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Why does decreasing pressure also decrease boiling point?

Discussion in 'Nerd Out Zone' started by joppiesaus, Nov 14, 2014.

  1. joppiesaus

    joppiesaus Infamous Space-Octopus

    • Member
    I once saw someone who had a cup of water and put it in a vacuum. It was just room temperature(293 K), and it started boiling.
    As soon as the presure rises, it stopped boiling.
    This is also the way pressure cookers work; increase pressure, increase boiling point.

    Why is that?
  2. tetryds

    tetryds cus tet, that's it

    • Member
    You mean decreasing pressure, right?
    It's because of how this stuff works.
    The pressure on the liquid "holds it from boiling", the air molecule generate a force on it that makes it stay together up to a higher temperature point.
    When you have less pressure or vacuum you don't have this pressure and there is nothing forcing it to stay together more strongly.
    So the boiling point drops.

    [​IMG]

    If you want more detail just ask, I tried to put it as simple as I can :p
    tetryds, Nov 16, 2014
    Last edited by tetryds; at Nov 16, 2014
    #2
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  3. joppiesaus

    joppiesaus Infamous Space-Octopus

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    Yeah that totally makes sense! That's an awesome explanation! Thanks!
    (And yes I meant pressure, not temperature. What was I thinking?)

    So at lower pressure the molecules can move faster, because there's no "pressure" of not doing so.
    So this also works with ice, but the graph shows it's harder to do that.
  4. tetryds

    tetryds cus tet, that's it

    • Member
    Well, they actually move at the same intensity (the vibration intensity is given by the temperature), but they don't have anything preventing them from boiling.
    And yes, you got it :)

    When ice goes straight into a gas form, this is called sublimation.
    That is what happens to dry ice.

    Edit: Now I re-read what you said...
    Yes, that is right, they CAN move faster without boiling because something is holding them.
    Sorry for correcting something you said right.
    Last edited: Nov 16, 2014
    tetryds, Nov 16, 2014
    Last edited by tetryds; at Nov 16, 2014
    #4
  5. PsychoticLeprechaun

    PsychoticLeprechaun Designer & Web Developer

    • Dev Member
    If you want a slightly more fuller description:

    The vapour pressure of a substance is the pressure of a vapour of a thing acting on the none-vaporised phases of itself (i.e. liquids and gases). This vapour pressure must be equal to the pressure surrounding the none-vaporised substance in order for this substance to vaporise. Hence the definition of the boiling point of a substance is the temperature at which this vapour pressure is equal to the atmospheric pressure. The vapour pressure will increase with temperature, as more particles in the substance have sufficient kinetic energy to escape to bonds holding the substance together. Since this is the case, in situations where the surrounding pressure is greater, it stands to reason that the boiling point of that substance must increase.

    That said, I have a ponderance (not a word in the official English language, but damned well should be...) based on Feynman's description of Atomic Theory:

    It is commonly assumed that the total pressure acting on a substance affects the boiling point of a substance, and this can be seen empirically.


    However, the actual case (as I stated) is that the vapour pressure of the substance (which is a function of temperature) must be equal to the surrounding pressure, now in all sources I can find, they only discuss the situation of the air we are used to and (usually) water.

    Given that the thermodynamic equilibrium is reached when the vaporised molecules per unit time equals the condensed molecules per unit time, does it stand to reason that in, say, a gas of pure Nitrogen at 1atm, the water would boil as if (almost) in a contained vacuum (as the partial pressure of water in the system is 0 to start with), or would it require the same 100 degrees celsius input heat to boil?

    I personally feel like the latter should be the case, however the more I think about it, the less I can see a reason why. In fact, the more I think about it, the more I become confident the former is the case regardless of my instinctual intuition.
    Last edited: Nov 16, 2014
    PsychoticLeprechaun, Nov 16, 2014
    Last edited by PsychoticLeprechaun; at Nov 16, 2014
    #5
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  6. PsychoticLeprechaun

    PsychoticLeprechaun Designer & Web Developer

    • Dev Member
    After a small discussion with a bunch of friends studying physics alongside me at uni, we have come to the general opinion of the following:

    The thermodynamic equilibrium point is unaffected by whatever gas (or lack thereof) is present surrounding the liquid substance, however it is affected by the partial pressure of the vaporised part of the substance surrounding the none-vaporised part of the substance (this partial pressure is the pressure that the particular gas would exert if all other gases in the mixture were removed).

    However, the rate of change of thermodynamic equilibrium would be affected by two things: the initial partial pressure of the vaporised part of the substance, as this would reduce the net number of molecules vaporising per unit time as some of those gaseous molecules would condense while others vaporise. As well as it being affected by the total pressure of the gas surrounding the none-vaporised substance, as this would tend to occasionally knock some of the vaporised molecules back into the liquid part of the substance, as well as "sap" away energy from the most energetic molecules in the liquid - these two things occurring more often with higher total pressures.

    In terms of quantifying this, I couldn't, but conceptually this makes sense, and would explain why vaporisation occurs so much more rapidly in a vacuum than in a gas that contains no gaseous amount of the substance that is to be vaporised.
    PsychoticLeprechaun, Nov 16, 2014
    Last edited by PsychoticLeprechaun; at Nov 16, 2014
    #6
  7. tetryds

    tetryds cus tet, that's it

    • Member
    You scared him.
    Why did you do that?
    He asked why that happens to start with, how do you expect him to react to "partial pressure" and talks like that?
    What I mean is, you overdid it, the ones who know what you are talking about already knew it, the ones who don't didn't understand what you said :p

    Also, your last statement is not scientific (because citation needed).
    But I don't want to be rude, just saying.
    Last edited: Nov 16, 2014
    tetryds, Nov 16, 2014
    Last edited by tetryds; at Nov 16, 2014
    #7
  8. PsychoticLeprechaun

    PsychoticLeprechaun Designer & Web Developer

    • Dev Member
    Perhaps, but I don't think so; better to give all the information and then people choose how much they understand (by reading further around the subject or asking more) than limit the amount of information provided and preclude that from ever happening! After all, giving a less complete answer sometimes precludes a correct one (your answer isn't correct as far as detailed explanation goes, but more just gives a simple mental image for those that don't want to explore the reason why pressure affects boiling point in depth).

    Also, I have a feeling that most people who understand that total pressure does indeed affect boiling points will never have actually tried to determine why it is the case - I know that none of my friends had (and we're all studying physics!).

    In fact, that made me realise I never gave a conclusion as to how the different things affect boiling point:

    (Ignoring the more complicated parts of what I went into above)

    The boiling point of a substance is simply as defined; the vapour pressure of the thing to be vaporised is equal to the surrounding pressure (and increasing the temperature of the substance increases its vapour pressure), which has a simple effect when the system that includes the substance and surrounding gas is in thermodynamic equilibrium (number of molecules that become part of the liquid = number of molecules that become part of the gas) before the boiling begins: the total pressure increasing will increase the number of collisions with the liquid's surface, and so reduce the kinetic energy of the surface of the liquid, as well as increasing the likelihood that a molecule of the liquid that does escape would collide and rebound back into the liquid. This means that the idea of thinking of the pressure as a "force" that acts to keep the liquid from boiling is only true in a very simplistic sense, which most likely comes from trying to just extrapolate the macroscopic aspects of pressure to the atomic scenario of vaporisation.
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  9. tetryds

    tetryds cus tet, that's it

    • Member
    The interactions between the gas and the fluid are also put in a very simplistic sense on your phrasing.
    But you can never explain/know everything of everything.

    His knowledge on this subject is now what I told him + what he researched by himself.
    There is a massive gap between what he knows now and the point where you start your explanation, rather than make sense it will easily confuse him.
    Not only that, but also several specific words with physical meanings most people are not aware of.
    This makes it impossible for him to expand his knowledge by following your posts, that is what I mean.

    Unfortunately the only way I see this could be done would be introduct every basic topic required for the understanding of this subject.
    If you can do that then you are amazing, but I don't think it's easily feasible.
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  10. PsychoticLeprechaun

    PsychoticLeprechaun Designer & Web Developer

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    It isn't difficult, just would take a long time. That said, most people surely get training in the physical sciences up to a certain standard. To understand what I said, one only needs to understand some simple energy physics (kinetic energy being related to velocity and also to temperature), dynamics (pretty much just momentum), and that a liquid is a bunch of loosely bonded molecules and a gas is a bunch of none-bonded molecules. Every thermodynamic term I mentioned, I also explained (i.e. what is partial pressure, total pressure and vapour pressure).

    That said, I may have explained some of them poorly, I cannot make comment on that myself, as I already understand those terms. So if that is the case, and anyone is also interested in making sure they understand the details I gave, then let me know!

    Also, my explanation is pretty much as complete as it gets conceptually, and I could not give a quantification of those concepts because it isn't exactly simple to do so.
    PsychoticLeprechaun, Nov 16, 2014
    Last edited by PsychoticLeprechaun; at Nov 16, 2014
    #10
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  11. joppiesaus

    joppiesaus Infamous Space-Octopus

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    Thank you very much y' all! :thumbup:

    Soooooo
    if pressure of boiled liquid = surrounding pressure, cook.
    Well that was simple.

    And tetryds, you are right, I don't understand everything, but studying the terms and just think about as an image helps a lot.
    The "advanced" answers help explaining stuff, telling it's background. If I don't understand it, that's OK, maybe someone else likes it.
    And, mostly important, this is the "nerd-out zone" afterall :cool:
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  12. tetryds

    tetryds cus tet, that's it

    • Member
    No problem, you can ask anything else :)
    You summed it up in a very simple way to understand.

    Smashed my argument, lol :p
  13. Jpr

    Jpr Industrial Re-revolutionist

    • Member
    Aw. I was going to explain too...
    Caught it too late :(
    I'll catch the next one :)

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