A question about terminology of phase state & criticality

In summary, a supercritical fluid is a state in which both the temperature and pressure are above the critical point, making it behave as both a gas and a liquid. The term subcritical is used for a gas or liquid with lower temperature and pressure than the critical point. There is no specific term for a gas with higher temperature but lower pressure, or a liquid with higher pressure but lower temperature than the critical point. They would be considered normal gas and liquid, respectively.
  • #1
swampwiz
571
83
I'm looking at the diagram (Figure 1) at

http://en.wikipedia.org/wiki/Supercritical_fluid

I get that the region in which both the temperature & pressure are higher than the critical point is called a supercritical fluid, which really means it is both a gas & a liquid, thermodynamically speaking.

And as I understand it, the term subcritical gas or liquid is for the region in which the temperature & pressure are both less than the critical point - with the subcritical gas also termed a vapor here.

But what about a gas that is a higher temperature but lower pressure, or a liquid at a higher pressure, but lower temperature, than the critical point. It seems that there should be some term for this that is something like a "saddle" state. I don't think the term supercritical should be used, but something with critical should be.

Any idea what the terminology of this is?
 
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  • #2
a gas that is a higher temperature but lower pressure...

Isn't it just normal gas? How does it differ?

a liquid at a higher pressure, but lower temperature,

Isn't it just a normal liquid? Ditto?
 

FAQ: A question about terminology of phase state & criticality

What is phase state and criticality?

Phase state refers to the physical state of a substance, such as solid, liquid, or gas, under certain conditions of temperature and pressure. Criticality refers to the point at which a substance undergoes a phase transition, such as from liquid to gas, due to changes in temperature and pressure.

How do you determine the critical point of a substance?

The critical point of a substance can be determined by plotting its phase diagram, which shows the relationship between temperature and pressure at which a substance exists in different phases. The critical point is the point at which the phase boundary lines between the different states of matter merge into one.

How does criticality affect the properties of a substance?

At the critical point, the properties of a substance, such as density, heat capacity, and compressibility, undergo significant changes. These changes are due to the fluctuations in the molecular structure of the substance as it transitions between phases.

Can a substance have multiple critical points?

Yes, a substance can have multiple critical points, depending on its molecular structure and properties. For example, water has two critical points, one for the transition from liquid to gas and another for the transition from liquid to solid.

How is criticality important in industrial processes?

Criticality is an important factor in many industrial processes, such as chemical reactions and material synthesis. Understanding the critical point of a substance can help optimize these processes and improve the efficiency of production. Additionally, some substances, such as supercritical fluids, have unique properties at their critical points that make them useful in various applications, such as in pharmaceuticals and cleaning products.

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