What is the Critical Pressure?

The pressure that corresponds to the critical point of the substance is termed as the critical pressure of that substance. It is defined as the point on the temperature and pressure scale in which a liquid substance can coexist with its vapour. For temperatures that exceed the critical temperature, it cannot be liquified no matter the amount of pressure applied. Hence, we can also define critical temperature as the pressure applied to a substance to liquefy it at its critical temperature. Critical pressure is denoted by 'PC.'


Liquefaction of Gases

Gas enters the liquefaction state when the intermolecular forces of attraction become so high that they bind the gas molecules together, forming the liquid state. By increasing the pressure on the molecules, the intermolecular forces of attraction can be increased. Hence, increasing and decreasing the amount of temperature and pressure in the liquefaction of gases is extremely important. As the temperature of gases increases, so makes the difficulty of it to liquify. Thus, they require more pressure to complete the process.


Triple Point

The matter has three common states, namely solid, liquid, and gas. The physical states of these substances are influenced by two primary factors, temperature, and pressure. Each substance has a phase boundary, and therefore by adjusting the temperature-pressure combinations, we can pinpoint the phase boundary of any substance. Some substances have a triple point at which the substance can exist in all three states of matter. 


Critical State

In thermodynamics, the critical state is said to be the endpoint of a phase equilibrium curve of any substance. The critical state is the endpoint of the pressure-temperature curve that determines when liquid and its vapour can coexist. We know that, at higher temperatures, gas cannot be liquefied by pressure alone. Therefore, the critical temperature and a critical pressure pc, the phase boundaries vanish. 


Critical Pressures of Some Common Substances

  • The critical pressure of water is 217.7 atm or 22,060 kiloPascals.

  • The critical pressure of ammonia (chemical formula: NH3) is approximately 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) is 2.24 atm or 227 kiloPascals.

  • The critical pressure of nitrogen corresponds to 33.5 atm or 3390 kiloPascals.

  • The critical pressure of Methane is 45.79 atm or 4,640 kiloPascal.

  • The critical pressure of Oxygen corresponds to 49.8 atm or 5,050 kiloPascal

  • The critical pressure of sulfur corresponds to 207 am or 21,000 kiloPascal.

  • Gold has a critical pressure of about 5,000 atm or 510,000 kiloPascal.

  • Mercury's critical pressure is 1,720 atm or 174, 000 kiloPascal.


Critical Pressure of Water

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The critical pressure of water is defined as the vapour pressure at its critical temperature, which is above the point at which the distinct gas and liquid phases do not exist. When the temperature of water approaches the critical temperature, then the gaseous and liquid phases' properties become the same. It results in the occurrence of only one phase. It is a well-known fact the critical point of water occurs at a temperature point of 647 Kelvin. It is equal to 374 degrees Celsius and 705 degrees Fahrenheit. The critical pressure is equal to 22.064 MPa, which is approximately equal to 218 atmospheres of pressure.

FAQs (Frequently Asked Questions)

1.What is the Difference Between the Critical Point and the Triple Point?

Ans. Critical point refers to the coexistence of two phases of the same substance. It is the point at which saturated liquid and saturated vapour lines meet on a phase diagram. 


Whereas, the triple point determines the coexistence of three phases of the same substance. On a phase diagram, the point at which the sublimation line and fusion line, and vapourization meet and is known as the triple point. 

2.What is the Importance of a Critical Temperature?

Ans. The critical temperature is the measure of the strength of intermolecular forces of attraction. It is extremely important because it determines the liquefaction of solids. The weaker the intermolecular force, the more difficult it will be to liquefy the gas. For example, weak intermolecular forces are present in helium as well as hydrogen and therefore they have a low critical temperature and they are difficult to liquefy. Whereas Carbon dioxide and ammonia have strong intramolecular forces of attraction; hence they can be easily liquefied because their critical temperature is higher than room temperature.