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Liquefaction of Gases

  • Last Updated : 22 Dec, 2021

Physics and chemistry are both concerned with the study of matter, energy, and their interactions. Scientists know that matter can change states and that the sum of a system’s matter and energy is constant because of thermodynamic rules. Matter changes state when energy is added or removed, forming a state of matter. One of the ways in which matter interacts with itself to generate a homogeneous phase is called a state of matter.

“State of matter” and “phase of matter” are interchangeable terms. This is fine for the most part. A system can technically have multiple phases of the same condition of matter. A solid bar of steel, for example, could comprise ferrite, cementite, and austenite. There are two liquid phases in a mixture of oil and vinegar (a liquid).

Liquefaction of Gases

Liquefaction is the transformation of a gaseous substance into a liquid condition.

For instance, oxygen is usually a gaseous substance that may be changed to a liquid by exerting enough pressure and lowering the temperature.

To liquefy a gas, the molecules must be brought closer together. By lowering the temperature and increasing the pressure, this can be accomplished. When the pressure on a gas is increased, the molecules get closer and closer until they merge to create liquids at a specific pressure. 

When the temperature of a gas is lowered, however, the molecules lose kinetic energy, resulting in a significant fall in velocity. Slow-moving molecules can’t resist the force of attraction, so they get closer and closer until they clump together to create a liquid.

The liquefaction of gases is caused by a decrease in temperature and an increase in pressure.

Conditions Necessary for Liquefaction of Gases

The following are two conditions that must be met in order for gases to be liquefied:

  • Low temperature
  • High pressure

Critical Temperature

T. Andrews investigated the phenomenon of gas liquefaction in 1869 and was successful in liquefying a number of gases. He discovered that no matter how high the pressure applied, each gas has a specific temperature above which it cannot liquefy. This temperature is referred to as the gas’s critical temperature. It is possible to define it as follows:

The critical temperature of a gas is the temperature above which it is impossible to liquefy it with any amount of pressure.

 for example, Carbon dioxide has a critical temperature of 30.98oC. This means that no amount of pressure given to carbon dioxide will cause it to liquefy above 30.98oC.

Tc is a symbol for the critical temperature. It is provided by-

Tc=8a / 27bR

Where R is the gas constant and a and b are Van der Waal’s constants.

Critical pressure and critical volume:

The critical pressure of a gas is the minimal pressure required to liquefy it at the critical temperature. Pc is the symbol for it, and it is given by-

Pc=a / 27b2

Where a and b are Van der Waal’s constant.

The critical volume of a gas is the volume occupied by one mole of the gas at critical conditions. Vc is the symobl for it, and it is supplied by-


The effective volume of the molecules per mole of gas is denoted by b.

The critical constants of gas are Tc, Pc, and Vc.

Isotherms of Carbon dioxide

T. Andrews conducted a series of carbon dioxide tests, analyzing the pressure-volume relationship for the gas at various temperatures and plotting the results. P–V isotherms of carbon dioxide are the names given to these curves. At 0°C, 21°C, 31.1°C, and 50°C, the isotherm was obtained.

  • Carbon dioxide occurs as a gas at point A, the lowest temperature used, i.e. 13.1oC at low pressure.
  • The volume of the gas drops as the pressure is increased along the curve, as can be seen.
  • Until point B, carbon dioxide behaves like gas when it reaches 21.5oC.
  • The gas exists in a dual state at point B, i.e. it is both liquid and gas.
  • At point C, all of the carbon dioxides condenses, resulting in an increase in pressure.
  • Because the liquid has a much smaller volume than the gas, as liquefaction begins, the volume of the gas rapidly decreases.
  • Because liquids are relatively little compressible, once the liquefaction is complete, the rise in pressure has very little influence on volume. As a result, a steep curve is produced. The steep line depicts the liquid isotherm.
  • The behaviour of the gas on compression is considerably different below 30.98oC, and each curve follows a similar pattern. At lower temperatures, just the length of the horizontal line rises, and at the critical point, the horizontal portion of the isotherm merges into one point.
  • The gas cannot liquefy over 30.98oC, despite the fact that considerable pressure can be applied. As a result, the critical temperature of carbon dioxide is 30.98oC.
  • Because of the isothermal compression, it was discovered that all gases behave identically to carbon dioxide.

Importance of Critical Temperature: In the liquefaction of gases, the critical temperature is crucial. Only when gas is below its critical temperature can it be liquefied. Gases with a high critical temperature, such as NH3, CO2, SO2, and others, can be liquefied by applying sufficient pressure.

Gases with low or very low critical temperatures, such as H2, He, and others, cannot be liquefied simply by adding pressure to them. They can only liquefy if they are chilled below their critical temperature and then exposed to sufficient pressure.

Conditions required to achieve Liquefaction of Gases

The following principles can usually be applied to gas cooling:

  1. By compressing the gas below its critical temperature.
  2. Joule-Thomson effect: When a highly compressed gas is moved through a throttle (a porous plug or jet) from a zone of high pressure to a region of low pressure under adiabatic circumstances, it suffers a temperature drop below its inversion temperature. The Joule-Thomson effect is a phenomenon that is commonly exploited in the liquefaction of gases.
  3. Adiabatic expansion involving mechanical work: When a gas undergoes adiabatic expansion involving mechanical work, some of its kinetic energy is lost, and the temperature drops.

Methods of Liquefaction of Gas

  • Linde’s Method- The Joule-Thomson effect is used in this procedure. Pure, dry air is fed into a compressor, which compresses it to around 200 atmospheres. The heat of compression is then removed by passing it through a conduit cooled by a refrigerating liquid such as liquid ammonia. The pressurized air is then routed via an insulated chamber and into a spiral pipe with a jet at one end. The compressed air expands as it passes through the jet, resulting in a significant reduction in temperature. The expanded air rises through the chamber, cooling the fresh air that enters through the spiral tube. It is then collected and returned to the compressor via a conduit. When the air is sufficiently chilled and liquefied, the procedure is performed again and again.

Linde’s Method

  • Claude’s Method- This method involves mechanical work and leverages both the Joule-Thomson and adiabatic expansion effects of the gas. The compressor accepts only clean, dry air, which is compressed to around 200 atmospheres. The heat of compression is then removed by cooling it with a refrigerating liquid. A tube transports the compressed gas to an insulated chamber. It is divided into two parts here. One component is carried through a spiral tube with a jet at the end, where it undergoes Joule-Thomson expansion and a temperature drop is recorded. The second half is inserted into an engine’s cylinder, where it does mechanical work by pushing the piston back and is cooled. It then enters the insulated chamber and mixes with the jet’s air. The pipe carrying the incoming air is subsequently cooled. Cooled air is gathered and returned to the compressor. When the air is sufficiently chilled and liquefied, the entire process is performed again and again.

Claude’s Method

Sample Questions

Question 1: What is the process of liquefaction?


Liquefaction is the transformation of a gaseous substance into a liquid condition.

For instance, oxygen is usually a gaseous substance that may be changed to a liquid by exerting enough pressure and lowering the temperature.

Question 2: What role does critical temperature play in the liquefaction of gases?


In the liquefaction of gases, the critical temperature is crucial. Only when a gas is below its critical temperature can it be liquefied.

Question 3: What are the uses of the liquefaction of gases?


The most major benefit of gas liquefaction is that it allows for considerably more compact storage and transportation than is possible in the gaseous condition. LPG and Liquid Oxygen, for example, are two of the most significant gases.

Question 4: What role does pressure play in gas liquefaction?


When enough pressure is applied to a gas, the space between its particles shrinks, the gas begins to compress, and the gases begin to liquefy.

Question 5: What is the principle of the liquefaction of gases?


The liquefaction of gases requires high pressure and low temperature, according to the principle. Increasing the pressure leads the gas molecules to move closer together, while lowering the temperature causes the attractive forces to rise.

Question 6: What are the two most important steps in the gas liquefaction process?


The following are the two primary steps involved in the liquefaction of gases: 

  • Increase the pressure, which pushes the gas molecules closer together.
  • The temperature is dropped, which causes the attraction forces to increase.

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