Phase transition

Things can change how they look and feel.

This is called a change of state. Matter can be a solid. It can be a liquid. It can even be a gas.

Heat or cold makes this happen. For example, heat can turn liquid into gas. This can happen very fast. A change in heat or pressure causes the change. It is fun to watch things change!

Everything around us is made of matter. Matter can change its form. This is called a phase transition.

Most people know the three main states. These are solids, liquids, and gases.

A phase transition happens when conditions change. Usually, we change the temperature or the pressure.

When you heat a liquid, it can turn into a gas. This happens at the boiling point. At this point, the liquid and gas are equally likely to exist. If you cool a liquid, it can turn into a solid. This happens at the freezing point.

Changes can also happen inside solids. A solid can change its shape without changing what it is made of. This is called allotropy. Some metals change their structure when they get hot or cold. Even magnets can change. A magnet can lose its power at a special point called the Curie point.

These changes help us understand how the world works.

A phase transition is a way that matter changes its form. This can happen to living things, the physical world, or even space. Most often, we use this term for changes between solids, liquids, and gases. Sometimes, matter can even become a plasma. During these changes, the physical properties of the substance change. This happens because external conditions change. Usually, these conditions are temperature or pressure.

How does this work? It happens when a system crosses from one region to another. For example, water turns into solid ice when the temperature drops below the freezing point. At a specific transition point, like the boiling point, two forms are equally likely to exist. Below the boiling point, the liquid is more stable. Above the boiling point, the gas is more stable. Sometimes, a substance can stay in a state that is not its most stable form. This is called a metastable state. This can happen during superheating or supercooling.

Scientists have studied these changes for a long time. Paul Ehrenfest created a way to classify these transitions. He looked at how the energy of a system changes. He labeled them as first-order or second-order transitions. Later, Lars Onsager found a new way to look at these changes. In 1944, he found an exact solution for the Ising model. This model helps us understand magnetic changes. His work showed that some changes are more complex than Ehrenfest thought.

There are many different types of transitions to learn about. In elements, a solid changing its structure is called allotropy. In compounds, this is called polymorphism. For example, iron can change its crystal structure. This can happen in carbon steel during a martensitic transformation. Magnetic materials also change at a point called the Curie point.

Mixtures are even more complex than pure substances. They can have a temperature span where solid and liquid exist together. Some mixtures undergo a eutectic transformation. This is when a liquid turns into two different solid phases while cooling.

Phase transitions are all around us in the real world. You see them when ice melts or water boils. They even happen in very tiny ways. Some metals become superconductors when they are cooled below a critical temperature. Even the early universe had phase transitions as it cooled down. Scientists also study how metals like titanium and nickel mix together. These mixtures create different stable compounds.

Understanding these changes helps us understand how everything in our universe is built.

A phase transition is a physical process where a medium moves from one state to another. In physics, chemistry, and biology, this describes how matter changes its fundamental form. Most people use the term to describe changes between solids, liquids, and gases. In some rare cases, matter can also transition into a plasma. During these changes, the physical properties of the medium change. This occurs because external conditions, such as temperature or pressure, are altered.

To understand how this happens, we must look at the stability of different states. At a specific phase transition point, such as a boiling point, two different phases have identical free energies. This means both forms are equally likely to exist at that exact moment. Below the boiling point, the liquid phase is the more stable state. Above the boiling point, the gaseous form becomes the more stable state. A phase transition usually occurs when a system crosses from one region to another on a phase diagram. For example, water turns from a liquid to a solid as soon as the temperature drops below the freezing point.

Sometimes, matter behaves in unexpected ways through metastable states. A system can be changed diabatically so that it passes a transition point without actually changing its state. This results in a metastable state, which is less stable than the equilibrium state but not completely unstable. Examples of this include superheating and supercooling. In structural transitions, solids can change their arrangement without changing their chemical makeup. In elements, this process is called allotropy. In chemical compounds, it is known as polymorphism. One specific example is the martensitic transformation, which occurs in carbon steel.

Phase transitions also describe changes in magnetic ordering. A well-known example is the transition between ferromagnetic and paramagnetic phases. This specific change occurs at a temperature known as the Curie point. Scientists use the Ising model as a simplified but useful way to study these magnetic transitions.

Beyond magnetism, transitions can involve complex mixtures. While pure compounds have a single melting point, mixtures can have a temperature span. In this span, solid and liquid phases can coexist in equilibrium. This is common in solid solutions where the two components are isostructural.

There are several specialized types of transformations involving mixtures. A eutectic transformation occurs when a single-phase liquid is cooled and turns into two different solid phases. If the process starts from a solid instead of a liquid, it is called a eutectoid transformation. A peritectic transformation happens when a single-phase solid is heated and turns into a solid and a liquid. There are also peritectoid, monotectic, and spinodal decomposition processes. These complex interactions allow different chemical compositions to separate or combine in specific ways.

Historically, scientists have used different systems to classify these events. Paul Ehrenfest created a classification based on how thermodynamic free energy behaves. He identified first-order transitions, which involve discontinuous changes like density. He also identified second-order transitions, which are continuous in their first derivative. For instance, the ferromagnetic transition in iron is a second-order transition. The magnetization increases continuously from zero as temperature drops below the Curie temperature. However, the magnetic susceptibility shows a discontinuity.

The Ehrenfest classification was later challenged by new discoveries. In 1944, Lars Onsager found an exact solution for the Ising model. His work showed that the specific heat diverged logarithmically at the critical temperature. This was different from earlier approximations. Because of this, modern science uses a different classification scheme. Phase transitions are now divided into two broad categories. These transitions can also be non-thermodynamic, such as quantum or topological phase transitions. These connections show how phase changes influence everything from tiny atoms to the early history of the universe.