Phase (matter)

In physical chemistry, thermodynamics, and material science, the phase is a spatial area in which the determined physical parameters ( order parameter, such as the density or refractive index ), and the chemical composition of the material are homogeneous. A stage in the sense of thermodynamics is any homogeneous part of a system.

Often the term is used to denote units of a system in which a substance in different states (solid, liquid and gaseous) is present, eg, water (liquid phase) with ice (solid phase) in a glass. Nevertheless, several phases with different order can also occur within a material in the same physical state, such as the transition from disordered carbon to graphite or in the transition to superfluidity in liquid helium.


The homogeneity of the phase can be illustrated further by the example of good ice in the water glass. Thus the water in the glass appears as a homogeneous entity. The ice is characterized by a different refractive index (you can see the edges go under water) and a different density from ( the ice floats on top ). But in itself it is again homogeneous. As well as the glass which forms a homogeneous phase itself.

Use of the term

The term phase is primarily used to distinguish two or more regions of homogeneous composition and properties from each other. This is due to that in thermodynamics typically interested in matters that relate to the exchange of energy or matter. Thus, a material in the melting of a solid to liquid phase transition ( phase change ). Likewise, for example pass in solving salt matter from the solid salt phase into the liquid aqueous phase.

A special case of the application of the term occurs in the description of different physical states and modifications of a Einstoffsystems, where the phase is used here mostly synonymous with these two terms.

Stability of phases

If different phases interact with each other, so they are stable only under certain conditions. These conditions (usually the pressure, temperature and composition) may be applied in a phase diagram. Here you can read quickly, how many and which phases are stable under certain conditions.

Clearly, this is especially for component systems, due to the differences, the phases only by their physical state or its modification here. Being on one of the faces in these diagrams, only one phase is present, one is on a line, two phases are present ( eg liquid and solid water ). When nodes can accordingly also be three ( the water of the triple point ) or more phases.

In multi-component systems, the charts are correspondingly more complicated, since one example, in the binary system already has to consider three variables. It can be pure phases are formed ( a Einstoffphase among other phases ) or mixed phases in this case. The more components are included, the more variants for the training of phases there. Generally, it can be calculated with the Gibbs phase rule, the maximum number of coexisting phases.

If two or more phases coexist, to phase boundaries, where the properties change, and thus the order parameter abruptly form. The phase boundaries between different phases are an exciting area of ​​research, since, for example, chemical reactions may proceed or change the properties of the substance compared to the interior of the phase.

In addition to the stable phases and metastable phases can exist. The phases are not thermodynamically stable, but may exist over a longer time. Examples are diamond under normal conditions or glass.

Difficulty of homogeneity concept

Homogeneity is here difficult to umbrella term: Depending on how exactly you look at the substance, it can appear as one or more phases:

  • Milk is mainly composed of an emulsion of small droplets of fat in water. So it represents a two-phase system with a water and a fat phase, even if it looks at first glance like a homogeneous liquid.
  • A pile of salt grains is comprised of a plurality of individual solid phase (salt grains), which are all of a gas phase (air) is surrounded, like the fat droplets in the milk. In the parlance refers to the set of small, similar to each other grains of salt as a solid phase.
  • Another example is a water- vapor, which acts overall homogeneous, but consists of two phases: the condensate droplets and gaseous water.
  • Micelles are indeed extremely small, they only consist of a few molecules, but they can be considered as condensed phase, while a solution of micelles on the eye makes a homogeneous impression.
  • A stone is at first glance a phase. However, many rocks are composed of different minerals - this is evident upon closer inspection. Each mineral forms a separate solid phase.

It can be seen that is often referred to in a particular arrangement of the phases ( evenly distributed small droplets or particles in the dispersions ) of only one phase.

In addition, inhomogeneities can also occur in homogeneous systems, by definition, to a certain extent. For example, minimum concentration and pressure differences in gases due to gravity ( concentration gradient, gravity pressure ) are possible, or the composition of crystals is slightly different on their surfaces. In such cases, one does not speak of two phases.