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Imagine a simple experiment - we have a high-pressure container partially filled with water, and then pump the air pressure inside to a high level, about 1000+ bar. At room temperature, air density inside the container will get to the neighborhood of 1225 kg/m3, while water density should still be near 1000 kg/m3.

Does this mean that water will be actually splashing at the TOP of the container, while the air occupies the space at the bottom?

This question is directly related to Is it possible to have a Gas heavier than a liquid?, but more specific.

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  • $\begingroup$ Interesting question. Air isn't a gas at room temperature and 1000 atmospheres, though. $\endgroup$ – Chris Nov 28 '17 at 2:54
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No. You run into a pretty general problem here- if you increase the pressure of a gas enough that it has the density of a liquid (of comparable molecular mass), it won't be a gas anymore! At some point while you are raising the pressure, one of a few things will happen:

  • The gas will undergo a phase change to a liquid (if at a temperature above the triple point)
  • The gas will undergo a phase change to a solid (if at a temperature below the triple point)
  • The gas will become a supercritical fluid, which is not clearly a liquid or a gas, but has properties of both (if at a temperature above the critical point)

In the case of both nitrogen and oxygen (and so the vast majority of air), it is the last one that occurs, as both have critical temperatures at much below room temperature.

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  • $\begingroup$ Also in general the two substances may interact non trivially, for example it should be studied how much air will go into water and thus what will become of it. $\endgroup$ – Yrogirg Nov 28 '17 at 5:08

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