Why does frozen CO2 melt into a gas instead of liquid?

Discussion in 'Chemistry' started by Magical Realist, May 15, 2014.

  1. Magical Realist Valued Senior Member

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    Is there something about the structure of the CO2 molecule that makes it skip liquidity and go straight to a gas?

    Also, why does metal touching dry ice vibrate and make noise?
     
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  3. Aqueous Id flat Earth skeptic Valued Senior Member

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    All materials have a characteristic way they change phase from solid to liquid to gas. This characteristic is a set of curves that are plotted over temperature and pressure. It just so happens that CO[sub]2[/sub] has a particular curve that allows for sublimation at one atmosphere of pressure. I think it's probably more correct to say this happens with dry ice just because we live on Earth. If we were on some other world, we might notice water doing the same thing. Note, this is kind of like asking if there is something about the atomic structure of Mercury that allows for it to exist as liquid at room temperature. And in fact if you place certain materials in a vacuum they will also sublimate. Water can even be made to boil at room temp. merely by lowering the pressure.

    In fact the amount of energy required to liberate one molecule of a material from its solid form is indeed directly tied to the molecular structure that binds it . . . but there is nothing in particular about CO[sub]2[/sub] that is remarkably different from any other material. It just happens to be one that sublimes at standard temp. and pressure. It's just a coincidence that there is as much energy as needed, at room temp and 1 atm, to liberate the surface molecules.
     
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  5. elte Valued Senior Member

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    MR, I'll throw in my interpretation after looking at Wikipedia. The melting point is -78.5degC and the boiling point is -56.6degC, both way colder than room temperature, and close together compared to (familiar) water, so the window for liquid carbon dioxide is very small and remotely cold compared to what we are most familiar with.

    I'd say the metal vibration would be due to the carbon dioxide boiling off the ice's surface, like how water boiling in a pot gurgles.
     
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  7. Enmos Staff Member

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  8. Magical Realist Valued Senior Member

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    So why exactly does CO2 sublimate? What iow is there about the CO2 molecule that makes it sublimate? Does science have an answer? Or is this just an arbitrary given?
     
  9. elte Valued Senior Member

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    Basically because the environmental temperature we are familiar for using dry ice at isn't between -78.5degC and -56.6degC where it would remain liquid. That is, we usually use it at temps near room temp. If it were -62degC, say, the carbon dioxide would be in its liquid state.

    For liquid nitrogen, the liquid window is even fewer degrees C than for liquid carbon dioxide (less than 15). I like to wonder why water is so unusual.
     
  10. Captain Kremmen All aboard, me Hearties! Valued Senior Member

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    Water is a strange substance.
    Water is more dense as a liquid than a solid.
    So the molecules in liquid water are closer together than in ice.

    If you increase the pressure, will water turn to ice?
     
  11. Motor Daddy ☼☼☼☼☼☼☼☼☼☼☼ Valued Senior Member

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    1 lb of water at 70 degrees and 14.7 psi atmospheric pressure has a specific volume.

    1 lb of ice at 70 degrees and 14.7 psi atmospheric pressure has a different specific volume.

    The 1 lb of ice has a greater volume than 1 lb of water. That means ice is less dense than water, because there is more volume and the same mass, so there is less mass per cubic centimeter of volume.

    Heating the ice up to melt to water means energy is added to the ice....???
     
  12. Russ_Watters Not a Trump supporter... Valued Senior Member

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    The boiling point is below room temperature and the boiling and melting points and energies are close together.

    Ice sublimates too, under certain conditions. You should notice the ice cubes in your freezer shrinking.
     
  13. Captain Kremmen All aboard, me Hearties! Valued Senior Member

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    To answer my own question, yes

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    At just above 100 kbar water is ice at 350 degrees C.
    As 1 kbar is roughly 1000 X atmospheric pressure at 1 bar, that's a lot of pressure.
    A very high pressure tyre such as an aircraft tyre is 14 bar or less.
    Stepping up to another level, at a few hundred times 100 kbar , the water becomes hexagonal in structure.
    (would that make it like graphite?)
    From my reading of the graph only pressure matters in producing hexagonal water.
    The temperature can be anything ( within the limits of the graph anyway).
    I'd guess that any water in the body of the earth deeper than about 30 miles would be in this form, if it exists there.
     
    Last edited: May 16, 2014
  14. Trippy ALEA IACTA EST Staff Member

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    I'll see if I can find a video of it, however, once upon a time Woods-hole did an experiment. The basic idea was that if you pumped liquid carbondioxide deep enough under water the pressure and temperature would ensure that it stayed a liquid and it would be dense enough to sit in a pool on the ocean floor. From there the oceans could reabsorb it as they would anyway, but it gets it out of the atmosphere.
     
  15. Trippy ALEA IACTA EST Staff Member

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    This comparisson of phase diagrams illustrates some of the differences between water and carbon dioxide.

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    In this regards there is nothing unique about carbondioxide - all solids have the potential to sublime.

    All liquids and solids have a property called vapour pressure. As I recall, if you took a sufficient quantity of the substance, be it solid or liquid, and placed it in a vacuum, that substance would evaporate or sublime until enough of the substance existed in the gas phase that the rate of sublimation/evaporation was equal to the rate of deposition/condensation. The vapour pressure of the substance is the amount of pressure exerted by the gas phase once this equlibrium has been achieved. All solids sublime, this is why freeze-frying works. In essence, they sublime for the same reason they evaporate, because in any given volume of substance in the solid or liquid phase, some proportion of atoms/molecules near the surface have sufficient energy to escape to the gas phase.

    The next thing to understand is that the boiling point of a liquid is strongly dependent on pressure - this is why water boils at I think 60 or 80 degrees celsius at Mount everest. But the thing is, the melting point of a substance is only weakly dependent on pressure. This means that - and you can see this in the phase diagram I posted, it takes a greater change in pressure (a steeper line) to make a change in melting point then it does to make an equivalent change in the boiling point (a shallower line). This is why on phase diagrams the line between liquid and gas is shallower than the line between liquid and solid. This means that by lowering pressure it is actually possible to have a boiling point that is actually lower than the melting point, and when this happens sublimation occurs.

    The specific temperatures at which these various things occur is a function of molecular structure and intermolecular bonding. The stronger the intermolecular bonding the lower the vapour pressure.

    So there we have our answer carbon dioxide readily sublimes at standard temperature and pressure because at standard pressure the boiling point of carbon dioxide is below the melting point, which in turn is below the standard temperature. This is because of the weak intermolecular bonding between carbon dioxide molecules.

    There is one aside, some solids have such strong intermolecular bonding (eg silica, metals) that their vapour pressure is so low that they effectively do not sublime at STP.
     

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