Compounds having incongruent melting points

F irst we examine the behavior of the pure solid compound. If the temperature is raised, the state point moves along the line ab. At b liquid having the composition c forms. Since this liquid is richer in potassium than the original compound, some solid sodium d is left unmelted. Thus, on melting, the compound undergoes the reaction 

If a system of composition i is cooled, primary crystals of sodium form at j the liquid composition moves along jc as more sodium crystallizes. At k solid Na2K forms because of the peritectic reaction. 

The amount of sodium in the composition i is insufficient to convert the liquid c completely into compound. Hence the primary crystals of sodium are consumed completely. After the sodium is consumed, the temperature drops, Na2K crystallizes, and the liquid composition moves along cm at /, the tie line shows that Na2K, n, coexists with liquid m. When the temperature reaches o, pure potassium begins to crystallize the liquid has the eutectic composition p the system is invariant until the liquid disappears, leaving a mixture of solid potassium and solid Na2K. 

The sodium sulfate-water system forms an incongruently melting compound, Na2S04 IOH2O (Fig. 15.15a). The line eb is the solubility curve for the decahydrate, while the line ba is the solubility curve for the anhydrous salt. The figure shows that the solubility of the decahydrate increases, while that of the anhydrous salt decreases with 

If an unsaturated solution of composition g is heated, anhydrous salt will crystallize at/ if it is cooled, the decahydrate will crystallize at h. It is possible to supercool the solution to a temperature below /i then the heptahydrate will crystallize at i Fig. 15-15(b). The curve e b is the solubility curve for the heptahydrate, Na2S04 7H20. The peritectic temperature for anhydrous salt-heptahydrate-saturated solution is at 24.2 °C. In Fig. 15.15(b), the dashed lines are the curves for the decahydrate. The solubility curve for the heptahydrate lies for the most part in the region of stability of solid decahydrate-saturated solution. Therefore the equilibrium between solid heptahydrate and its saturated solution is a metastable one the system in such a state can precipitate the less soluble decahydrate spontaneously. 

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In certain cases, the melting point of the compound is not observed, because the mixture decomposes completely at a temperature below its melting point. Such a compound is said to have an incongruent melting point. A typical melting point-composition curve for this type... 

The system BaO — T1O2 comprises 5 compounds, three of which have an incongruent melting point. The lowest eutectic melts at 1317 °C. Only two of the compounds mentioned find practical applications BaTi03 and BaTi409. The former is especially significant and will be discussed in detail below. The other compound is one of the correcting phases used in rutile ceramics (see above). In addition, it constitutes abase for linear dielectrics with a low temperature dependence of permittivity. The properties of some of the materials dealt with above are listed in Table 27. 

The solid compound in Fig. 9-11 is said to have a congruent melting point since it melts into a liquid of the same composition as the compound. A solid compound is said to have an incongruent melting point if it does not melt into a liquid phase of the same composition but decomposes. It is of interest that solid-compound formation can occur with ideal-liquid behavior. 

The solute and solvent of a binary system may, and frequently do, combine to form one or more different compounds. In aqueous solutions these compounds are called hydrates for non-aqueous systems the term solvate is sometimes used. Two types of compound can be considered one can coexist in stable equilibrium with a liquid of the same composition, and the other cannot behave in this manner. In the former case the compound is said to have a congruent melting point in the latter, to have an incongruent melting point. 

The BioHi4-bcnzene system is a simple eutectic (eutectic point 20mol% Bi()Hi4, -7.0 °C). In BioH 14-toluene an incongruently melting compound is formed. Another group of workers have reported equilibrium diagrams for the Bi()H]4-arene systems (arene = benzene, durene, naphthalene, or biphenyl). ... 

Liquid mixtures freeze at lower temperatures than the pure components of which they are comprised. The lowest possible temperature in which we only have liquid is called the eutectic point. The presence of solid compounds of stoichiometry aj y increases the complexity of their phase diagrams. A compound with a definite melting point is termed congruent, while a compound that is not stable all the way up to a well-defined melting point is said to have an incongru-ent melting point. The state at which the latter case dissociates is called the peritectic point. 

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