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Q & A: gas to liquid

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Most recent answer: 06/13/2008
Q:
Do all liquid gases change state at room temperatures? Is this change instantaneous for all gases? Thank you for your time and consideration.
- trish
turnersville, new jersey,usa
A:
Nice questions.

The boiling points of liquids depend both on the particular liquid and the pressure. In some cases (say water at much less than atmospheric pressure) those boiling points are near room temperature. In other cases (say helium) the boiling point at any reasonable pressure is far below room temperature. In other cases (say mercury) the boiling point is well above room temperature.

The second question is even better. At low pressure the transition from liquid to gas is generally abrupt- each little region is either gas or liquid. At fixed pressure, you can find an exact boiling point temperature, with all liquid below that temperature and all gas above it. At high pressure, however, the transition from liquidy to gassy is continuous, with no sudden change as the fluid is heated. You can sort of see why. Gas is much more compressible than liquid. As you increase the pressure, the gas density increases a lot more than the liquid density. When they catch up to each other, there's no distinction between the two. High-pressure carbon dioxide in this "super-critical" state is used as a cleaning fluid.

Mike W.

(published on 06/13/2008)

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