Critical Point

[MedPR]

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The critical point is the specific pressure and temperature where a gas and liquid can no longer be distinguishable from each other, correct?

How do we know that the critical point is also the point at which gas density = liquid density for a given substance?

 

[milski]

The critical point is the specific pressure and temperature where a gas and liquid can no longer be distinguishable from each other, correct?

How do we know that the critical point is also the point at which gas density = liquid density for a given substance?

If they were different, you would be able to distinguish between liquid and gas based on that.

 

[MedPR]

If they were different, you would be able to distinguish between liquid and gas based on that.

But don't supercritical fluids have solubilities similar to the liquid phase rather than the gas phase? Can't we distinguish them based on that?

 

[mcloaf]

But don't supercritical fluids have solubilities similar to the liquid phase rather than the gas phase? Can't we distinguish them based on that?

I'm not sure I see what you're getting at here. As Milski pointed out, by definition above the critical point your substances liquid and gas phases are indistinguishable. Perhaps an easier way to think about it is that above the critical point the liquid and gas phases simply do not exist, there is only one phase of supercritical fluid (this is as per Wikipedia). It's not a mix of liquid and gas acting similarly, it is a single, distinct phase.

 

[SaintJude]

And what about at the triple point, where all three phases coexist? Does density of gas = density liquid=density of solid?

 

[milski]

And what about at the triple point, where all three phases coexist? Does density of gas = density liquid=density of solid?

It's not really three states coexisting. It is a single state. As such there is nothing to distinguish between.

 

[SaintJude]

So yes? I see that you're saying that there may be no such thing as the density of a seperate phase such as that of "gas" or "density" of solid at that moment. But this could also argued for the critical point as well. So, say the passage did give the density of the gas at a triple point, then we could we also assume it's the same density for all the other phases?

 

[TXKnight]

At thw triple point the 3 phases coexist that is, you'll find all 3 states (with their distinct individual properties) at the same time. Beyond the critical point, liquid and gases are no longer distinct. They share the same properties because they are neither one or the other, they behave as a mixed entity with physical properties that are between the values of the their other states.

 

[SaintJude]

Ah, thank you for putting it in clear terms.