Phase diagram
The phase diagram illustrates states and their corresponding phases as a function of state variables (e.g. pressure and temperature) dar. alternative designations include state diagram, or phase diagram equilibrium diagram.
It is a commonly used tool in chemistry, physics and materials science specifically. Find application phase diagrams mostly in solutions and alloys, but in principle, any other type of substance or mixture of substances.
Phases are much more common defined as the three states of matter (solid, liquid, gas) and therefore only in the special case of their synonyms. There are various forms of phase diagrams, depending on how many substances, and phase variable is considered. Also, phase transitions occur within a physical state between different states of order.
Pressure - temperature phase diagram of pure substances
A pT diagram for the three different states of aggregation of a pure substance is very well suited to explain the scheme underlying the phase diagrams. The diagrams contain some curves, the areas of different phases, or also physical states, diffuse. These curves, which are called phase boundary lines represent the mixing regions of these phases dar. Under the given conditions through them consequently there are several phases in thermodynamic equilibrium. These areas are mathematically described by the Clausius-Clapeyron equation, and variations thereof, in particular the Clausius -Clapeyron equation for the phase transition is condensed / gaseous. The lines are the boiling point curve ( between the triple point and critical point phase boundary liquid / gaseous ), sublimation ( solid / gas between zero and triple point phase boundary ), and melting pressure curve ( phase boundary solid / liquid) respectively. Boiling point and sublimation can be summarized here also to the vapor pressure curve. The critical point and in this case have the triple point on the Gibbs' phase rule, no degree of freedom, all the lines have a degree of freedom (temperature and pressure) and in the phase areas, there are two degrees of freedom ( temperature and pressure).
The case of the water, shown in the lower right of the phase diagram, is particularly critical to an understanding of the dynamics of the atmosphere, and thus of the weather with respect to the humidity and the water vapor. The phase diagram of water is due to its and its importance for many areas the most common phase diagram and also has an important peculiarity. The anomaly of water leads to the phase diagram of water, a special feature is observed, which it shares only with a few other substances. The melting pressure curve deviates to the left, that is at lower temperature values than the triple point. This is unusual and ultimately leads to the fact that ice is less dense and lighter than the surrounding water, hence can float on water. This anomaly is due to the physical properties of the water molecules and the consequent hydrogen bonds.
Multicomponent systems
Especially in materials science and materials science, chemistry and chemical engineering, but also in geology, is the multi-component systems and for purposes of illustration, especially the two-component systems is of particular importance, in which case metals and their alloys or minerals are decisive. These are mixtures of substances of various components with respect to their mixing behavior in the corresponding isobaric temperature - mole fraction phase diagrams ( isobaric change of state) or temperature - mass fraction phase diagrams are described. The following considerations restrict to simplify to binary systems, where multi-component systems, quadratic mole fraction - square diagrams ( four components), etc., or three-dimensional isobaric Tx phase diagrams ( three components) can be described in isobaric and isothermal mole fraction triangle diagrams ( three components).
If two substances, usually same physical state, mixed together, then one or more mixing phases are formed, which are dependent on the general miscibility of materials whose respective concentration, pressure and temperature. Depending, it may happen that complete miscibility of substances only a mixed phase occurs, or trained with limited solubility, two different mixed phases, which are referred to as miscibility gap. The substances are not mixed, then this mixture gap extends over the whole Tp phase diagram.
Ideal binary mixture, Siedediagramm, melting diagram
In the case of an ideal mixture, the mixture gap always has a lens shape. Characteristic for ideal behavior are the lack of volume and temperature change in the mixing procedure. The mixture diagram (melting chart) of forsterite () and fayalite ( ) represents an example of this, said the composition of olivine.
On the left side of such a diagram, the first component (A) is present as a pure substance, and consequently on the right side, the second component (B) also, so it inscribes a vertical limit also here in order to illustrate that the abscissa in both directions is limited. The resulting curves meet at a point on the left and right margins, thus describing the dependency of the phase transition temperatures of the composition of the mixed phase, the edge points representing the respective transition temperature of the pure substances. The transition temperatures for pure substances are by definition independent of the direction of phase change, which explains the outer points. However, If the substances into a mixture, then the transition values , depending on whether the substance evaporates or condenses or freezes or melts differ. Since all three states form a common equilibrium only in the triple point, one distinguishes the Tx diagrams of two occurring aggregate states, the transition being solid-gas not listed here due to the low relevance.
Melting diagram ( solid-liquid)
The boundary line that separates the liquid region is called the liquidus line, and those delimiting the fixed range, as solidus line. Above the liquidus line is the substance completely liquid ( shortcut: L), below the solidus line is completely determined (abbreviation ). The region between the liquidus and solidus line is referred to as miscibility gap (abbreviated ).
At high temperature the liquid phase has a lower Gibbs free energy in the normal case ( ) as the solid phase; this implies a minimum of a lower enthalpy H () and / or higher entropy S. Therefore, it is " preferred " by the laws of thermodynamics, and the solid phase melts in favor of the liquid completely. However, the lower the temperature, the better the energy conditions of the solid phase, after which the liquid phase progressively solidifies. It happens that the energetic " upper hand " depends on the mixing ratio of the substances, and consequently emerge more corresponding mixed phases. Only when the temperature drops to such an extent that the solid phase, regardless of the mixture ratio has the higher energy, the isotherm is completely in the solid phase portion of the diagram.
It should be noted that without a change in temperature after a certain time ( which may be several millions of years of geology ) a dynamic equilibrium is established. The phase transition processes thus cancel each other out, but in principle continue to exist. Particularly impressive this is in the mixed crystals, which, when immersed in a miscibility gap, very slow different phases forming what is known as segregation.
On cooling from the melt, some of the substance solidifies at a certain temperature-dependent ratio. This ratio can be determined using the tie line. In fact, in the miscibility gap does not exist thermodynamically stable composition of the two substances, which would correspond to the " Entrance Composition ". During the cooling wander the thermodynamically stable compositions of melt and solid phase along the isotherm. The composition of the melt moves along the liquidus line downward while the composition of the solid phase move with the solidus line also parallel to the bottom. Both remain always in the same isotherm, ie lie in the Tx diagram opposite horizontally. During the melting of the solid phase, the same applies, except that the compositions migrate with the rising temperature in the diagram above. The case passed through temperatures of the respective line is called the liquidus and solidus temperature.
Siedediagramm ( liquid-gas )
The upper line represents the Siedediagramm, in contrast to the melt diagram of the condensation curve (also called vapor line ), while the lower line represents the boiling point. Consequently, the upper phase is gaseous and the lower liquid. Everything else is analogous to the melting diagram. Apply the temperature - mole fraction (Tx ) or pressure - mole fraction plots (px diagrams) about the interpretation and calculation of a distillation.
Real binary mixture
Since real mixtures often behave differently than ideal mixtures, the phase diagrams may differ from almost any idealized image. Phase diagrams of eutectic compositions have a eutectic point. One example is the binary system of the diopside mineral anorthite. More to the area associated terms are peritectically and monotektisch.
Are particularly frequent downward (convex isobar ) or rare upward ( concave isobars ) unlimited miscibility gaps, the maxima and minima are referred to as upper and lower critical Entmischungspunkte. Example is a mixture of phenol and water. A method for determining the composition of the individual phases of a two-component system provides the Konodenregel.