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Critical Temperature

Updated Jun 14, 2026

Critical temperature is the specific temperature above which a substance cannot exist as a liquid, regardless of the amount of pressure applied. At this threshold, the gas phase and the liquid phase of a substance become indistinguishable, forming a single, homogeneous state known as a supercritical fluid.

On This Page

  1. Short Definition
  2. Detailed Explanation
  3. Key Formula / Syntax / Principle
  4. Important Characteristics
  5. Practical Example
  6. Common Confusions or Misconceptions
  7. Related Terms
  8. Why It Matters

Short Definition

The critical temperature (##T_c##) represents the upper limit of the temperature range in which a substance can undergo a phase change from gas to liquid. When a substance is heated beyond its critical temperature, its kinetic energy becomes so high that intermolecular attractive forces are insufficient to maintain the liquid state, even under extreme compression.

Detailed Explanation

In thermodynamics, the behavior of a substance is governed by its position on a phase diagram, typically plotted as pressure versus temperature (##P##-##T## diagram). For most substances, the boundary between liquid and vapor phases ends at a specific point called the critical point, defined by the critical temperature (##T_c##) and the critical pressure (##P_c##).

As a substance approaches its critical temperature, the density of the liquid phase decreases while the density of the vapor phase increases. At the exact moment the substance reaches ##T_c##, the meniscus—the visible boundary between the liquid and the gas—vanishes. The substance then enters the supercritical state. Supercritical fluids possess unique properties: they exhibit the diffusivity of a gas and the solvent power of a liquid, making them highly valuable in industrial extraction processes, such as decaffeinating coffee or cleaning precision components.

Key Formula / Syntax / Principle

The critical temperature is intrinsically linked to the Van der Waals equation of state, which accounts for the volume occupied by gas molecules and the attractive forces between them. The critical constants can be derived from the parameters ##a## (representing attractive forces) and ##b## (representing excluded volume):

###T_c = \dfrac{8a}{27Rb}###

Where:

  • ##T_c## is the critical temperature (in Kelvin).
  • ##a## is the Van der Waals constant for attraction.
  • ##b## is the Van der Waals constant for molecular volume.
  • ##R## is the universal gas constant.

Important Characteristics

The following table summarizes the behavior of substances relative to their critical temperature:

State Condition Physical Behavior Phase Status
##T < T_c## Liquefaction possible with pressure Subcritical
##T = T_c## Phase boundary vanishes Critical Point
##T > T_c## Liquefaction impossible Supercritical Fluid

Practical Example

Consider water, a substance with a relatively high critical temperature. The ##T_c## for water is approximately ##374^\circ\text{C}## (##647\text{ K}##) at a critical pressure of ##22.06\text{ MPa}##.

In a standard domestic pressure cooker, water is heated under pressure, allowing it to reach temperatures above ##100^\circ\text{C}## while remaining liquid. However, if one were to attempt this in a system exceeding ##374^\circ\text{C}##, no amount of pressure could force the steam back into a liquid state. The water would exist as a supercritical fluid, which behaves as a highly reactive, dense gas. This property is utilized in supercritical water oxidation (SCWO) to destroy hazardous organic waste.

Common Confusions or Misconceptions

A frequent misconception is that pressure is the only factor required to liquefy a gas. While pressure is essential, it is entirely ineffective if the system is above the critical temperature. Another point of confusion involves the difference between "critical temperature" and "boiling point." The boiling point is pressure-dependent and refers to the phase transition at a specific pressure, whereas the critical temperature is an intrinsic material constant that defines the absolute limit of the liquid phase.

Term Definition
Critical Pressure The minimum pressure required to liquefy a gas at its critical temperature.
Supercritical Fluid A state of matter where distinct liquid and gas phases do not exist.
Critical Volume The volume occupied by one mole of a substance at its critical temperature and pressure.

Why It Matters

Understanding critical temperature is vital for several fields of science and engineering:

  • Refrigeration and HVAC: Engineers must select refrigerants with appropriate critical temperatures to ensure that the cooling cycle remains efficient within the operating range of the equipment.
  • Chemical Engineering: Supercritical fluid extraction (SFE) is a green technology used in pharmaceutical and food industries, relying on the unique solvent properties of substances above their ##T_c##.
  • Geology: Deep-crustal fluid movement and hydrothermal vent activity are governed by the behavior of water and minerals at supercritical conditions.

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