Critical point A to Z: Why do at critical point the liquid-vapor equilibrium break?

Summary: The thermodynamics of water-steam changes dramatically near the critical point. The direct relationship between pressure and temperature [P and T] of saturated water and steam ceases to exist near the critical point. When water expands due to the breaking of H bonds, the resulting repulsion begins to compress the vapour phase. There comes a point when the vapour can no longer withstand further compression. The vapour sub-cools. The volume of the vapour decreases as it cools. The density of the vapour phase increases as it contracts. This process is repeated until water and vapour have the same density. The two phases become indistinguishable.

This is the critical point.

Further clarification

The critical temperature is the temperature at which a substance transitions from a gas to a liquid as pressure is increased, while the critical pressure is the pressure at which this transition occurs at the critical temperature.

The critical pressure measures the force required to compress the gas into a liquid at the critical temperature. At a critical point, the compression of the vapor phase exceeds the vapor pressure of the liquid phase and the liquid-vapor equilibrium break.

Detail

Above a certain temperature called the critical temperature, liquid and vapour combine to form a single phase known as a supercritical fluid. Near the critical point, the physical properties of the liquid and vapour change dramatically, with both phases becoming increasingly similar. Liquid water is nearly incompressible under normal conditions, has a low thermal expansion coefficient, and a high dielectric constant, and is an excellent electrolyte solvent. All of these properties reverse near the critical point: water becomes compressible, expandable, a poor dielectric, a poor electrolyte solvent, and prefers to mix with nonpolar gases and organic molecules.

The following fundamental knowledge is required to comprehend critical temperature.

The phase diagram of water [PT diagram]: Pressure and temperature are represented by the axes. The phase diagram depicts the lines of equilibrium or phase boundaries between the three pressure-temperature phases of solid, liquid, and gas. On the phase diagram, the curves represent the points that separate the two phases. Along this line, the two phases are in equilibrium, with Gibbs free energy = 0.

The phase boundary between liquid and gas does not continue indefinitely in the diagram on the left. Instead, it comes to an end at a point on the phase diagram known as the critical point. This reflects the fact that, at extremely high temperatures and pressures, the liquid and gaseous phases of a supercritical fluid become indistinguishable. Tc = 647.096 K (373.946 °C), pc = 22.064 MPa (217.75 atm) are the critical points in the water.

Critical point: The critical point defines the boundary where the water-steam [ subcritical] region ends on the phase diagram and the supercritical phase region begins. The critical point (or critical state) is the end point of a phase equilibrium curve.

Phase boundaries vanish at the critical point, which is defined by a critical temperature Tc and a critical pressure pc. At temperatures above the critical temperature, the substance cannot exist as a liquid, regardless of pressure. At temperatures and pressures above the critical point, the substance is classified as a fluid, which is neither a gas nor a liquid. The substance is considered a gas at pressures lower than the critical pressure (but at higher temperatures). Near the critical point, many physical properties of pure fluids exhibit unusual behaviour.

Below the critical point, the region is called a subcritical region. Above the critical point, the region is known as the supercritical region.

What happens when water is heated?

Part 1: Water: Liquids are incompressible because there is comparatively less free space between molecules. Gases are compressible because gases have plenty of free space between molecules.

Please follow the attached TS diagram of the water- steam

As you heat water and as long as the water is saturated with steam that is water and steam are inside the bell curve, they are in equilibrium. Inside the bell curve, the temperature and pressure of both phases increase at constant volume. P and T follow a direct relation inside the bell curve. The topmost point of the bell is the critical point.

At a critical point, the compression of the vapor phase exceeds the vapor pressure of the liquid phase and the liquid-vapor equilibrium break.

P and T direct relations are lost near the critical point. The standard EOS is not valid at the critical point. At the critical point, it is different thermodynamics. Near critical points water and steam, behaviour changes dramatically. Water breaks its H-bonds leaving the positively charged H ions of H-O-H to repel each other. Water expands at critical pressure and its density reduces. The density of water reduces to 0.322 g/cm3.

At temperatures and pressures above the critical point, the substance is classified as a fluid, which is neither a gas nor a liquid. The liquid and gas phases combine into a single phase.

Part 2: Vapor

When water expands due to the breaking of H bonds, the resulting repulsion, begins to compress the vapour phase. There comes a point when the vapour can no longer withstand further compression. The vapour sub-cools. The volume of the vapour decreases as it cools. The density of the vapour phase increases as it contracts. This process is repeated until water and vapour have the same density.