Reactions between Solids and Solutions

Reactions between solids and solutions may be treated in a similar manner. Every solid has a definite solubility in every solvent, just as it has a definite vapour pressure at a given temperature. The solubility depends on the nature of the solvent and on the temperature, but is independent (in dilute solutions at least) of the presence of other dissolved or undissolved substances. So-called insoluble substances differ from soluble substances only in that their solubility is exceedingly small. 

MTien several solid substances react with one another in presence of a solvent, the condition for equilibrium may be obtained as follows. In homogeneous solutions which contain definite, although possibly very minute, concentrations of all the molecules taking part in the reaction, the law of mass action will hold, provided that the concentrations are not too high. At constant temperature, however, the concentrations of all the substances which are present in the solid state are determined by their solubilities. The constant values of these concentrations may therefore be included in the equilibrium constant, like the vapour pressures of the solid substances in the calculations of the previous paragraph. For reactions between solids and solutions the law of mass action therefore assumes the same form as for homogeneous solutions, viz. ah 

As an illustration of this, let us take the ionisation of a sparingly soluble salt (saturated solutions of soluble salts are not sufficiently dilute to permit of the application of the simple laws of solution). 

The ionisation of thallous chloride takes place according to the equation TlCl Tl -I- CF 

As the concentration of the unionised TlCl in the saturated solution has a constant value at each temperature, the law of mass action for the homogeneous solution, viz. 

---

For minerals crystaUizing from low-temperature aqueous solutions, the primary controls on their stability should be (a) the activity of the species in solution and G ) protonation reactions between solid and solution at the edges of polyhedron chains deprotonation of edge anions promotes attachment of aqueous cation species (i.e., crystallization), whereas protonation of edge anions weakens their bonds to the bulk structure and promotes dissolution. With regard to crystallization, the character and activity of the aqueous species is of interest as these provide groups of atoms that may attach to the solid during crystallization. 

---