# Critical temp - which phase?

1. Jul 4, 2004

### turin

I have just discovered that the critical temperature of (fluid) nitrogen is about 125 K. Since the nitrogen in the atmosphere is (I assume) at about 300 K (well above the critical temperature), does this mean that the nitrogen in the atmosphere isn't really in the gas phase, but rather in the superfluid phase?

2. Jul 4, 2004

### mathman

At normal pressures, nitrogen (as you may have noticed when you breath) is a gas at 300 K. Only at very high pressures will it behave strangely.

3. Jul 5, 2004

### turin

OK, that makes sense. Thanks. I'm still a little confused, though. What is the meaning of "critical temperature?" I used to think that it was the temperature above which a fluid is either or neither a gas or a liquid, just a fluid.

4. Jul 6, 2004

### salamander

I believe critical temprature is the temperature above which a gas cannot become liquid, whatever pressure you apply to it. I hope I'm right.

Cheers.

5. Jul 6, 2004

### turin

salamander,
Do you have a definition for "critical pressure?"

6. Jul 6, 2004

### salamander

I'm quite sure critical pressure is the pressure it takes to liqudize a gas at a certain temperature. It can also be seen as the pressure at which a liquid will vaporize at a given temperature...
I think that's it, but just don't listen to me if you're about to conduct some experimentation that might be hazardus to yourself, the general public or property if the procedures that you plan to withtake depend on the information I gave you above. There, just putting that in for the legal record

Cheers.

7. Jul 9, 2004

### quarkman

I thought the critical point, whether thought of in terms of pressure or temperature, is the point at which there is no way to distinguish between a liquid or gas phase of a substance. Solids and liquids do not have this point due to the crystalline nature of solids and the lack thereof in liquids, thus making them distinguishable from one another. I would look in a thermodynamics website/textbook for more info.

Note the basis of LCD (liquid crystal displays) could be considered an exception to the solid/liquid phase differences.

8. Jul 9, 2004

### Staff: Mentor

That is correct. HERE is water's phase diagram. edit: still looking for a decent one for nitrogen.

Last edited: Jul 9, 2004
9. Jul 9, 2004

### quarkman

Doesn't the Clausius-Clapeyron relationship give the entire phase boundary for any substance? Is the critical point determined experimentally or can you determine it from this relation? I had a difficult time working with this relation in my undergraduate thermo course.