Final answer:
The common ion effect occurs when adding an ion common to the ionic compound in a solution, which shifts the equilibrium and decreases the solubility of the ionic species or suppresses the ionization of a weak acid or base.
Step-by-step explanation:
Understanding the Common Ion Effect in Chemistry
The common ion effect refers to the effect on equilibrium when a substance that has an ion in common with the dissolved species is added to the solution. For example, if you have a saturated solution of calcium sulfate, CaSO4, adding a calcium ion, which is a common ion, will lead to a decrease in the solubility of the ionic compound. This is because the added common ion shifts the equilibrium in favor of the solid, causing more of the solid to precipitate out of the solution.
Another example is when we consider the dissolution of silver iodide:
AgI(s) ⇌ Ag+ (aq) + I−(aq)
Adding either silver ions (Ag+) or iodide ions (I−) to this system will push the equilibrium towards the solid AgI, resulting in precipitation and a lowered concentration of dissolved ions.
The same principle applies to weak acids or weak bases. If a common ion is added, it suppresses the ionization of the weak acid or base because the product of the dissociation is already present in higher concentrations due to the added common ion.