Phase Diagrams of Water & CO2 Explained - Chemistry - Melting, Boiling & Critical Point

00:10:27
https://www.youtube.com/watch?v=QrHlwgmMTq4

摘要

TLDRThe video explains the phase diagrams of carbon dioxide (CO2) and water (H2O), starting with basic phase changes like melting, freezing, vaporization, condensation, sublimation, and deposition. It details CO2's phase diagram, highlighting the triple point where all phases coexist, the critical point where a supercritical fluid can form, and the concept of sublimation at standard pressure. CO2 sublimates directly from solid to gas at pressures below the triple point. Unlike many substances, solid CO2 is denser than its liquid due to a positive slope in the melting point curve. Water's phase diagram is introduced to show differences. It has a negative slope in the melting point line, meaning liquid water is denser than ice, explaining why ice floats. Key features like the critical point and normal boiling point where the atmospheric pressure is 1 atm are discussed, helping to understand how pressure and temperature affect phase transitions.

心得

  • 🧊 Melting is solid to liquid transition.
  • 🔥 Vaporization is liquid to gas transition.
  • 🌬️ Sublimation is solid to gas skipping liquid phase.
  • 💧 Condensation is gas to liquid.
  • ❄️ Deposition is reverse of sublimation: gas to solid.
  • 🔺 Triple point is where solid, liquid, gas coexist.
  • ⚖️ CO2 solid is denser than liquid due to positive melting slope.
  • 🌡️ Critical point leads to supercritical fluid forms.
  • 🌊 Ice floats because liquid water is denser.
  • 🔄 H2O sublime below triple point pressure.
  • 📈 Normal boiling point is at 1 atm pressure.
  • ⬇️ H2O melting line has negative slope, unlike CO2.

时间轴

  • 00:00:00 - 00:05:00

    The video begins with an explanation of phase changes, including melting (solid to liquid), freezing (liquid to solid), vaporization (liquid to gas), and condensation (gas to liquid). Sublimation (solid to gas) and deposition (gas to solid) are also discussed. For CO2, sublimation occurs at standard pressure because it's below the triple point. The phase diagram's x-axis shows temperature, and the y-axis shows pressure, with critical points discussed, highlighting that beyond the critical temperature, gases can't be liquefied just by increasing pressure.

  • 00:05:00 - 00:10:27

    Moving on to the phase diagram for water, the melting point line has a negative slope, indicating differences in density between phases. At 1 atm, water transitions from solid to liquid to gas upon increasing temperature. Below the triple point, ice sublimates. Liquid water is denser than ice, explaining why ice floats. The critical point is where supercritical fluids form, having both gas and liquid properties. Boiling and melting points are further discussed, with 'normal' points being defined at 1 atm pressure.

思维导图

Mind Map

常见问题

  • What is melting?

    Melting is the phase change from a solid to a liquid.

  • What is sublimation?

    Sublimation is the phase change from a solid directly to a gas, skipping the liquid phase.

  • What is the triple point?

    The triple point on a phase diagram is where the solid, liquid, and gas phases coexist.

  • What is a supercritical fluid?

    A supercritical fluid has properties of both a gas and a liquid, existing beyond the critical temperature and pressure.

  • What determines whether a material's solid phase is more dense than its liquid phase?

    Observing the melting point curve slope; for CO2, a positive slope indicates solid CO2 is denser.

  • Why does ice float on water?

    Ice floats on water because liquid water has a higher density than solid ice.

  • What is the critical point?

    The critical point is the temperature and pressure at which a substance becomes a supercritical fluid.

  • How do phase diagrams of CO2 and H2O differ?

    CO2's melting point slope is positive, indicating solid is denser, while H2O's is negative, with liquid being denser.

  • When does sublimation occur for water?

    Sublimation happens when water is at a pressure below its triple point.

  • What is the normal boiling point of water?

    The normal boiling point is the temperature at which water boils at atmospheric pressure of 1 atm.

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  • 00:00:00
    in this video we're going to talk about
  • 00:00:02
    the phase diagrams for CO2 and water but
  • 00:00:06
    before we jump into that there are a few
  • 00:00:09
    things that we need to talk
  • 00:00:11
    about so let's go over phase changes if
  • 00:00:14
    you go from a solid to a
  • 00:00:16
    liquid what is this phase change called
  • 00:00:20
    going from a solid to a liquid this is
  • 00:00:23
    called
  • 00:00:25
    melting think of
  • 00:00:27
    ice when ice melts
  • 00:00:31
    into water it converts from a solid to a
  • 00:00:33
    liquid now what about the reverse liquid
  • 00:00:36
    to
  • 00:00:37
    solid so when water turns into ice it's
  • 00:00:41
    called
  • 00:00:47
    freezing now what about going from a
  • 00:00:49
    liquid to a
  • 00:00:51
    gas going from a liquid to a gas is
  • 00:00:54
    known as vaporization
  • 00:01:03
    now what about gas to a
  • 00:01:06
    liquid this is called
  • 00:01:09
    condensation now let's say if you place
  • 00:01:11
    a cold glass cup in a humid
  • 00:01:15
    environment you'll notice that water
  • 00:01:17
    condenses on the outside of the cup
  • 00:01:21
    you'll see like water droplets that's
  • 00:01:23
    the water vapor in the air condensing
  • 00:01:25
    into a
  • 00:01:28
    liquid now what about the conversion
  • 00:01:30
    from a solid directly into a gas
  • 00:01:33
    skipping the liquid
  • 00:01:35
    phase this is known as
  • 00:01:40
    sublimation a good example of
  • 00:01:41
    sublimation is dry ice
  • 00:01:44
    CO2 if you ever watch a YouTube video on
  • 00:01:46
    dry ice if you see solid
  • 00:01:50
    CO2 notice that it converts directly
  • 00:01:53
    into a gas it doesn't liquefy at room
  • 00:01:56
    temperature or rather at 1 atm at normal
  • 00:02:00
    pressure it goes directly from a solid
  • 00:02:03
    to a gas that's why it's called dry ice
  • 00:02:06
    now if you think about the other one gas
  • 00:02:08
    to a
  • 00:02:09
    solid this is simply the reverse of
  • 00:02:12
    sublimation this is called
  • 00:02:15
    deposition that's when the gas goes
  • 00:02:17
    directly back into the solid
  • 00:02:21
    form now let's start with the phase
  • 00:02:24
    diagram for
  • 00:02:28
    CO2 so on the x- axis we have uh the
  • 00:02:31
    temperature and the pressure is on the Y
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    AIS and typically the graph
  • 00:02:38
    looks something like
  • 00:02:47
    this so that's just a rough
  • 00:02:50
    sketch so here we have the
  • 00:02:54
    solid here is the liquid phas and here's
  • 00:02:56
    the gas phas
  • 00:03:01
    and here is the standard pressure at 1
  • 00:03:06
    atm the point where the solid liquid and
  • 00:03:09
    gas phases
  • 00:03:10
    coexist is called the triple
  • 00:03:14
    point the line between the solid and
  • 00:03:16
    liquid phase is known as the melting
  • 00:03:19
    point
  • 00:03:20
    line because to go from a solid to a
  • 00:03:23
    liquid it's melting and going backwards
  • 00:03:26
    from a liquid to a solid it's freezing
  • 00:03:31
    now the line between the liquid and the
  • 00:03:33
    gas phase is called the bowling Point
  • 00:03:36
    curve going from a liquid to a gas we
  • 00:03:38
    set it's uh vaporization and gas to a
  • 00:03:42
    liquid is uh
  • 00:03:47
    condensation now at 1 atm if you
  • 00:03:51
    increase the temperature for CO2 notice
  • 00:03:53
    that it goes directly from a solid to a
  • 00:03:55
    gas so CO2 sublimes
  • 00:03:59
    notice that at 1 atm the pressure is
  • 00:04:03
    below the triple point if the pressure
  • 00:04:05
    was above the triple point it would go
  • 00:04:07
    from a solid to a liquid to a gas but at
  • 00:04:10
    standard pressure it's below the triple
  • 00:04:11
    point so that's why CO2 sublimes from a
  • 00:04:14
    solid to a
  • 00:04:16
    gas as the temperature
  • 00:04:19
    increases now here's a question for you
  • 00:04:23
    which phase has a higher density the
  • 00:04:26
    liquid phase of CO2 or the solid phase
  • 00:04:30
    because for some materials the
  • 00:04:33
    liquid has a greater density and for
  • 00:04:36
    other materials the solids have a
  • 00:04:38
    greater density so how can you tell
  • 00:04:40
    based on the phase
  • 00:04:41
    diagram well pick a point let's start
  • 00:04:45
    with the liquid
  • 00:04:46
    phase as you increase the pressure the
  • 00:04:49
    density of the material increases
  • 00:04:52
    because you're compacting the atoms
  • 00:04:54
    together as you squeeze atoms into a
  • 00:04:57
    tighter space there's more mass unit
  • 00:04:59
    volume and so it has a higher density so
  • 00:05:02
    as we increase the pressure notice that
  • 00:05:05
    we're going to go from the liquid to the
  • 00:05:06
    solid phase so we're going to freeze the
  • 00:05:10
    substance and that means that the solid
  • 00:05:13
    phase is at a higher pressure than the
  • 00:05:15
    liquid phase so in the case of CO2
  • 00:05:19
    because
  • 00:05:20
    the the melting point curve has a
  • 00:05:22
    positive
  • 00:05:24
    slope the density of solid CO2 is
  • 00:05:28
    greater than that liquid CO2 at higher
  • 00:05:33
    pressures now there's another point of
  • 00:05:36
    interest and it's called the critical
  • 00:05:40
    point the critical point is location
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    where you beyond the temperature and
  • 00:05:47
    pressure of that point you can have a
  • 00:05:49
    super critical
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    fluid a super critical fluid is a fluid
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    that has properties of a gas and a
  • 00:05:57
    liquid it's not exactly a gas and it's
  • 00:06:00
    not exactly a liquid but it's in between
  • 00:06:05
    the
  • 00:06:07
    two like a gas it has a lower density
  • 00:06:11
    than a liquid but like a liquid it can
  • 00:06:13
    dissolve certain materials like a gas it
  • 00:06:17
    can euse through different materials so
  • 00:06:20
    it has properties of
  • 00:06:23
    both notice that beyond the critical
  • 00:06:27
    temperature which is this point
  • 00:06:30
    you can no longer liquefy a gas by
  • 00:06:32
    increasing the pressure If you increase
  • 00:06:34
    the pressure the gas will go directly to
  • 00:06:36
    the super critical phase it won't go
  • 00:06:38
    into liquid phase a gas can be
  • 00:06:42
    liquified when the temperature is below
  • 00:06:45
    the critical
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    temperature if it's above it you're
  • 00:06:48
    going to go straight to the super
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    critical fluid rather than liquefying
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    the gas at high
  • 00:06:57
    pressure now let's go over the phase
  • 00:07:01
    diagram for
  • 00:07:10
    water so it's a little different for H2O
  • 00:07:13
    than it is for
  • 00:07:21
    cl2 for one thing notice that the
  • 00:07:24
    melting point line has a negative slope
  • 00:07:26
    instead of a positive slope As you move
  • 00:07:29
    towards the right the line
  • 00:07:33
    decreases now this is still going to be
  • 00:07:35
    the solid phase this is the liquid and
  • 00:07:36
    this is the gas
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    phase and this
  • 00:07:42
    time the pressure of 1 atm is above the
  • 00:07:46
    triple
  • 00:07:47
    point so because standard pressure is
  • 00:07:49
    above the triple point as you increase
  • 00:07:52
    the temperature for let's say ice ice is
  • 00:07:55
    going to melt into a liquid and as you
  • 00:07:57
    continue to increase the temperature the
  • 00:07:59
    liquid will vaporize into a
  • 00:08:02
    gas so at a pressure of 1
  • 00:08:05
    atm you can increase the temperature and
  • 00:08:08
    get all three phases of water now if you
  • 00:08:10
    decrease the pressure below the triple
  • 00:08:13
    point when you heat up the solid it will
  • 00:08:16
    Sublime directly into a
  • 00:08:20
    gas so sublimation can occur when the
  • 00:08:23
    pressure is below the triple
  • 00:08:25
    point now let's talk about the density
  • 00:08:27
    of water
  • 00:08:30
    which phase has a higher
  • 00:08:32
    density ice or liquid
  • 00:08:35
    water so if we start at this point and
  • 00:08:38
    if we increase the pressure if we go
  • 00:08:41
    vertically upwards in the phase diagram
  • 00:08:43
    notice that we're going to travel from
  • 00:08:45
    the solid to the liquid so that means
  • 00:08:47
    that the liquid is at a higher pressure
  • 00:08:50
    and therefore is more dense uh than the
  • 00:08:53
    solid so liquid water has a higher
  • 00:08:56
    density than ice that's why ice floats
  • 00:08:59
    on
  • 00:09:00
    water now this is still the critical
  • 00:09:04
    point and above this uh region or in its
  • 00:09:07
    region you have the super critical fluid
  • 00:09:12
    again so the temperature at the critical
  • 00:09:15
    point is called the critical temperature
  • 00:09:18
    which is whatever that value is and the
  • 00:09:21
    pressure that corresponds to the
  • 00:09:23
    critical point is the critical pressure
  • 00:09:31
    now what is the difference between the
  • 00:09:33
    boiling point of
  • 00:09:34
    H2O and the normal boiling point of
  • 00:09:38
    H2O the boiling point of water can vary
  • 00:09:42
    and it's anywhere along this line
  • 00:09:44
    however the normal boiling point is a
  • 00:09:48
    specific location on that line the
  • 00:09:50
    normal boiling point occurs when the
  • 00:09:52
    pressure of the atmosphere is 1
  • 00:09:55
    atm so this particular point is the
  • 00:09:58
    normal boiling point of
  • 00:10:00
    water whereas anywhere along this line
  • 00:10:03
    is called the bowling Point curve now
  • 00:10:05
    the normal melting point occurs at a
  • 00:10:07
    pressure of 1 atm so this is the normal
  • 00:10:10
    melting
  • 00:10:12
    point and this is called the melting
  • 00:10:15
    point
  • 00:10:17
    curve so if you hear the word normal
  • 00:10:20
    attached to melting point or boiling
  • 00:10:23
    point the pressure has to be 1 atm
标签
  • phase diagrams
  • CO2
  • water
  • sublimation
  • triple point
  • critical point
  • density
  • melting
  • supercritical fluid
  • boiling point