In some two component, solid liquid systems, a solid compound forms.
In systems in which the components have an interaction for such other, a solid state compound of the two compounds of the two components can form.
Formic acid and formaide form a solid state, one-to-one compound. The effect on the freezing point diagram is shown in fig. 1, such diagrams are understandable on the basis of the discussion of the diagrammatic problems. Each half of the fig. corresponds to the simple eutectic diagrams treated there.
Solutions which on cooling reach line NM or RW of fig. give rise to solid formaide, respectively. Solutions which on cooling reach line PN or PQ give rise to a solid which is a compound containing equimolar amounts of formic acid, and at point N the solution is in equilibrium with the new compound and formaide. Points N and Q represent two eutectics that generally have different temperatures.
Again, as in the preceding section, the initial slopes of the lines at M, P and R can be interpreted in terms of the enthalpy of fusion and the freezing point of the substance that separates out as a solid near these points. Likewise, the curves can be interpreted in terms of the solubility of these components and can be compared with the ideal solution expectations given by the above equation.
Compound formation in the solid state is frequently encountered with hydrates, the formation of hydrated compounds of sulphuric acid in the solid state. Again, such diagrams are easily understood as a series of simple eutectic diagrams side by side.
A complication does occur when a solid compound does not have sufficient stability to persist up to the temperature at which it would melt. In such cases the unstable solid breaks down into a solution, and the solid state of one or the other of the two components. This is illustrated by the system calcium fluoride calcium chloride, as shown in the fig. the decomposition of such a solid is referred to as a peritectic reaction or an incongruent melting. Thus the equimolar crystal: CaF2. CaCl2 of fig. breaks down at 737 degree C into a solution of composition B and solid CaF2. The dashed line shows how the diagram might have looked if the compound had survived to a real or congruent melting point. This line is helpful for visualizing the phase behavior but has, of course, no real significance.
Miscible solids: brief mention can be made, particularly in view of their importance as alloys, of system forming only one solid phase which is a solid solution. Such behavior is a result of complete mutual solubility of the solid phases in each other affects the phase diagram of a system that shows a simple eutectic. Such a partial solubility frequently occurs when the atoms of one component are small and can fit into the interstices of the lattice of the major component. In this way an interstitial alloy is formed. The carbon atoms in a carbon containing alloy are usually so accommodated.
Complete solubility of two solid phases usually results when the atoms of the two components are about the same size and can substitute for each other in the lattice to form a substitutional alloy. The system of copper and nickel shows this behavior. The upper of the two curves shows the temperature at which solutions of various compositions start to freeze. The lower curve gives the comparison of the solid which separates out at that freezing point. In this system the solid is always richer melting component than the solution from which it separates. The alloy consisting of 60 percent copper and 40 percent nickel is known as constantan.