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Phase problem

  1. Jan 1, 2008 #1
    sorry to ask this stupid question....
    can anyone tell me exactly the difference between liquid and gas based on kinetic model????

    most textbooks give that in liquids inter molecular spaces are lesser than gases ,etc & etc

    but if compress the gas to much ,still it doesnt get converted into liquid
    surely its I.M.S decreases. i think

    do not include macroscopic properties...
  2. jcsd
  3. Jan 1, 2008 #2

    The inter molecuar spaces of liquid are not only lesser that gases, but also they are almost unchanged. That's why liquids are almost uncompressable, unlike gases.

    This depends on what kinds of gases. If you compress CO2 to about 80 bars, you have liquid CO2.
    Last edited: Jan 1, 2008
  4. Jan 1, 2008 #3


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    sure it does. and if you increase the pressure further it becomes a solid. Waver vapor, for example (at a fixed temperature, say 400 Kelvin) will become a liquid (water) as it is compressed (pressure increased) and then if pressure is increased still further it will become a solid (ice VII).

    There is a beautiful discussion in the introductory chapter of Chaiken and Lubensky's condensed matter book about the microscopic (and macroscopic) differences between water vapor, water, and ice.

  5. Jan 2, 2008 #4
    i have seen in phase diagrams that water vapour cont be converted to liquid if its temperature is more than the critical temperature which is 374.1 Celsius.

    well ihave seen that water vapour at around 374.1
    is as difficult to compress as a liquid at that temperature at very high pressure >261 atm , though it is not a liquid at that temperature .

    well liquids are negligibly compressible , not completely..

    that means only intermolecular distances cannot decide whether the substance is a liquid or gas...
  6. Jan 2, 2008 #5
    Anyway, the difference is that you can compress a gas to very high density, but when you release, the volume will increase infinitely. While as for a liquid, the volume will expand very little and then stop no mater how much you reduce the pressure.
    Probably the expansion of a liquid is similar to that of solid.
  7. Jan 2, 2008 #6


    Staff: Mentor

    The intermolecular spacing is small with a typical liquid, however there are also intermolecular interactions that essentially define the liquid phase. In the gas phase the molecules interact only briefly during collisions, but in the liquid phase the interactions are fairly continuous. It is these interactions that are broken when a liquid evaporates and the reason that heat is required despite no increase in temperature.
  8. Jan 2, 2008 #7
    if we decrease the pressure to much then will not liquid molecules leave the liquid surface
    and become gaseous molecules(if temperature is suitable)
    well you cant think of liquid at room temperature in vacuum it will certainly turn into gas and expand
  9. Jan 2, 2008 #8
    That is different. What we call phase transision. Liquid ==> Gas.
    Once in liquid state, the mater can not expand more than certain extent if you reduce the pressure.
  10. Jan 2, 2008 #9
    then what is the difference between liquid and solid since both have attraction but in one motion is vibratory and in other it is transilatory.
  11. Jan 3, 2008 #10
    The difference between solid, liquid and liquid - gas, I know. But in fact, I have been considering your question one of the mystery, at least for me !
    Hope to hear from others.
  12. Jan 3, 2008 #11
    not necessary if you compress the gas to much inter molecular distances will be less enough for intermolecular attraction.

    compress 100 kg of gas in 1 cubic cm volume , at fairly high temperature , it remains gas.

    to Mr. pixel01 your all answers talk about macroscopic properties , you never told about the kinetic model of them i.e at what energy , I.M.F,I.M.S,pressure the subs. exist in a given phase...actually this is what i asked for .

    but still thank for your views
  13. Jan 3, 2008 #12


    Staff: Mentor

    That is correct, at the critical point the density and viscosity of the liquid and the gas phases are equal and the two phases are indistinguishable. The molecules are close enough to be interacting fairly continuously. At this point much of the behaviour that characterizes the gas phase is lost because of these interactions. The gas is no longer very compressible and the deviations from the ideal gas law are quite extreme. It is certainly correct to say "it remains gas" as long as you understand that it is not an ideal gas and has some liquid-like properties due to the interactions.

    EDIT: Going back to your original question, in the kinetic theory a gas and a liquid at the same temperature have the same average kinetic energy, so the difference is something else. Also, energy is required to make the transition although this energy does not go to increasing the kinetic energy. So, where does it go? It goes to increasing the potential energy by breaking these intermolecular interactions (e.g. hydrogen bonds).
    Last edited: Jan 3, 2008
  14. Jan 3, 2008 #13
    thanks for that reply
  15. Jan 3, 2008 #14


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    a solid is rigid and has long range order; i.e. the atoms are fixed in a lattice so that if you know the position of one atom then you know the position of the other atoms relative to it even very far away. E.g., you know that there is another atom 100 lattice spacings to the right of your first atom.

    Whereas in a liquid there is no lattice and there is no correlation between the position of your atom and the position of a far away atom.
  16. Jan 4, 2008 #15


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    It looks like you've already got some good explanations; here's a little bit more.

    Let's consider the positions of the charges in an atom. Protons are concentrated together in one location: the nucleus. Electrons are different and instead inhabit the outside of the atom, at distances that are quite far from the nucleus (relative to their sizes anyways). So protons are generally on the inside and electrons on the outside.

    Now consider the electric force that is inversely proportional to the square of the distance. Gas species are likely to repel each other because as they approach each other individually from far away, the outer electrons + outer electrons approach each other first. This is also why gases tend to expand to fill their containers. This is only part of the story though...

    The fact that protons are concentrated in the nucleus has some other effects. As we increase in atomic number, the protons continue to be found in the nucleus yet the electrons are spread all throughout the voluminous space around. The consequences of this is that there are certain positions (much closer to nuclei) where the nucleus of one species attracts certain valence electrons of another species. If both species hold onto a valence pair of electrons then you have one form of intermolecular forces that is stronger than electron repulsion. You now have a liquid or a solid depending on fluidity and whether the substance has a definite shape.
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