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Should not all substances boil in pure vacuum?

  1. Aug 17, 2014 #1
    It is well known that when the vapor pressure of a substance is bigger than the external pressure, it boils. In vacuum there are no particles and therefore no pressure, shouldnt therefore, even diamond as for example, boil in complete vacuum?
  2. jcsd
  3. Aug 17, 2014 #2
    Why do you think diamonds do not boil in normal conditions on the Earth? And why oxygen does?
  4. Aug 17, 2014 #3
    It is of course due to the intermolecular forces acting. However all solid and liquid substances are in an equilibrium with their gas form, therefore, if we lower the external pressure just enough then the equilibrium will be displaced towards the gas phase. I think this is a consequence of the law off mass action.

    It seems also that the second law of thermodynamics support this. There will be an "urge" for the substance to fill up the empty space in order to raise the total entropy of the system.

    Edit; I am aware that absolute vacuum is very hard (impossible?) to reach, but you can view it as a thought experiment.
  5. Aug 17, 2014 #4
    That contradicts your previous statement, which assumes it is solely the external pressure that holds materials solid (because, otherwise, you would have to conclude that I.M. forces can prevent materials from boiling even when ext. pressure is zero).
  6. Aug 17, 2014 #5
    How is it a contradiction? The intermolecular forces are closely related to the vapor pressure of the substance. Substances with strong forces tend to have a high boiling point and therefore a very low valued vapor pressure. Take as for example water, which has a vapor pressure of approximately 25 torr, and compare it to methanol which has its value around 125 torr. Clearly the hydrogen bonding contributes.

    Edit; a better answer would have been that in normal conditions it does not contribute to an increase in total entropy.
  7. Aug 17, 2014 #6
    I am not talking about vapor pressure. I am talking about just pressure. You said:

    That very clearly assumes that it is (external) pressure that keeps materials from boiling.

    But then you said:

    I. M. forces within a body (e.g., a diamond) are clearly not responsible for (external) pressure. So what keeps a diamond solid? Its I.M. forces or external pressure? (Assuming the ordinary conditions here on planet Earth.)
  8. Aug 17, 2014 #7

    I guess its a combination of both and ultimately the entropy contribution
  9. Aug 17, 2014 #8
    Yes, it can be a combination of both. Then what makes you reject the idea that at least some substances can have I.M. forces alone strong enough to keep their solid shape?
  10. Aug 17, 2014 #9
    But is that the case? I am looking for a discussion and ultimately an answer.

    My thought is that if you have an infinitely big volume of complete vacuum, and a piece of diamond, or any substance in the center, then it must be broken into its smallest components in order to fill the space and create maximum disorder.

    If this is not the case, then why? For me it clearly seems that this is the implication of the thermodynamics.
  11. Aug 17, 2014 #10
    Yes, that is the case. Diamonds are forever or pretty close to that, even in the hardest vacuum you can think of.
  12. Aug 17, 2014 #11
    Do you have any references? I would love to read about the physics behind it, it is quite surprising if that is the case
  13. Aug 17, 2014 #12
    I just find it surprising, since I am aware that diamond has very strong kinetic barriers, but that it still, even in an infinite vacuum can resist being broken down.
  14. Aug 17, 2014 #13
  15. Aug 17, 2014 #14
    It still feels weird that it does not decompose to singly carbon atoms, spread in space. However I guess you are right. Thank you my friend.
  16. Aug 17, 2014 #15


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    Staff: Mentor

    You're thinking about vacuum backwards - it's not infinite vacuum, it's zero pressure. If the intermolecular forces are strong enough to hold the molecules together without any help from outside pressure, the object will remain together.
  17. Aug 17, 2014 #16
    Thank you for the explanation. However, shouldn't free atoms be more stable in absolute vacuum, compared with atoms in a lattice? Putting the kinetics aside.
  18. Aug 17, 2014 #17
    Atoms form a lattice because that arrangement has lower energy. Which means it is more stable.
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