Phase transition

Phase transition is a term used in physics to describe the transformation of a thermodynamic system from one phase or state of matter to another. A phase of a thermodynamic system and the states of matter have uniform physical properties. During a phase transition of a given medium, certain properties of the medium change, often discontinuously, as a result of the change of external conditions, such as temperature, pressure, or others. For example, a liquid may become gas upon heating to the boiling point, resulting in an abrupt change in volume.

Types of phase transitions

Phase transitions are classified into two broad categories, named after the Ehrenfest classification: first-order phase transitions and second-order phase transitions.

First-order phase transitions

First-order phase transitions are those that involve a latent heat. During such a transition, a system either absorbs or releases a fixed (and typically large) amount of energy. Because energy cannot be instantaneously transferred between the system and its environment, first-order transitions are associated with "mixed-phase regimes" in which some parts of the system have completed the transition and others have not. This phenomenon is familiar to anyone who has boiled water: the water does not instantly turn into gas, but forms a turbulent mixture of water and steam bubbles.

Second-order phase transitions

Second-order phase transitions are also called continuous phase transitions. They are characterized by a smooth and continuous change in the properties of the system, such as its specific heat capacity. Second-order phase transitions do not involve a latent heat: the changes occur smoothly and continuously from one phase to the other, with no energy being absorbed or released during the transition.

Examples of phase transitions

Some common examples of phase transitions include, but are not limited to, the melting of ice, the boiling of water, and the magnetization of a material in a magnetic field.

Melting of ice

The melting of ice is a first-order phase transition. When ice is heated, it absorbs heat energy that is used to break the hydrogen bonds between the water molecules. The heat absorbed is the latent heat of fusion. The temperature remains constant at 0°C until all the ice has melted.

Boiling of water

The boiling of water is another first-order phase transition. When water is heated to its boiling point, it absorbs heat energy to change from the liquid phase to the gas phase. The temperature remains constant at 100°C until all the water has vaporized.

Magnetization of a material

The magnetization of a material in a magnetic field is an example of a second-order phase transition. As the external magnetic field is increased, the magnetic moments of the atoms align with the field, causing the material to become magnetized. This transition is continuous and does not involve a latent heat.

See also

• Critical point (thermodynamics)
• Phase (matter)
• State of matter
• Thermodynamic system

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