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In physics, chemistry, and other related fields like biology, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic states of matter : solid , liquid , and gas , and in rare cases, plasma .
A reaction coordinate diagram may also have one or more transient intermediates which are shown by high energy wells connected via a transition state peak. Any chemical structure that lasts longer than the time for typical bond vibrations (10 −13 – 10 −14 s) can be considered as intermediate.
The concept of a transition state has been important in many theories of the rates at which chemical reactions occur. This started with the transition state theory (also referred to as the activated complex theory), which was first developed around 1935 by Eyring, Evans and Polanyi, and introduced basic concepts in chemical kinetics that are still used today.
The pressure on a pressure-temperature diagram (such as the water phase diagram shown above) is the partial pressure of the substance in question. A phase diagram in physical chemistry , engineering , mineralogy , and materials science is a type of chart used to show conditions (pressure, temperature, etc.) at which thermodynamically distinct ...
This process involves the dissipation of energy from the molecule to its surroundings, and thus it cannot occur for isolated molecules. A second type of nonradiative transition is internal conversion (IC), which occurs when a vibrational state of an electronically excited state can couple to a vibrational state of a lower electronic state. The ...
Reaction intermediates are often confused with the transition state. The transition state is a fleeting, high-energy configuration that exists only at the peak of the energy barrier during a reaction, while a reaction intermediate is a relatively stable species that exists for a measurable time between steps in a reaction. Unlike the transition ...
George Hammond developed the postulate during his professorship at Iowa State University. Hammond's postulate (or alternatively the Hammond–Leffler postulate), is a hypothesis in physical organic chemistry which describes the geometric structure of the transition state in an organic chemical reaction. [1]
Phase transitions (phase changes) that help describe polymorphism include polymorphic transitions as well as melting and vaporization transitions. According to IUPAC, a polymorphic transition is "A reversible transition of a solid crystalline phase at a certain temperature and pressure (the inversion point) to another phase of the same chemical composition with a different crystal structure."