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In chemistry, the lever rule is a formula used to determine the mole fraction (x i) or the mass fraction (w i) of each phase of a binary equilibrium phase diagram.It can be used to determine the fraction of liquid and solid phases for a given binary composition and temperature that is between the liquidus and solidus line.
The fourth condition (straight solidus/liquidus segments) may be relaxed when numerical techniques are used, such as those used in CALPHAD software packages, though these calculations rely on calculated equilibrium phase diagrams. Calculated diagrams may include odd artifacts (i.e. retrograde solubility) that influence Scheil calculations.
This equation of state of the mixture is called the lever rule. [5] [6] [7] The dotted parts of the curve in Fig. 1 are metastable states. For many years such states were an academic curiosity; Callen [8] gave as an example, "water that has been cooled below 0°C at a pressure of 1 atm. A tap on a beaker of water in this condition precipitates ...
English: An phase diagram designed to explain the lever rule. Based on an image from Smith, William F.; Hashemi, Javad (2006), Foundations of Materials Science and Engineering (4th ed.), McGraw-Hill, p. 319, ISBN 0-07-295358-6 .
Only a lever rule extends the point to a diagram. Gibbs: If a homogenous body [a single phase system] has C independently variable components [in addition to the thermodynamic variables p and T], the phase of the body [a rim of crystal or bubble] is evident capable [by itself] of [only] C+1 independent variations, [because the Gibbs-Duhem ...
At the peritectic decomposition temperature the compound, rather than melting, decomposes into another solid compound and a liquid. The proportion of each is determined by the lever rule. In the Al-Au phase diagram, for example, it can be seen that only two of the phases melt congruently, AuAl 2 and Au 2 Al, while the rest peritectically decompose.
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In thermodynamics, the phase rule is a general principle governing multi-component, multi-phase systems in thermodynamic equilibrium.For a system without chemical reactions, it relates the number of freely varying intensive properties (F) to the number of components (C), the number of phases (P), and number of ways of performing work on the system (N): [1] [2] [3]: 123–125