Answer:
decrease the molar solubility of the solid relative to its molar solubility in pure water.
Step-by-step explanation:
The common ion effect is defined as a decrease in the solubility of a solute because of the addition of a second solute with which it has a common ion. If a solution already contains a solute MX and another ionic solid containing BX is added to the solution, the X^- anion is common to the both species. Hence the presence of MX will decrease the solubility of BX compared to the solubility of BX in pure water.
Generally, when a soluble ionic solid is placed in a solution containing a common ion, the molar solubility of the solid decreases compared to its molar solubility in pure water.