critical point

History

Thomas Andrews first discovered the critical point in 1861 while studying carbon dioxide. He noticed that above 31°C, the gas would not liquefy under any pressure. Later, Van der Waals developed the theory, linking it to molecular forces.

How it works

Under normal conditions, liquid molecules attract each other and stay together, while in a gas they fly apart. With increasing temperature and pressure, the kinetic energy of molecules rises, and intermolecular forces can no longer hold them in an ordered state. At the critical point, these forces become equal: the liquid ceases to be dense, and the gas ceases to be rarefied. It's like a crowd at a concert: when there are too many people and everyone is moving, you can't tell who is standing still and who is dancing—everyone blends into a homogeneous mass.

History

Thomas Andrews first discovered the critical point in 1861 while studying carbon dioxide. He noticed that above 31°C, the gas would not liquefy under any pressure. Later, Van der Waals developed the theory, linking it to molecular forces.

How it works

Under normal conditions, liquid molecules attract each other and stay together, while in a gas they fly apart. With increasing temperature and pressure, the kinetic energy of molecules rises, and intermolecular forces can no longer hold them in an ordered state. At the critical point, these forces become equal: the liquid ceases to be dense, and the gas ceases to be rarefied. It's like a crowd at a concert: when there are too many people and everyone is moving, you can't tell who is standing still and who is dancing—everyone blends into a homogeneous mass.

History

The critical point was discovered by Charles Cagniard de la Tour in 1822, but systematically studied by Thomas Andrews in 1869. Andrews showed that for each gas there is a temperature above which it cannot be liquefied. In 1873, Johannes van der Waals proposed an equation of state for real gas that for the first time explained the existence of a critical point based on molecular forces. In the 20th century, Landau developed the theory of second-order phase transitions, and Wilson applied renormalization group to describe universal critical phenomena.

How it works

From a microscopic perspective, the critical point is the result of a balance between intermolecular attraction, which tends to condense the substance, and thermal motion, which tends to expand it. At the critical temperature, the attraction energy equals the thermal energy, and the phase boundary disappears. This is described by the van der Waals equation, which includes parameters a and b. At the critical point, compressibility tends to infinity, and the correlation length of fluctuations becomes macroscopic, leading to critical opalescence. In modern understanding, critical phenomena are universal and do not depend on system details, but are determined only by the dimensionality of space and the symmetry of the order parameter, as described by renormalization group theory.

Open problems & fun facts

Complete description of critical exponents for all universality classes; Unification of the theory of critical phenomena with quantum effects at low temperatures; Prediction of critical points for complex liquids, such as ionic liquids or polymer solutions; Study of the critical point in nuclear matter (quark-gluon plasma phase)

💡 At the critical point, water becomes an excellent solvent for organic matter and can even dissolve glass. Supercritical water is used for eco-friendly dry cleaning and destruction of toxic waste.