The critical point is one of the most important features on a phase diagram in IB Chemistry Topic 1 and Topic 5. It marks a special temperature and pressure at which the distinction between liquid and gas disappears. Above this point, a substance becomes a supercritical fluid, a unique state of matter with properties of both liquids and gases. Understanding the critical point helps explain industrial extraction processes, unusual phase behavior, and why increasing pressure cannot always liquefy a gas.
What Is the Critical Point?
The critical point is the temperature and pressure at which the liquid–gas boundary ends, and the liquid and gas phases become indistinguishable.
At the critical point:
- Density of liquid = density of gas
- Surface tension falls to zero
- Meniscus disappears
- Substance becomes a supercritical fluid
Above this point, there is no longer a clear separation between liquid and gas.
Key Features of the Critical Point
The critical point consists of two values:
1. Critical Temperature (Tc)
The highest temperature at which a substance can exist as a liquid, no matter the pressure.
2. Critical Pressure (Pc)
The minimum pressure needed to liquefy a gas at the critical temperature.
A gas cannot be liquefied above its critical temperature, even if the pressure is extremely high.
Why the Critical Point Exists
As temperature increases:
- Molecules move faster
- Intermolecular forces weaken
- Liquid density decreases
- Gas density increases
Eventually, the densities meet in the middle.
At that moment, the boundary between liquid and gas disappears, creating a new state: the supercritical fluid.
This explains why:
- You cannot compress steam at high temperatures into a liquid
- Liquids gradually lose their distinguishable surface near the critical point
The Supercritical Fluid Region
Above the critical point, a substance exists as a supercritical fluid, which has unusual properties:
Liquid-like properties:
- High density
- Good dissolving power
Gas-like properties:
- High compressibility
- Rapid diffusion
This makes supercritical fluids extremely useful in industrial and environmental chemistry.
Examples of Critical Points for Common Substances
1. Water
- Tc ≈ 374°C
- Pc ≈ 218 atm
Above these conditions, water becomes supercritical and behaves neither fully like a liquid nor a gas.
2. Carbon dioxide
- Tc ≈ 31°C
- Pc ≈ 73 atm
CO₂ becomes supercritical at relatively low temperatures, making it very useful in industry.
Uses of Supercritical Fluids
1. Supercritical CO₂ extraction
Used for:
- Decaffeinating coffee
- Extracting essential oils
- Producing herbal supplements
Supercritical CO₂ is non-toxic, efficient, and leaves no solvent residues.
2. Supercritical water oxidation
Used to destroy hazardous waste with high efficiency.
3. Enhanced oil recovery
Supercritical CO₂ dissolves hydrocarbons and improves extraction.
4. Material synthesis
Used to produce nanoparticles and advanced materials.
The Critical Point on a Phase Diagram
On a phase diagram:
- The liquid–gas equilibrium line ends at the critical point
- Beyond this point, no line separates liquid and gas
- The upper-right region is the supercritical zone
This is the end of phase boundaries—not just another transition point.
Why the Critical Point Matters
The critical point helps explain:
- Why gases cannot always be liquefied
- Why high-pressure steam behaves abnormally
- How supercritical fluids can dissolve substances efficiently
- Trends in intermolecular forces and density
It also appears frequently in IB exam diagrams and conceptual questions.
Common IB Misunderstandings
“Critical point means extremely high temperature only.”
No—pressure is equally important.
“Supercritical fluid is just hot gas.”
Incorrect—it behaves differently from both liquids and gases.
“Liquefying a gas is always possible with enough pressure.”
False—above Tc, it is impossible.
“Critical point is a melting point.”
It has nothing to do with melting; it relates only to liquid–gas behavior.
FAQs
Why can't a gas be liquefied above its critical temperature?
Because particles move too fast for intermolecular forces to pull them into a liquid state.
What happens to the meniscus at the critical point?
It disappears—the interface between liquid and gas vanishes.
Are supercritical fluids used in real chemistry?
Yes—supercritical CO₂ is widely used in extraction, cleaning, and industrial processes.
Conclusion
The critical point marks the temperature and pressure at which the liquid–gas boundary disappears, producing a supercritical fluid with hybrid properties. It is an essential concept for understanding phase diagrams, intermolecular forces, and industrial applications in IB Chemistry. Knowing how the critical point works helps students interpret physical behavior at extreme conditions and understand why liquids and gases become indistinguishable.
