. But it’s good to know
this word enthalpy. Especially in a chemistry
context, because it’s used all the time and it can be very
confusing and non-intuitive. Because you’re like, I don’t
know what enthalpy is in my everyday life. Just think of it as heat
contact, because that’s really what it is. But anyway, on this
axis, I have heat. So this is when I have very
little heat and I’m increasing my heat. And this is temperature. Now let’s say at low
temperatures I’m here and as I add heat my temperature
will go up. Temperature is average
kinetic energy. Let’s say I’m in the
solid state here. And I’ll do the solid
state in purple. No I already was using purple. I’ll use magenta. So as I add heat, my temperature
will go up. Heat is a form of energy. And when I add it to these
molecules, as I did in this example, what did it do? It made them vibrate more. Or it made them have higher
kinetic energy, or higher average kinetic engery, and
that’s what temperature is a measure of; average
kinetic energy. So as I add heat in the solid
phase, my average kinetic energy will go up. And let me write this down. This is in the solid phase, or
the solid state of matter. Now something very interesting
happens. Let’s say this is water. So what happens at
zero degrees? Which is also 273.15 Kelvin. Let’s say that’s that line. What happens to a solid? Well, it turns into a liquid. Ice melts. Not all solids, we’re talking
in particular about water, about H2O. So this is ice in our example. All solids aren’t ice. Although, you could think of
a rock as solid magma. Because that’s what it is. I could take that analogy a
bunch of different ways. But the interesting thing that
happens at zero degrees. Depending on what direction
you’re going, either the freezing point of water or
the melting point of ice, something interesting happens. As I add more heat, the
temperature does not to go up. As I add more heat, the
temperature does not go up for a little period. Let me draw that. For a little period, the
temperature stays constant. And then while the temperature
is constant, it stays a solid. We’re still a solid. And then, we finally
turn into a liquid. Let’s say right there. So we added a certain amount
of heat and it just stayed a solid. But it got us to the
point that the ice turned into a liquid. It was kind of melting
the entire time. That’s the best way
to think about it. And then, once we keep adding
more and more heat, then the liquid warms up too. Now, we get to, what
temperature becomes interesting again for water? Well, obviously 100 degrees
Celsius or 373 degrees Kelvin. I’ll do it in Celsius
because that’s what we’re familiar with. What happens? That’s the temperature at which
water will vaporize or which water will boil. But something happens. And they’re really getting
kinetically active. But just like when you went from
solid to liquid, there’s a certain amount of energy that
you have to contribute to the system. And actually, it’s a good
amount at this point. Where the water is turning
into vapor, but it’s not getting any hotter. So we have to keep adding heat,
but notice that the temperature didn’t go up. We’ll talk about it
in a second what was happening then. And then finally, after that
point, we’re completely vaporized, or we’re
completely steam. Then we can start getting hot,
the steam can then get hotter as we add more and more
heat to the system. So the interesting question, I
think it’s intuitive, that as you add heat here, our
temperature is going to go up. But the interesting thing is,
what was going on here? We were adding heat. So over here we were turning our
heat into kinetic energy. Temperature is average
kinetic energy. But over here, what was
our heat doing? Well, our heat was was
not adding kinetic energy to the system. The temperature was
not increasing. But the ice was going
from ice to water. So what was happening at that
state, is that the kinetic energy, the heat, was being
used to essentially break these bonds. And essentially bring
the molecules into a higher energy state. So you’re saying, Sal,
what does that mean, higher energy state? Well, if there wasn’t all of
this heat and all this kinetic energy, these molecules
want to be very close to each other. For example, I want
to be close to the surface of the earth. When you put me in a plane
you have put me in a higher energy state. I have a lot more potential
energy. I have the potential to fall
towards the earth. Likewise, when you move these
molecules apart, and you go from a solid to a liquid,
they want to fall towards each other. But because they have so much
kinetic energy, they never quite are able to do it. But their energy goes up. Their potential energy is higher
because they want to fall towards each other. By falling towards each
other, in theory, they could do some work. So what’s happening here is,
when we’re contributing heat– and this amount of heat we’re
contributing, it’s called the heat of fusion. Because it’s the same amount
of heat regardless how much direction we go in. When we go from solid to liquid,
you view it as the heat of melting. It’s the head that you need
to put in to melt the ice into liquid. When you’re going in this
direction, it’s the heat you have to take out of the
zero degree water to turn it into ice. So you’re taking that potential
energy and you’re bringing the molecules closer
and closer to each other. So the way to think about it
is, right here this heat is being converted to
kinetic energy. Then, when we’re at this phase
change from solid to liquid, that heat is being used
to add potential energy into the system. To pull the molecules
apart, to give them more potential energy. If you pull me apart from the
earth, you’re giving me potential energy. Because gravity wants to pull
me back to the earth. And I could do work when I’m
falling back to the earth. A waterfall does work. It can move a turbine. You could have a bunch
of falling Sals move a turbine as well. And then, once you are fully a
liquid, then you just become a warmer and warmer liquid. Now the heat is, once again,
being used for kinetic energy. You’re making the water
molecules move past each other faster, and faster,
and faster. To some point where they want
to completely disassociate from each other. They want to not even slide
past each other, just completely jump away
from each other. And that’s right here. This is the heat of
vaporization. And the same idea
is happening. Before we were sliding next to
each other, now we’re pulling apart altogether. So they could definitely
fall closer together. And then once we’ve added this
much heat, now we’re just heating up the steam. We’re just heating up
the gaseous water. And it’s just getting hotter
and hotter and hotter. But the interesting thing there,
and I mean at least the interesting thing to me when I
first learned this, whenever I think of zero degrees water I’ll
say, oh it must be ice. But that’s not necessarily
the case. If you start with water and you
make it colder and colder and colder to zero degrees,
you’re essentially taking heat out of the water. You can have zero degree
water and it hasn’t turned into ice yet. And likewise, you could have 100
degree water that hasn’t turned into steam yeat. You have to add more energy. You can also have 100
degree steam. You can also have zero
degree water. Anyway, hopefully that gives you
a little bit of intuition of what the different states
of matter are. And in the next problem, we’ll
talk about how much heat exactly it does take to
move along this line. And maybe we can solve some
problems on how much ice we might need to make
our drink cool.

About James Carlton

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88 thoughts on “States of matter | States of matter and intermolecular forces | Chemistry | Khan Academy

  1. Thank You for making such a simple video about a complex subject. I wasn't sure if my students would like it, but they did…and a lot of them understood some things about bonding that they hadn't before. I also used the beginning myself for some 3rd grage students..used different colored markers on the white board..they drew as I did..they loved it. Thanks

  2. Awesome, thanks, a very good reminder for me and it will come in very handy for my teaching assignment 🙂

  3. "Enthalpy is a word that was introduced to confuse chemistry student", I totally agree Sal 😀 Why can't people just use simple words we are all know instead of making things seem so complicated?

  4. @malaikaswm I must disagree with Sal on this one: Heat is a way of transfering ENERGY, systems do not have heat, systems have energy, so you could not and can't say you have a change in heat, because it does not make sense, since heat is just a way of transfering energy. And since ppl needed an easier way to say "oh now the rate of energy being added by heat is larger that it was before", they created enthalpy, and now they can just say "increase in enthalpy" instead of all that bs.

  5. #1 Between HF and HCl, which would be a stronger acid and why?
    #2 Wouldn't the Bose-Einstein condensate be classified as a 5th state of matter?

  6. thx wish u r my chemistry teacher i love it so much. my teacher just say read it in the book but i dont under stand -.- thx keep uploading these things 😉

  7. Hey, I have a question. You said that at its boiling point (373K), water starts to turn into a gas, but that made me wonder, why is it that a pretty low temperature, water still evaporates (as well as ice actually, that's called sublimation, right?)? To me it doesn't quite fit into the theory addressed in this video. Thanks

  8. What is the difference vapor and steam? Since english is not my mother tongue and my dictionary says it's the same I was wondering what the exact difference is.

  9. I watch all your biology videos for my exams and stuff. I don't know why, but i never thought of checking out your chemistry playlist. I just did my chem exam yesterday and today i look at this playlist. Every single thing that i needed was here. My exam would've been sooooo much easier if I'd just bothered to look at this. So pissed with myself! 🙁 You're awesome Sal! Lemme just go have a look at physics…

  10. Differently from what Sal says in 1:10, steam is actually not a gas in physics and chemistry, though people equate both in common language. Steam is a mixture of tiny liquid water droplets with air.

  11. —————————————————————————–

  12. There are 42 states of matter. To name a few, Degen. Matter, Liquid Crystals, Quantum Hall State, Quark Gluon Plasma and Symmetric Matter.

  13. 1:47 Both pairs of electrons are actually on top… that's why the hydrogen are angled like that, they're not top and bottom. The only way that would happen is if you were looking at it in a weird way (since it is in 3D space), but the most common way to represent the dot structure is to put both pairs on top.

  14. You forgot quark glueing plasma and superfluids and super solids ect…..

    There are a total of 20

    Look up superfluous they are really cool

  15. HAHAHAHAH 99% of the universe is plasma the 1% is us but we have made 3 states of matter you can’t find outside

  16. 10 gram of ice at minus 10 degree Celsius is dropped into 10 gram of water at 60 degree Celsius find the final temperature of the mixture how much I could melt I want to solution for this problem

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