<em>Le Châtelier's principle</em> states that when there is an dynamic equilibrium, and this equilibrium is disturbed by an external factor, the equilibrium will be shifted in the direction that can cancel the effect of the external factor to reattain the equilibrium.
The compound that has a common ion effect is Na₂CO₃.
When it is added to the solution, it is dissolved to produce Na⁺ and CO₃²⁻ which increases the concentration of CO₃²⁻ that will increase the concentration of the products side, so the reaction will be shifted to the left side to suppress the increase in the concentration of CO₃²⁻.
