Phase transition
Physical process of transition between states of a medium.
In physics, chemistry, and biology, a phase transition—also called a phase change—describes the physical shift of a medium from one state to another. Most often, this term refers to changes between the fundamental states of matter: solid, liquid, gas, and, less commonly, plasma. A thermodynamic phase and the states of matter each have uniform physical properties. When a phase transition occurs in a given medium, some of its properties change because external conditions like temperature or pressure shift. This change can be abrupt and discontinuous; for instance, a liquid heated to its boiling point turns into a gas, causing a sudden jump in volume. The specific external conditions where this transformation happens define the phase transition point.
**Types**
**States of matter** Phase transitions commonly involve a substance moving between the four states of matter. At the phase transition point—for example, the boiling point—the two phases involved, such as liquid and vapor, have identical free energies, making them equally likely to exist. Below the boiling point, the liquid is the more stable phase; above it, the gas is more stable. For a single component, the most stable phase at various temperatures and pressures is shown on a phase diagram, which typically depicts states in equilibrium. A phase transition usually occurs when pressure or temperature changes cause the system to cross from one region to another, like water turning from liquid to solid when the temperature drops below freezing. An exception is when a system is changed diabatically (rather than adiabatically), allowing it to pass a phase transition point without actually transitioning. The resulting state is metastable—less stable than the phase it would have become, but not unstable. This happens in superheating and supercooling. Metastable states do not appear on standard phase diagrams.
**Structural** Phase transitions can also happen when a solid changes its internal structure without altering its chemical composition. In elements, this is called allotropy; in compounds, it is polymorphism. Examples include shifting from one crystal structure to another, from a crystalline solid to an amorphous solid, or from one amorphous structure to another (polyamorphs). The martensitic transformation is a key phase change in carbon steel and serves as a model for displacive transformations. Order-disorder transitions, such as in alpha-titanium aluminides, are another type. As with states of matter, structural phase transitions can involve metastable-to-equilibrium transformations. A metastable polymorph that forms quickly due to lower surface energy will convert to an equilibrium phase if enough thermal energy is supplied to overcome an energetic barrier. Many dynamic, or soft, microporous materials—especially metal–organic frameworks—show structural phase transitions between closed and open forms. The phase landscape can be more complex, with intermediate phases discovered. Additionally, guest molecules can stabilize specific phases, leading to crystal configurations that depend on the surrounding atmosphere's composition.
**Magnetic** Phase transitions can also describe changes between different types of magnetic ordering. The best-known example is the transition between ferromagnetic and paramagnetic phases in magnetic materials, occurring at the Curie point. Another example is the transition between differently ordered magnetic structures—commensurate or incommensurate—such as in cerium antimonide. A simplified but highly useful model for magnetic phase transitions is the Ising model.
**Mixtures** Phase transitions involving solutions and mixtures are more complex than those for a single compound. While chemically pure compounds have a single melting point between solid and liquid, mixtures can either melt congruently at one temperature or have different liquidus and solidus temperatures, creating a temperature range where solid and liquid coexist in equilibrium. This often occurs in solid solutions, where the two components are isostructural. There are also phase transitions involving three phases: a eutectic transformation, where a two-component single-phase liquid cools and turns into two solid phases; the same process starting from a solid is called a eutectoid transformation. A peritectic transformation occurs when a two-component single-phase solid is heated and transforms into a solid and a liquid. A peritectoid reaction is similar but involves only solid phases. A monotectic reaction involves a liquid changing into a solid and a second liquid, where the two liquids have a miscibility gap. Separation into multiple phases can happen via spinodal decomposition, where a single phase is cooled and splits into two different compositions. Non-equilibrium mixtures, such as supersaturation, can also occur.
**Other examples** Other phase changes include: a transition to a mesophase between solid and liquid, such as a liquid crystal phase; the dependence of adsorption geometry on coverage and temperature, as seen with hydrogen on iron (110); and the emergence of superconductivity in certain metals.
- field
- Physics, chemistry, biology
- known_for
- Transitions between states of matter, structural, magnetic, and mixture phase changes
- types
- First-order, second-order (Ehrenfest classification), metastable transitions
- examples
- Boiling, freezing, ferromagnetic transition, eutectic transformation, superconductivity
Lore & Background
Phase transitions commonly refer to when a substance transforms between one of the four states of matter to another. At the phase transition point, such as the boiling point, the two phases involved have identical free energies and are equally likely to exist. Below that point, one phase is more stable; above it, the other is more stable. Metastable states, such as superheating and supercooling, can occur when a system is brought past a phase transition point without undergoing the transition.
Phase transitions are not limited to changes among solid, liquid, gas, and plasma. They also encompass structural changes within a solid, where a substance alters its crystal structure without changing its chemical composition. In elements, this is called allotropy; in compounds, polymorphism. Examples include transitions from one crystal structure to another, from a crystalline to an amorphous solid, or between amorphous forms. The martensitic transformation in carbon steel is a classic model of such displacive transformations. Order-disorder transitions, as seen in alpha-titanium aluminides, are another type. Metastable polymorphs can form rapidly due to lower surface energy and may later transform to an equilibrium phase with sufficient thermal energy to overcome an energetic barrier. Some dynamic, soft microporous materials, particularly metal–organic frameworks, exhibit structural phase transitions between closed and open forms, sometimes involving intermediate phases; guest molecules can stabilize specific phases, making crystal configurations dependent on the surrounding atmosphere.
Magnetic phase transitions also occur, such as the change between ferromagnetic and paramagnetic states at the Curie point, or transitions between commensurate and incommensurate magnetic structures, as in cerium antimonide. The Ising model provides a simplified but highly useful description of such magnetic transitions.
For mixtures and solutions, phase transitions are more complex than for pure compounds. While pure substances have a single melting point, mixtures may exhibit congruent melting at a single temperature or have separate liquidus and solidus temperatures, creating a range where solid and liquid coexist. This is common in solid solutions where components are isostructural. Additionally, three-phase transitions occur: a eutectic transformation, where a two-component l
Reader's Guide
Phase transitions are fundamental to understanding the behavior of matter under varying conditions. They encompass not only changes between solid, liquid, and gas but also structural changes in solids (allotropy, polymorphism), magnetic ordering transitions (e.g., ferromagnetic to paramagnetic at the Curie point), and complex transitions in mixtures (eutectic, peritectic, spinodal decomposition). The Ehrenfest classification distinguishes first-order transitions, which involve a discontinuous change in a first derivative of free energy (like density), from second-order transitions, which are continuous in the first derivative but discontinuous in a second derivative. Phase transitions also occur in non-thermodynamic systems, such as quantum phase transitions and topological transitions, where parameters other than temperature drive the change. Their study is essential in materials science, condensed matter physics, and cosmology.
Did You Know?
- Phase transitions can occur adiabatically, leading to metastable states like superheating or supercooling.
- The martensitic transformation in carbon steel is a model for displacive phase transformations.
- Isotope fractionation occurs during phase transitions, enriching heavier isotopes in the liquid phase during condensation.
- Phase transitions can occur in non-thermodynamic systems, such as quantum phase transitions and topological phase transitions.
The Core Mechanism: How Matter Shifts State
A phase transition is the physical process by which a medium moves from one state to another. While the term most often describes the familiar shifts among solid, liquid, and gas—and in rarer instances plasma—it fundamentally captures any transformation between distinct phases of a thermodynamic system, each characterized by uniform physical properties. The driving force behind such a shift is a change in external conditions, typically temperature or pressure. What makes these transitions striking is that they can be genuinely discontinuous: heat a liquid past its boiling point and it abruptly becomes vapor, accompanied by a sudden jump in volume. The precise external conditions at which this transformation takes place define what we call the phase transition point. At that exact point, the two coexisting phases—say, liquid and vapor—possess identical free energies, making them equally probable. Below that threshold, the liquid is the thermodynamically favored state; above it, the gaseous form takes over as the more stable configuration. This elegant balance of free energies is what governs which phase a system will occupy under given conditions.
Mapping the Landscape: Phase Diagrams and Metastability
For a single-component substance, the most stable phase at any given combination of temperature and pressure can be charted on a phase diagram. These diagrams typically depict states in thermodynamic equilibrium, and a phase transition is visualized as the system crossing a boundary from one region to another—water freezing into ice the moment temperature dips below the freezing point being a classic illustration. Yet nature is not always so orderly. In certain circumstances, a system can be driven past a transition point diabatically without actually undergoing the expected transformation. The resulting state is metastable: less stable than the equilibrium phase it would eventually become, but not unstable either. Superheating a liquid beyond its normal boiling point or supercooling it below the freezing point are everyday examples of this phenomenon. Crucially, metastable states do not appear on standard phase diagrams, which means the full picture of a material's behavior extends beyond what those familiar charts display. This gap between the idealized diagram and real-world behavior is where much of the practical complexity in materials science and engineering resides.
Beyond the Obvious: Structural and Magnetic Transformations
Phase transitions are not limited to the familiar melting and boiling of substances. A solid can reorganize its internal structure without any change in chemical composition. In pure elements this is called allotropy; in compounds, polymorphism. The shift might be from one crystal lattice to another, from crystalline to amorphous, or between different amorphous arrangements known as polyamorphs. In carbon steel, the martensitic transformation serves as a textbook example of a displacive phase change, while alpha-titanium aluminides illustrate order-disorder transitions. A metastable polymorph that forms quickly because of lower surface energy will, given enough thermal energy to surmount an energetic barrier, relax into the equilibrium structure. Microporous materials and metal–organic frameworks display structural shifts between closed and open configurations, and guest molecules in the surrounding atmosphere can even stabilize particular crystal phases. In the magnetic realm, the transition from ferromagnetic to paramagnetic ordering at the Curie point is the most celebrated example, and the Ising model offers a simplified yet powerful framework for understanding such magnetic phase changes.
Complex Systems: Mixtures, Multi-Phase Transitions, and Beyond
When mixtures and solutions are involved, phase transitions grow considerably more intricate than those of pure compounds. A chemically pure substance has a single melting temperature, but a mixture may exhibit congruent melting or, more commonly, a temperature span between a liquidus and a solidus where solid and liquid coexist in equilibrium. Three-phase transformations add further layers: a eutectic reaction converts a two-component liquid into two distinct solid phases upon cooling; a eutectoid reaction does the same starting from a solid; a peritectic reaction transforms a single solid into a solid plus a liquid; and a monotectic reaction splits a liquid into a solid and a second liquid exhibiting a miscibility gap. Spinodal decomposition describes a single phase that, when cooled, separates into two compositions. Non-equilibrium phenomena like supersaturation also complicate the picture. Beyond these, phase transitions encompass the emergence of liquid crystal mesophases, the onset of superconductivity below a critical temperature, Bose–Einstein condensation in bosonic fluids such as liquid helium, and even the breaking of fundamental symmetries in the laws of physics as the early universe cooled.
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