What happens to gases above the critical temperature?

Short Answer

Above the critical temperature, gases cannot be liquefied by applying any amount of pressure. Even if pressure is extremely high, the gas will not change into a liquid because the molecules move too fast and the intermolecular forces cannot pull them together.

In this condition, the gas behaves like a highly compressed fluid. It may become very dense, but it will not form a separate liquid phase. Instead, it exists as a supercritical fluid, which has properties of both gases and liquids.

Detailed Explanation

Gases Above the Critical Temperature

When a gas is heated above its critical temperature, it enters a special region where it can no longer exist as a normal liquid, no matter how much pressure is applied. Critical temperature is the highest temperature at which a gas can be liquefied by pressure alone. Above this point, the kinetic energy of gas molecules becomes so high that intermolecular attractions cannot hold them together to form a liquid.

This behaviour is important in understanding real gases, phase changes, and the limitations of liquefaction. The state of matter above the critical temperature is often referred to as a supercritical state, where the gas has both gas-like and liquid-like properties.

Why Liquefaction Becomes Impossible

To liquefy a gas, two changes are usually needed:

  • Decrease in temperature
  • Increase in pressure

Cooling reduces the speed of molecules so that attractive forces become effective. However, above the critical temperature:

  • Molecules move extremely fast
  • Intermolecular forces cannot slow or pull them together
  • Even enormous pressure cannot overcome their high kinetic energy
  • No phase boundary appears

Thus, the gas remains in a non-condensed state.

Behaviour of Gas Molecules Above Critical Temperature

Above the critical temperature:

  • Molecules have high kinetic energy
  • Intermolecular attractions are almost negligible
  • Gas molecules remain widely spaced
  • Compression increases density but not phase change
  • No clear boundary between liquid and gas forms

This behaviour shows that temperature has a stronger effect on liquefaction than pressure.

Formation of Supercritical Fluid

When pressure is increased above the critical pressure while the temperature remains above the critical temperature, the gas forms a supercritical fluid.

A supercritical fluid has special properties:

  • It spreads like a gas
  • It flows like a liquid
  • It can dissolve substances like a liquid
  • It has no definite boundary between liquid and gas
  • Its density can be controlled by changing pressure

Supercritical fluids are used in many industries because they behave in ways neither gases nor liquids can.

Characteristics of Gases Above Critical Temperature

  1. Cannot be liquefied
    No matter how much pressure is applied, the gas will stay in a non-liquid state.
  2. High molecular motion
    Molecules move too fast for attractive forces to work effectively.
  3. Becomes dense under pressure
    Increasing pressure will make the gas denser, but not convert it into a liquid.
  4. No phase boundary
    The distinction between liquid and gas disappears.
  5. Forms supercritical state
    At very high pressures, the gas turns into a supercritical fluid.

Examples of Gases Above Critical Temperature

  • Carbon dioxide above 31°C
  • Nitrogen above −147°C
  • Oxygen above −118°C
  • Ammonia above 132°C

For these gases, if temperature exceeds their critical temperatures, liquefaction becomes impossible unless temperature is lowered first.

Importance of Understanding Gas Behaviour Above Critical Temperature

  1. Industrial Gas Liquefaction

Factories must cool gases below critical temperature before applying pressure.
For example, carbon dioxide must be cooled below 31°C to become a liquid.

  1. Refrigeration and Air Conditioning

Refrigerants must have suitable critical temperatures for efficient cooling.

  1. Storage and Transport of Gases

LPG, liquid oxygen, and LNG must be stored below their critical temperatures.

  1. Supercritical Fluid Technology

Supercritical carbon dioxide is used in:

  • Extraction of essential oils
  • Decaffeination of coffee
  • Dry cleaning
  • Chemical reactions

These processes rely on gases being above their critical temperature.

  1. Thermodynamics and Real Gas Studies

Understanding behaviour above critical temperature helps explain deviations from ideal gas laws.

Difference Between Gas and Supercritical Fluid Above Critical Temperature

Even above the critical temperature, pressure determines whether the substance behaves more like:

  • A gas (low pressure)
  • A supercritical fluid (high pressure)

Both states do not show liquid behaviour, but supercritical fluids have liquid-like density and gas-like flow.

Conclusion

Above the critical temperature, gases cannot be liquefied no matter how much pressure is applied. The molecules have too much kinetic energy for intermolecular forces to bring them together into a liquid state. Instead, the gas becomes denser under pressure and may form a supercritical fluid with unique properties. This concept is essential in gas liquefaction, refrigeration, industrial processing, and understanding real gas behaviour.