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Q & A: Liquid phase?

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Most recent answer: 10/22/2007
Q:
Can You Make a liquid out of all gases?????
- Anonymous
A:
That's a pretty interesting question. I think the answer is no, but I can't think of an example off-hand. Here's the reasoning.

Whether some substance forms a solid or liquid or gas (we'll ignore other possibilities here) depends on both its temperature and its pressure. The possibilities are described by a "phase diagram" like the one shown in another answer: .

In many cases. at low pressure the solid goes directly to a gas without going through the liquid phase in between as temperature is raised. Dry ice (carbon dioxide) is a familiar example. At higher pressure, there typically is a liquid phase between the solid and gas, as the temperature is raised. At (usually) still higher pressure, the dsitinction between a liquid and a gas is lost. As you heat up that fluid, it gradually gets less dense, but it doesnít have any abrupt change from a dense "liquid" to a less dense "gas". I suspect there may be siome substances where the distinction between a liquid and a gas is gone over the whole pressure-temperature range that isnít solid. Also, for substances made of molecules, we know that some molecules fall apart as the temperature is raised. Thereís no reason in principle why that couldnít happen at temperatures lower than those needed to get a liquid. So Iím guessing that there must be some substances for which thereís no liquid phase. Unfortunately, I donít know of any examples. If I hear of one, weíll update this answer.

Mike

(published on 10/22/2007)

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