What is critical temperature?

Short Answer

Critical temperature is the highest temperature at which a gas can be liquefied by applying pressure alone. Above this temperature, no amount of pressure can convert the gas into a liquid. This means cooling is essential to bring the gas below its critical temperature before liquefaction can occur.

Every gas has its own critical temperature. Gases with higher critical temperatures are easier to liquefy, while gases with very low critical temperatures, such as helium and hydrogen, are difficult to liquefy and require special cooling methods.

Detailed Explanation

Critical Temperature

Critical temperature is a very important concept in the study of real gases and their liquefaction. It refers to the maximum temperature at which a gas can be converted into a liquid by applying pressure. If the gas is above its critical temperature, it cannot be liquefied, no matter how much pressure is applied. At or below this temperature, increasing pressure will gradually bring the molecules close enough to form a liquid.

The concept of critical temperature helps explain why some gases are easily liquefied while others require very low temperatures before they can be converted into liquid form. It also plays a major role in industrial processes such as refrigeration, air liquefaction, production of LPG, and cryogenic technology.

Meaning of Critical Temperature

Critical temperature is the boundary between liquefiable and non-liquefiable gas states. At temperatures below the critical point:

  • Molecules move slowly
  • Intermolecular attraction becomes effective
  • Pressure can compress the gas into a liquid

Above the critical temperature:

  • Molecules move too fast
  • Attraction forces are too weak
  • Pressure alone cannot bring particles close enough
  • Liquefaction becomes impossible

Thus, critical temperature sets the limit for gas liquefaction.

Why Critical Temperature Matters

Liquefaction depends on two factors:

  1. Low temperature → reduces molecular movement
  2. High pressure → forces molecules close

However, if temperature is above the critical value, intermolecular forces cannot overcome the high kinetic energy of molecules. Even if pressure is increased to extremely high levels, the gas will still not condense into a liquid. Therefore, gases must be cooled to or below their critical temperature before applying pressure.

This is why air liquefaction plants first cool gases before compressing them.

Relation Between Molecular Forces and Critical Temperature

A gas with strong intermolecular forces has a higher critical temperature because less cooling is required to liquefy it.
A gas with weak forces has a very low critical temperature, meaning it needs extreme cooling.

Examples:

  • Carbon dioxide has a critical temperature of 31°C
  • Ammonia has a high critical temperature because of hydrogen bonding
  • Nitrogen has a critical temperature of −147°C
  • Helium has an extremely low critical temperature (−268.9°C), making it very hard to liquefy

This relationship shows how strength of attraction between molecules affects liquefaction.

Behaviour of Gases at Critical Temperature

At critical temperature:

  • The distinction between gas and liquid begins to disappear
  • The gas becomes extremely dense
  • The gas exists in a special state called a supercritical fluid
  • Properties of both liquid and gas are observed
  • Surface tension becomes zero

This is a unique region of matter, useful in advanced scientific applications.

Critical Pressure and Critical Volume

Critical temperature is part of a set of three critical constants:

  • Critical temperature (Tc)
  • Critical pressure (Pc)
  • Critical volume (Vc)

Critical pressure is the minimum pressure needed to liquefy a gas at its critical temperature, and critical volume is the volume of one mole of gas at the critical point.

Together, these constants describe the exact conditions under which the gas changes from gaseous to liquid state.

Applications of Critical Temperature

Critical temperature has many practical uses:

  1. Gas Liquefaction

Industrial liquefaction of oxygen, nitrogen, and carbon dioxide depends on cooling below critical temperature.

  1. Refrigeration

Systems use knowledge of critical temperature to choose suitable refrigerants.

  1. Storage of Gases

LPG, LNG, and industrial gases are stored as liquids only when kept below their critical temperatures.

  1. Cryogenic Engineering

Ultra-low temperature technologies work with gases like helium and hydrogen.

  1. Chemical Industry

Supercritical fluids are used for extraction, purification, and chemical reactions.

Why Some Gases Are Hard to Liquefy

Gases with very low critical temperatures (such as helium and hydrogen) are difficult to liquefy because:

  • They have extremely weak intermolecular forces
  • They need very low temperatures
  • Simple compression is not enough
  • Special cooling cycles like Linde or Claude processes are required

This explains why liquefied helium is expensive and used only in high-tech applications.

Understanding Gas Behaviour with Critical Temperature

Critical temperature helps understand:

  • Why gases condense
  • How intermolecular forces influence states of matter
  • Why ideal gases do not liquefy
  • Why real gases deviate from ideal behaviour at low temperature
  • How pressure and temperature interact in phase changes

It connects the microscopic behaviour of molecules with macroscopic properties like pressure and liquefaction.

Conclusion

Critical temperature is the highest temperature at which a gas can be liquefied by applying pressure. Below this temperature, gases can condense when compressed, but above it, liquefaction is impossible without cooling. This concept is essential in understanding gas behaviour, designing refrigeration systems, selecting refrigerants, and liquefying industrial gases. It highlights the role of molecular forces and kinetic energy in phase changes.