The term soluble is arbitrary. How soluble? Soluble compared to what? One often used criterion is that a substance is soluble if a saturated solution has a concentration >= 0.1 M. Of course, that would never work for soluble proteins.
Many substances dissolve to a lesser degree. Calcium carbonate ( CaCO3), found in seashells, is characterized as not soluble. In fact, it dissolves to make a saturated solution containing about 6.2 mg in 1.0 L of solution.
Although calcium carbonate dissolves poorly, it dissociates (nearly) entirely into ions when it does dissolve. In other words, there is no CaCO3 in solution. When the solution is saturated, the dissolution of CaCO3 into dissolved, solvent separated ions is matched by the rate at which the ions react to reform CaCO3. This equilibrium is written as follows:
Calcium carbonate is a complicated situation because the carbonate ion can react further to form other ions in water solution. A less common substance, AgCl(s), has fewer complications:
AgCl Ag+ + Cl-
We might write the mass action expression for this as:
[Ag+] [Cl-] [AgCl]
At equilibrium, this would equal some constant, Keq. However, since AgCl is a solid whose concentration (activity) is not easily changed, its value is taken into the Keq to form a new type of equilibrium constant, Ksp, the solubility product constant.
Ksp = [Ag+] [Cl-]
To estimate the solubility of a substance use the Ksp expression and the value of Ksp. For example, the Ksp value for AgCl is 1.8 x 10-10. The dissociation equation of silver chloride is written as:
Several types of problems are used in conjunction with Ksp. We determine the solubility given the Ksp. We determine the Ksp given the solubility. Most important, we look at the interactions of the ions in special situations. In one case, we see what the presence of excess of one ion (the common ion) has on solubility. In another, we see what happens if one of the ions reacts to form either a complex (with a Lewis base like ammonia) or reacts as a base (or acid) with H+ (or OH-).
