How phase change at the critical point is different from the water-vapor phase transition?

Summary

A phase change at a critical point occurs when a substance has zero degrees of freedom to choose another point at a fixed temperature and pressure. Water-vapor phase change occurs on an equilibrium line between two phases with one degree of freedom [phase diagram]. That is, by varying either temperature or pressure independently while holding the other constant, there are infinite possibilities for a phase change. In a nutshell, that is the distinction.

What exactly is molecular symmetry?

Energy levels differ between the two phases. Both phases of a phase molecule have the same energy level at maximum symmetry.

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A phase change can occur in two ways [1] when two existing phases in a system find out a point of pressure and temperature where they have minimum molecular symmetry and [2] when two existing phases in a system find out a point of temperature and pressure where both phases have the minimum free energy.

The first one is called the second-order phase change. The second one is called the first-order phase change.

Critical point phase change – 2nd order phase change

A critical point is a property of a pure substance. Every pure substance has a critical composition at a fixed temperature of pressure for example water has a critical point at 374 degc and 217.7-atmosphere pressure. The critical point is a point where the two phases have minimum molecular structural symmetry.

Let us elaborate on the second-order phase change.

Let's?start?with?the?basics. When?the?energy?difference?between?two?phases?is?the?smallest,?a?phase?change occurs. They?achieve?equilibrium. Entropy?or?disorder?is?important?whether?the?phase?change?is?first?or?second?order.

There?are?two?points?to?be?made?here.?The?first?is?that?a?phase?is?more?disordered?at?high temperatures?than?at?low temperatures?due?to?molecular?motions.

Due?to?spontaneous?symmetry?breaking,?the?high-temperature?phase?typically?contains?fewer symmetries?or more entropy than?the?low-temperature?phase. This minimizes the free energy for phase change.

This happens abruptly when two phases interact and reach critical temperature and pressure. Critical point phase change is an example of 2nd order phase change. Phase change at a critical point does not involve the role of latent heat. At a critical point, all phases exist at minimum free energy with degrees of freedom F = 0.[ F = C -P+ 2, at the critical point there are 3 phases [1] Sub-critical vapor, [2] Sub-critical liquid, and [3] critical fluid, F = C-P +2 = 1 -3 +2 =0]

Therefore, the critical point does not have an option or freedom to reverse its phase. A superfluid is a unique collection of particles that has no kinetic energy and no viscosity. The superfluid component has zero viscosity and zero entropy. Since all the atoms in the superfluid are in the same energy level the total entropy of the system is zero.

Water-vapor phase change

First-order phase change

The water-vapor phase change is a first-order phase change.

Any transition involving latent heat in which the system absorbs the heat but the temperature remains constant is referred to be first order.

-The system does not fully shift to the new phase in a seamless manner (regions of both phases exist simultaneously)

-The state variables of the system have a discontinuity (such as temperature or pressure)

Melting ice and boiling water are two common instances of first-order phase transitions (not all ice melts instantaneously nor does all water turn to steam immediately).

What triggers 1st order phase change?

The fundamental unit of matter we meet on a daily basis is an atom, which is made up of protons, neutrons, and electrons in particular ratios. Atoms have a strong interaction with one another and can produce many phases of matter. In a nutshell, a phase of matter is a group of interacting constituents with macroscopic properties that are not accessible through constituent analysis. A phase is the existence of molecules at the lowest free energy.

So, when a huge number of ingredients are present, how do they decide which phase to form?

The answer is they all want to achieve minimum free energy. That is how different phases arise. ?G = ?U - T?S. or, ?G =0. ?

When two phases in equilibrium are struggling to maintain equilibrium as in the case when heat is applied to water, or you are adding enthalpy to water, the entropy will force more intermolecular bonds of water to break to reduce its free energy and let water-vapor stay in equilibrium by pulling down the energy of water.

The first-order phase change is not a property of a pure substance as we saw at the critical point. The degree of freedom for 1st order phase change, F = C -P + 2 = 1 -2 +2 [ C = Components, P = Phases and F =Degrees of freedom] 1st order phase change can occur at any temperature or pressure. In other words, 1st order phase change has the freedom to change its location by independently varying either or pressure. By varying either temperature or pressure independently while holding the other constant, there are infinite possibilities for a phase change.