Short Answer

The critical point is the specific temperature and pressure at which the liquid and gas phases of a substance become indistinguishable. At this point, the substance changes into a supercritical fluid that has properties of both liquid and gas.

Above the critical temperature, no amount of pressure can convert a gas back into a liquid. The critical point is shown on a phase diagram at the end of the liquid–gas boundary line and is an important concept in understanding phase behaviour.

Detailed Explanation

Critical Point

The critical point is a special and important condition in phase diagrams where the distinction between liquid and gas disappears. At this point, the substance reaches a temperature and pressure where its liquid and gas states have the same density, making them merge into a single phase known as a supercritical fluid. This phase has unique properties—it can flow like a gas but dissolve substances like a liquid.

The critical point is different for every substance and gives valuable information about how the substance behaves under extreme temperature and pressure conditions. It also marks the upper limit at which a liquid can exist. Beyond this point, the substance cannot be liquefied no matter how much pressure is applied.

1. Meaning and Concept of the Critical Point

The critical point occurs when:

• The critical temperature is reached
• The critical pressure is applied
• The densities of liquid and gas become equal

At this exact point, the liquid and gas phases meet and become identical.
There is no visible boundary between them, unlike at normal conditions where the surface separating liquid and gas (known as the meniscus) is clearly seen.

For example, when water reaches its critical point at:

• Critical temperature: 374°C
• Critical pressure: 218 atm

It forms a supercritical fluid instead of boiling into gas.

2. Critical Point on a Phase Diagram

On a phase diagram, the critical point is found at the upper end of the liquid–gas line.
This point marks:

• The highest temperature at which liquid can exist
• The beginning of the supercritical region
• The end of the phase boundary between liquid and vapour

After the critical point, the liquid–gas boundary disappears because the two phases become the same.

The region beyond this point is labelled the supercritical fluid region.

3. Characteristics of the Critical Point

The critical point has several defining features:

1. Critical Temperature

This is the highest temperature at which a substance can exist as a liquid.
Above this temperature, even very high pressure cannot liquefy the gas.

1. Critical Pressure

This is the minimum pressure needed to liquefy a gas at its critical temperature.

1. No Phase Boundary

At the critical point, the liquid and gas phases merge, so there is no clear surface between them.

1. Formation of Supercritical Fluid

The substance behaves partly like a liquid and partly like a gas.
It becomes:

• As dense as a liquid
• As free-flowing as a gas
• Very good at dissolving substances

These special features make supercritical fluids useful in many technologies.

4. Importance of the Critical Point

The critical point is important for several reasons:

1. Understanding Phase Behaviour

It explains how substances behave under extreme conditions and why liquids cannot exist above a certain temperature.

1. Industrial Applications

Supercritical fluids are used in:

• Extraction of caffeine from coffee (supercritical CO₂)
• Production of medicines
• Cleaning processes
• Polymer manufacturing

These processes depend on the properties of substances above their critical points.

1. Scientific Research

Critical point data helps chemists and engineers design equipment that operates at high pressures and temperatures.

5. Examples of Critical Points

Some well-known critical points include:

Water

• Critical temperature: 374°C
• Critical pressure: 218 atm
  Water becomes a supercritical fluid used in power plants and chemical reactions.

Carbon Dioxide

• Critical temperature: 31°C
• Critical pressure: 73 atm
  Supercritical CO₂ is widely used in extraction and cleaning because it is safe and non-toxic.

Ammonia

• Critical temperature: 132°C
• Critical pressure: 113 atm
  Used as a refrigerant and in fertiliser production.

6. Properties of Supercritical Fluids (Above the Critical Point)

A substance above the critical point behaves as a supercritical fluid with unique characteristics:

• High density like a liquid
• Low viscosity like a gas
• Good ability to dissolve materials
• Ability to diffuse quickly

This combination makes supercritical fluids very efficient for chemical processes.

Conclusion

The critical point is the specific temperature and pressure at which the liquid and gas phases of a substance become identical and form a supercritical fluid. It marks the limit beyond which a liquid cannot exist. The critical point plays an important role in phase diagrams, scientific research, and industrial applications, especially those involving supercritical fluids such as supercritical CO₂ and water.