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B Pressure and boiling

  1. Dec 2, 2018 #1
    Hi everyone,
    I have been mulling over the relationship between pressure and boiling for some time, and I am still slightly confused. I shall attempt to provide an overview of my current understanding in the hope that I can get some corrections/clarification on my current conceptual understanding.

    I shall start with the following: Imagine I have some fixed amount of water, say 1 mole, in the gaseous state. I apply an external pressure to this mole of water, and the gaseous water applies its own pressure on the sorroundings. It would seem to me that, if the external pressure is greater, then the gaseous water will be compressed. Let's say that the external pressure is greater. The gaseous water is compressed and begins to condense in to the liquid state. As it is compressed, it exerts increasing pressure on the surroundings. Let's say that I can apply sufficient external pressure to force the water in to a liquid state, but that the external pressure I can apply is limited. At some point, the pressure the water applies to the surrounding should equal the external pressure; at this point, the water ceases to be compressed. It seems to me that this would have to be the case: if the external pressure was greater, then the liquid water would continue to compress.

    Okay, so the two are equal, the pressure that the water applies to its surroundings (I am going to call this internal pressure) is equal to the pressure from the surroundings (I am going to call this external pressure). Now I am going to heat the liquid water, increasing its thermal energy and increasing its internal pressure. It therefore begins to exceed the external pressure and expands. As its volume increases, its pressure decreases; if it again equals the external pressure, it will cease to expand. If it expands sufficiently, the pressure within the bulk of the liquid should drop enough for the liquid water to enter the gas phase. This is what boiling is.

    Does this explanation in anyway make sense?
    I have a sneaking suspicion it is wrong, but I am not sure how.
    Thanks for any responses.
  2. jcsd
  3. Dec 2, 2018 #2


    Staff: Mentor


    Your narrative is contradictory. You start with a gas, then later you assume it is a liquid. Also, you can't treat the pressure vs temperature nature of a gas to the pressure vs temperature nature of a liquid or a solid.

    I think what you are trying to ask is about phase transitions gas/liquid/solid.
    From https://en.wikipedia.org/wiki/Phase_transition

    This wiki page may help. Read that first, then come back with questions about the parts you don't understand.
  4. Dec 2, 2018 #3


    User Avatar
    Science Advisor
    Gold Member

    Your question doesn't include how the temperature affects things. From the above phase diagram, you can see that temperature / pressure combinations both affect what phase the water is in. Energy has to flow in or out of the water for a phase change. Read up what @anorlunda has suggested (and as many more sources as you can manage). It will all come clear but don't try to rely too much on making your own conclusions - that's a potentially wasteful way of learning.
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