Critical Pressure

What is Critical Pressure?

The critical pressure of a substance is the pressure corresponding to the critical point (or the critical state) of the substance. The critical point of a substance can be defined as the point on the temperature and pressure scale in which a liquid substance can coexist with its vapour. At temperatures above the critical temperature of a substance, it cannot be liquified with the application of any amount of pressure. The critical temperature of a substance can, therefore, be defined as the pressure that must be applied to a substance in order to liquify it at its critical temperature. The critical pressure of a substance is often denoted by the symbol ‘PC’.

The three most common states of matter, namely solid, liquid, and gas, are known to be separated by phase boundaries. These phase boundaries are set by the two primary factors that influence the physical state of a substance – temperature and pressure. Therefore, the adjustment of the temperature-pressure combinations can help pinpoint a phase boundary. For example, the triple point of a substance is a point at which a substance can exist in all three states (solid, liquid, and gaseous). The temperature and pressure values at the triple points of different substances are different. For example, the triple point of water corresponds to 0.01 degrees celsius (or 273.16 Kelvin) and 4.58 mm of mercury.

In a similar fashion, by finding a similar temperature-pressure combination, the phase boundary that separates the liquid and the vapour state of a substance can be estimated. This temperature-pressure combination is commonly referred to as the critical point of the substance. The pressure corresponding to the critical point of a substance is the critical pressure of that substance.

Critical Pressures of Some Common Substances

  • The critical pressure of water corresponds to 217.7 atm or 22,060 kiloPascals.
  • The critical pressure of ammonia (chemical formula: NH3) corresponds to 111.3 atm or 11,280 kiloPascals.
  • The critical pressure of chlorine (symbol: Cl) corresponds to 76 atm or 7,700 kiloPascals.
  • The critical pressure of helium (symbol: He) corresponds to 2.24 atm or 227 kiloPascals.
  • The critical pressure of nitrogen corresponds to 33.5 atm or 3390 kiloPascals.

Frequently Asked Questions on Critical Pressure

What is the Critical Pressure of a Fluid?

The critical pressure of a fluid can be defined as the vapor pressure of the fluid at its critical temperature (above which point, distinct gas and liquid phases do not exist). While approaching the critical temperature of a substance, the properties of the gaseous and liquid phases are known to become the same, resulting in only one phase.

What is the Critical Point of Water?

The critical point of water is known to occur at a temperature point of 647 Kelvin (which is equal to 374 degrees celsius and 705 degrees fahrenheit) and a critical pressure of 22.064 MPa (which is roughly equal to 218 atmospheres of pressure).

What happens at temperatures above the critical temperature?

At temperatures that are higher than the critical temperature of a substance, the molecules of the substance are known to have too much kinetic energy for the intermolecular forces of attraction to bind them together in the liquid phase. Thus, above the critical temperature, the substance cannot be liquefied.

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