What is liquefaction of gases?

Short Answer

Liquefaction of gases is the process of converting a gas into a liquid by applying high pressure, low temperature, or both. When gases are cooled, their molecules lose energy and come closer. When pressure is increased, the molecules are forced together.

This process is important for storing gases like oxygen, nitrogen, LPG, and carbon dioxide in compact containers. Liquefied gases take less space and are easier to store, transport, and use in industries, households, and laboratories.

Detailed Explanation

Liquefaction of Gases

Liquefaction of gases refers to changing a gas into its liquid form by reducing its temperature, increasing its pressure, or applying both at the same time. In a gaseous state, particles move freely and remain far apart. To convert them into a liquid, the movement of molecules must slow down, and the distance between them must decrease. This can be achieved only when the gas is sufficiently cooled or compressed.

Gases like oxygen, hydrogen, nitrogen, ammonia, chlorine, and carbon dioxide can all be liquefied under the right conditions. The process has many industrial and scientific applications, such as producing liquid fuels, storing medical oxygen, transporting LPG cylinders, and making dry ice.

How Liquefaction Occurs

Liquefaction occurs when the conditions bring gas molecules close enough for intermolecular attraction to hold them together. A gas becomes a liquid when:

  • The molecules move slowly
  • The intermolecular attraction becomes stronger
  • The distance between particles decreases
  • The gas loses enough kinetic energy

These changes happen when the gas is cooled or compressed.

Role of Low Temperature

Lowering the temperature reduces the kinetic energy of gas molecules.
As the gas cools:

  • Molecules move more slowly
  • Collisions become weaker
  • Gas particles come closer
  • Attractive forces become effective

Eventually, the gas reaches a point where it cannot remain in gaseous form and changes into a liquid.

This is why gases like oxygen and nitrogen are liquefied by cooling them to very low temperatures, sometimes below −180°C.

Role of High Pressure

Increasing pressure forces gas molecules closer by reducing the space available.
When pressure is increased:

  • Distance between molecules decreases
  • Attractive forces become stronger
  • Gas becomes denser
  • The gas eventually liquefies

Liquefied petroleum gas (LPG) is a common example. Propane and butane are gases at normal conditions, but they become liquids when compressed into cylinders.

Combined Effect of Cooling and Compression

Most gases require both cooling and compression for successful liquefaction.
Cooling lowers kinetic energy, and compression pushes the particles together.
This combined method works more efficiently and is used in industries to liquefy gases in large quantities.

Critical Temperature and Liquefaction

Each gas has a specific critical temperature.
This is the highest temperature at which a gas can be liquefied, no matter how much pressure is applied.

  • If the gas is above its critical temperature, liquefaction will not occur even at very high pressures.
  • If it is below critical temperature, it can be liquefied easily by applying pressure.

For example:

  • Carbon dioxide has a critical temperature of 31°C.
    Above this, it cannot be liquefied unless cooled first.

Understanding critical temperature helps decide what conditions are needed for liquefaction.

Intermolecular Forces and Liquefaction

Liquefaction is directly related to the strength of intermolecular forces.
Gases with strong intermolecular forces are easier to liquefy.
Gases with weak forces require extremely low temperatures.

  • Ammonia (NH₃) and sulphur dioxide (SO₂) are easy to liquefy.
  • Helium (He) and hydrogen (H₂) are very difficult to liquefy because they have very weak forces.

This is why special equipment is needed to liquefy helium.

Methods Used for Liquefaction

Several practical methods are used:

  1. Linde’s Process

Uses cooling and pressure through repeated expansion and compression of gas.

  1. Claude’s Process

Uses external cooling and expansion to reach very low temperatures.

These methods help industries liquefy large amounts of gases safely.

Applications of Liquefied Gases

Liquefaction has many real-world uses:

  1. Domestic Use

LPG cylinders used for cooking contain liquefied propane and butane.

  1. Medical Use

Hospitals use liquid oxygen for patients and surgeries.

  1. Industrial Use

Liquid nitrogen is used for cooling metals and preserving food.

  1. Transport

Liquefied natural gas (LNG) is easier to store and transport in tankers.

  1. Scientific Research

Liquid helium is essential for cooling superconducting magnets.

Liquefaction makes gas storage practical and economical.

Advantages of Liquefaction

  • Saves storage space
  • Makes gases easier to transport
  • Allows gases to be used in controlled ways
  • Enables refrigeration and cryogenic applications
  • Reduces explosion risk by controlling pressure

These advantages make liquefaction a major process in modern industries.

Conclusion

Liquefaction of gases is the process of converting gases into liquids by applying low temperature, high pressure, or a combination of both. This reduces molecular motion and allows attractive forces to pull molecules closer. Liquefaction makes it possible to store, transport, and use gases in compact and efficient forms. It is essential in daily life, medicine, industry, and scientific research.