The effect of simple additives on solubility

3 The effect of simple additives on solubility Solubility products 

For poorly soluble materials such as silver chloride and barium sulfate the concept of the solubility product can be used. The following equilibrium exists in solution between crystalline silver chloride AgCb and ions in solution  

Conceptually, the solid salt (sodium chloride, for instance) is converted to the gaseous (g) state, Na+(g) -n Cl (g), and each unit is then hydrated to form the species Na (aq) and Cl (aq). If the heat of hydration is sufficient to provide the energy needed to overcome the lattice forces, the salt will be freely soluble at a given temperature and the ions will readily dislodge from the crystal lattice. If the partial molal enthalpy of solution of the substance is positive, the solubility will increase with increasing temperature if it is negative the solubility will decrease, in agreement with Le Chatelier s principle. 

Strictly, K should be written in terms of thermodynamic activities and not concentrations, but activities can be replaced by concentrations (denoted by square brackets) because of the low solubilities involved (see section 3.3.1). At samration the concentration of the crystalline silver chloride [AgClJ is essentially constant and the solubility product, may 

The solubility product is useful for evaluating the influence of other species on the solubility of salts of low aqueous solubility. Some values of solubility products are quoted in Table 5.7. 

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