Faasisääntöjä

Faasisääntöjä, often translated as phase rules, are fundamental principles in thermodynamics that describe the conditions under which different phases of a substance can coexist in equilibrium. The most well-known is the Gibbs phase rule, formulated by Josiah Willard Gibbs.

The Gibbs phase rule relates the number of degrees of freedom (F) of a system to the number of phases (P) present and the number of independent components (C). The rule is expressed by the equation F = C - P + 2. The '2' in the equation represents the temperature and pressure as intensive variables that can be independently changed.

Degrees of freedom refer to the number of intensive variables (such as temperature, pressure, or concentration) that can be altered independently without changing the number of phases in equilibrium. For instance, in a system with only one phase (P=1) and one component (C=1), such as pure water above its boiling point, the degrees of freedom are F = 1 - 1 + 2 = 2. This means both temperature and pressure can be changed independently while still maintaining a single liquid phase.

If two phases are in equilibrium (P=2) and there is one component (C=1), like liquid water and its vapor at the boiling point, then F = 1 - 2 + 2 = 1. This indicates that only one variable, either temperature or pressure, can be changed independently. If the temperature is fixed, the pressure is also fixed (e.g., the boiling point at a given atmospheric pressure).

When three phases coexist in equilibrium (P=3) for a one-component system (C=1), such as solid ice, liquid water, and water vapor at the triple point of water, then F = 1 - 3 + 2 = 0. This means there are no degrees of freedom; the system exists only at a specific temperature and pressure.

Faasisääntöjä are crucial in understanding phase diagrams and predicting the behavior of materials under various conditions, finding applications in chemistry, materials science, and engineering.