Phase transition
A phase transition is an abrupt change in the state of a thermodynamic system when external conditions, such as temperature or pressure, change. First-order transitions, like ice melting or water boiling, involve the release or absorption of latent heat and a jump in density. Second-order transitions occur without latent heat but with a sharp change in heat capacity; examples include the disappearance of iron's magnetization upon heating (the Curie point) or the transition to a superconducting state. Near the critical point, where differences between phases vanish, critical phenomena are observed, described by renormalization group theory.
History
Research began in the 19th century with the work of James Joule and William Thomson. Lev Landau created a general theory of phase transitions in 1937, and later Kenneth Wilson applied the renormalization group to explain critical phenomena.
How it works
The molecules of a substance are in constant motion. Heating increases their energy, and the bonds holding molecules in an ordered structure break—solid melts or liquid evaporates. Cooling, on the contrary, slows motion, and molecules arrange into a regular lattice.
💡 At the critical point, a liquid and its vapor become indistinguishable: one can observe opalescence—a cloudiness due to intense density fluctuations.